Article

Petro geoscience 2. In situ stresses in sedimentary rocks (part 2): Applications of stress measurements

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Abstract

This paper addresses the applications of in situ stress measurements in sedimentary rocks, with emphasis on Canadian case histories. Combined with other data, the measurements allow stress regimes in sedimentary basins to be characterized as attached or detached to underlying rocks. With information on stress orientations and relative magnitudes, hydraulic fracture propagation azimuths can be predicted, as can preferred flow directions in hydrocarbon reservoirs. If enough data are available to diagnose anomalous stress orientations in a basin, a number of significant geomechanical inferences can be made. Stress orientations, together with magnitudes, help assess the likelihood of serious borehole wall collapse during drilling, whereas stress magnitudes alone will help determine in which rock units hydraulic fractures will advance. Fluid production rates appear to be inversely related to stress magnitudes in coalbed methane deposits and it is likely that a similar situation exists with respect to conventional oil and gas reservoirs. Overpressuring mechanisms are particularly sensitive to stress magnitudes and vice versa, depending on the tectonic setting. Induced earthquakes may be triggered by changes in effective stress, a function of stress magnitude and fluid pressure. Today's stress regimes give us a window into the recent geological past, and possibly the future, in that they are manifestations of the ongoing structural evolution of the earth's surface. Horizontal stress orientations appear to reflect both the present motions of tectonic plates and their overall geometry: in short, today's geotectonics. However, at least in sedimentary rocks, stress orientation measurements give minimal indications of remnant (past) stresses inherited from former regimes. Although stress magnitudes seem to be controlled largely by today's lithostatic loads and by contemporary tectonic compression, they also appear to involve components of remnant stresses derived from former loads (such as ice sheets), crustal rebound and a region's thermal history.

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... Previous research studies prove the importance of in-situ stress studies in drilling (e.g., Abdelghany et al., 2021;Brudy and Zoback, 1999) production optimization (e.g., Sen, 2021a, 2021b;Khaksar Manshad et al., 2019), wellborestability (e.g., Gholami et al., 2015;Karatela et al., 2016;McLean and Addis, 1990), and hydraulic-fracturing (e.g., Lakirouhani et al., 2016;Nasehi and Mortazavi, 2013). Previous authors have proposed many classical methods for in-situ stress orientations and magnitudes, such as the field measurement method, focal mechanism method, geological data analysis method, experimental simulation method, logging data calculation method (e.g., Bell, 1996bBell, , 1996aZoback, 2007;Zoback et al., 2003), and the hydraulic fracturing method (e.g., Lee and Ong, 2018;Zang and Stephansson, 2010). Using image logs to identify wellbore deformation and failure, particularly the breakouts and fractures, is among the most frequently used methods for determining the orientation and magnitude of horizontal in situ stresses and has shown valuable results (e.g., Abdelghany et al., 2022;Cui et al., 2009;Haghi et al., 2018;Han, 2021;Li et al., 2019aLi et al., , 2019bRajabi and Tingay, 2014;Riedel et al., 2016;Tang et al., 2021;Zhang et al., 2018;Radwan et al., 2021;Xie et al., 2022). ...
... This region is located along the northeastern boundary of the Arabian Plate with the Anatolian and Eurasian plates and is considered one of the most prominent regions of convergent deformation on Earth ( Fig. 1 and Fig. 2, Allen et al., 2004;Berberian, 1995;Doski, 2021;Tatar et al., 2004). Comprehensive knowledge of the magnitudes and directions of principal stresses plays a vital role in every stage of oil and gas field development, from the early stages to well abandonment (Barton and Moos, 2010;Bell, 1996aBell, , 1996bRajabi et al., 2016a;Tingay et al., 2005aTingay et al., , 2005bZoback, 2007). The five essential principal components of any geomechanical study are the magnitudes of the three principal in-situ stresses (vertical stress (S v ), minimum horizontal stress (S h ), and maximum horizontal stress (S H )), the distribution of formation pore pressure (P P ), and the orientation of S H (Baouche et al., 2020b(Baouche et al., , 2020aBell, 1996b;Busetti and Reches, 2014;Rajabi et al., 2016aRajabi et al., , 2016bTingay, 2015;Zoback, 2007). ...
... Comprehensive knowledge of the magnitudes and directions of principal stresses plays a vital role in every stage of oil and gas field development, from the early stages to well abandonment (Barton and Moos, 2010;Bell, 1996aBell, , 1996bRajabi et al., 2016a;Tingay et al., 2005aTingay et al., , 2005bZoback, 2007). The five essential principal components of any geomechanical study are the magnitudes of the three principal in-situ stresses (vertical stress (S v ), minimum horizontal stress (S h ), and maximum horizontal stress (S H )), the distribution of formation pore pressure (P P ), and the orientation of S H (Baouche et al., 2020b(Baouche et al., , 2020aBell, 1996b;Busetti and Reches, 2014;Rajabi et al., 2016aRajabi et al., , 2016bTingay, 2015;Zoback, 2007). Gravitational and tectonic forces dominate the in-situ stresses, and are particularly associated with horizontal tectonic movements (Kang et al., 2010). ...
Article
The study area is in the Kurdistan Region of Iraq, including the northwestern extension of the Zagros suture zone, where present-day stress data derived from actual measurements is rare. The magnitudes and orientations of principal in-situ stresses were determined using well logging data. The deepest drilled formation in this study was the Albian upper Qamchuqa Formation in northeastern Iraq, which is a carbonate hydrocarbon reservoir. In the present study, several wireline logs including borehole image logs, conventional wireline logs, and six-arm caliper logs for two vertical wells provided detailed borehole breakout and drilling-induced tensile fracture (DITF) data between depths of 1600 and 2240 m. Two oil wells (wells A and B) were evaluated, and in well A, several breakouts occurred in eight distinct zones. Below the depth of 1600 m, 26 pairs of breakouts with considerable length were observed. The standard deviation of breakout azimuths was 11°, and the mean azimuth of the breakouts was N162°E. Using breakout and DITF data, the orientations of the maximum and minimum horizontal stresses (SH and Sh) were determined, and the results were validated by six-arm caliper measurements and lithological evaluation from wireline log data. The mean azimuth of maximum horizontal in-situ stress was N72°E, which showed relative consistency with the NE–SW to E–W direction of tectonic movement and previous studies in the nearby Zagros suture zone. Ultimately, the magnitudes of the three principal in-situ stresses were determined by two methods: poroelastic strain theory and breakout analysis. The stress regime below the depth of 1600 m was a reverse faulting stress regime, and above this depth, it likely changed to a strike-slip faulting stress regime. The determined stress regime was consistent with the nature and dynamics of the tectonic plate movement of the Arabian and Eurasian (Iranian) plates. Consequently, results suggest that the pattern of present-day tectonic stress is controlled mainly by the collision between the Arabian and Eurasian plates.
... Both these primary and secondary sources of stress control the regional stress field within the sedimentary basin and stress field within the basement rocks directly underlying the basin (Zoback, 1992;Tingay, 2009) (Figure 2.15). The local stress resulting from the other sources are superimposed on the regional stress field (Bell, 1996a). plate-scale indicated large blue arrows and "2 nd order" regional-scale indicated by the small blue arrows; from Tingay (2009) modified after Zoback (1992). ...
... The present-day in-situ stress orientation can be significantly impacted by the existing geological structures such as folds, faults, and salt diapirs (Aleksandrowski et al., 1992;Bell, 1996a;Finkbeiner et al., 1997b;Zoback, 2010). The variation of in-situ stress pattern due to geological structures can be observed at different spatial scales from a couple of meters to several kilometers (Bell, 1996a;Yale, 2003;Tingay et al., 2005;Brooke-Barnett et al., 2015;Mukherjee et al., 2017;Rajabi et al., 2017c;Rajabi et al., 2017d;Heidbach et al., 2018). ...
... The present-day in-situ stress orientation can be significantly impacted by the existing geological structures such as folds, faults, and salt diapirs (Aleksandrowski et al., 1992;Bell, 1996a;Finkbeiner et al., 1997b;Zoback, 2010). The variation of in-situ stress pattern due to geological structures can be observed at different spatial scales from a couple of meters to several kilometers (Bell, 1996a;Yale, 2003;Tingay et al., 2005;Brooke-Barnett et al., 2015;Mukherjee et al., 2017;Rajabi et al., 2017c;Rajabi et al., 2017d;Heidbach et al., 2018). A geological structure such as open fracture or fault acts as a mechanical discontinuity (Bell, 1996a) and is unable to sustain shear stresses (Tingay, 2009) which causes the reorientation of the in-situ stress field close to the structure. ...
Thesis
The Middle to Late Jurassic Walloon Coal Measures in the Surat Basin in Queensland, eastern Australia are prolific sources of Coal Seam Gas (CSG). The CSG development and production within the Surat Basin have grown exponentially in the last decade to support the Liquefied Natural Gas projects and domestic gas supply. Most of the current CSG production and development are within the fairway areas along the eastern and northwestern margin of the Surat Basin and compartmentalised by major folds and faults. The CSG reservoir performance within the Walloon Coal Measures is a function of gas saturation and permeability variation, and this thesis focusses on their relationship with in-situ stress and subsurface fractures within the different domains, to provide an insight into the CSG reservoir performance. This thesis investigates the in-situ stress variations, fracture orientations and intensity, and their relationship with initial reservoir permeability to understand the rheological behaviour of coal seams within the Walloon Coal Measures. In addition, the coal rank, composition and lithotype were analysed against gas adsorption capacity and saturation variation within the Walloon Coal Measures. Data available in the public domain were utilised and analysed for the study. Analysis of vitrinite reflectance, geothermal gradient and basement lithological description within coal and interburden of Walloon Coal Measures showed that higher rank coals, albeit bordering between sub-bituminous and high volatile bituminous, occur where sedimentary cover to the pre-Jurassic basement is thin. It is interpreted that the rank enhancement is associated with the heat flow from the metasedimentary and granitic rocks of the pre-Jurassic aged basement, without the thermal insulation of the underlying Permo-Triassic Bowen Basin sediments. The rank/depth gradient and palaeotemperature analysis showed widespread uplift at the basin scale with higher uplift at the basin edge. The elevated ranks, along with maceral composition, would influence gas generation and sorption capacity. Analysis of petrographic data from 89 wells across the Surat Basin corroborates that the Walloon coals are generally vitrinite rich with some variability observed between the younger Upper and Lower Juandah, and older Taroom coal measures. Lower Juandah coals have a relatively higher liptinite content and show higher gas content and saturation in comparison to the Upper Juandah and Taroom coal measures. There is substantial variation in adsorption capacity for coals at a similar rank, which makes rank a poor regional predictor. In contrast, maceral composition shows low to moderate correlation with the gas adsorption capacity, regardless of rank, and can be used as a regional predictor, albeit a weak one. Coal composition and lithotype both affect the fracture abundance within Walloon Coal Measures and, hence, control the inherent permeability along with stress. However, the weak to moderately positive relationship between coal composition and depth normalised permeability in each coal measures suggests higher order of control (geological domains, in-situ­ stress, fractures orientation) besides coal composition. The mean in-situ stress orientation (SHmax) from the Australian and World Stress Map projects show a significant variation of SHmax orientation in the eastern and northwestern part of the Surat and underlying Bowen basins from the regional NE-SW orientation. The SHmax pattern and fracture data (joints) from borehole image log analysis of 33 vertical wells along with the interpreted permeability data in the eastern Surat Basin were used to understand the influence of local structures (folds, oblique-slip faults) on the spatial variations of in-situ stress and permeability in the study wells. The results of this component of the thesis reveal that wells in the relatively simple structural areas, with subtle faulting, show higher permeability even at depth (>600 m) when the coal fracture trends are parallel and up to 40 degrees to the present-day SHmax orientation. The high permeability CSG reservoirs are mostly in a transitional normal to strike-slip tectonic stress regime within the study area and differ from the overall stress regime, due to the rheological differences between coal and interburden rocks within the Walloon Coal Measures. This thesis demonstrates that the reservoir permeability variation within the Walloon Coal Measures depends on the coal type, in-situ stress, and fracture relationship. Sub-optimal relationship between in-situ stress and fractures, higher effective vertical stress, and mineralisation within fractures causes low permeability. Interaction between meteoric recharge and some Walloon Coal Measures coals may also have introduced microbially charged water and potential gas migration or leakage that enhances or reduces saturation. The combination of gas saturated areas with low permeability results in poor reservoir production. Therefore, the ability to predict these likely poor production areas would enhance production efficiencies through better planning and well design to overcome low permeability. The comparative analysis of in-situ stress, fracture and coal rheological behaviour within prominent Surat Basin CSG fields, not only assists in testing the conceptual models of reservoir performance, but also provides a holistic framework that can be used in identifying the production variability in other eastern Australian basins.
... Knowledge of the variation of S Hmax orientation at these scales contributes to a better understanding of the petroleum applications. Previous studies have reported that different local structural patterns, variable mineral compositions, rock mechanical properties, faults are the main geological factors affecting the S Hmax orientation in many sedimentary basins worldwide (Bell, 1996a(Bell, , 1996b(Bell, , 1996bCuss et al., 2003;Shen et al., 2007;Brooke-Barnett et al., 2015;Alt and Zoback, 2016;Rajabi et al., 2017). ...
... Knowledge of the variation of S Hmax orientation at these scales contributes to a better understanding of the petroleum applications. Previous studies have reported that different local structural patterns, variable mineral compositions, rock mechanical properties, faults are the main geological factors affecting the S Hmax orientation in many sedimentary basins worldwide (Bell, 1996a(Bell, , 1996b(Bell, , 1996bCuss et al., 2003;Shen et al., 2007;Brooke-Barnett et al., 2015;Alt and Zoback, 2016;Rajabi et al., 2017). ...
... The orientation of the present-day S Hmax is an essential component of the stress tensor which has obtained extensive attention since the exploration and development of unconventional resources in recent years. (Bell, 1996b;Tingay et al., 2005b;Zoback, 2007;Barton and Moos, 2010). Interpretation of drilling induced fractures from borehole imaging logs are considered as the most common way to obtain the orientation of the S Hmax (Bell and Gough, 1979;Bell, 1996a;Zoback, 2007). ...
Article
Determining the present-day stress orientation is essential for the exploration and development of shale gas. The Longmaxi Formation is the most significant shale gas reservoir in the Jiaoshiba Area of the Sichuan Basin, China. However, little is known about the orientation of the maximum horizontal stress (SHmax) and the factors influencing it within the formation. In this study, the orientation of the SHmax in the Jiaoshiba Area was determined based on observations of drilling-induced fractures on resistivity image logs. Then, the tectonic activity, the mechanical properties of the rock, and finite element simulations were studied in detail to determine the major factors influencing the SHmax orientation in the Jiaoshiba Area. The results show that the SHmax orientation within the study area ranges from nearly EW to NEE-SWW. The vertical variations of the in-situ stress orientation are controlled by the structural strength and the mechanical properties of the rock. In the area that has experienced intense tectonic movements, the deviation in the SHmax orientation (from 30◦ to 80◦) was caused by the differences in the mechanical properties of the rocks. The finite element simulations indicate that the lateral variations in the SHmax orientation was mainly controlled by the faults. The stress rotation near the end of the fault is 14◦–21◦ higher than that in the middle of the fault. Furthermore, the stress variation caused by the mechanical properties of the fault zone ranges from 12◦ to 18◦, and this deviation increases as the Young’s modulus of the fault zone decreases. In the study area, the azimuth deflection (18◦–46◦) of the SHmax orientation influenced by the NE faults is higher than that (12◦–36◦) influenced by the nearly NS faults, which is the main reason for the lateral stress rotation.
... The state of stress is commonly described by stress tensor. In sedimentary basins, one of the principal stresses is considered vertical because the Earth's surface cannot transmit shear stresses (Bell, 1996). Hence, the stress tensor can be simplified to four components, the magnitudes of the vertical, maximum and minimum horizontal stresses in addition to the orientation of the maximum horizontal stress (Bell, 1996;Tingay et al., 2009). ...
... In sedimentary basins, one of the principal stresses is considered vertical because the Earth's surface cannot transmit shear stresses (Bell, 1996). Hence, the stress tensor can be simplified to four components, the magnitudes of the vertical, maximum and minimum horizontal stresses in addition to the orientation of the maximum horizontal stress (Bell, 1996;Tingay et al., 2009). Of these four components, the maximum horizontal stress orientation has received extensive attention, particularly in geodynamic and plate motion (e.g. ...
... Borehole breakouts and drilling induced tensile fractures can be interpreted in different types of image logs. For example, in acoustic images borehole breakouts appear (primarily the travel time image) as a pair of wellbore elongation zones parallel to the borehole axis and separated by approximately 180°, while breakouts appear as broad, parallel, often poorly resolved conductive zones separated by 180° and exhibiting caliper enlargement in the direction of the conductive zones (Bell, 1996;Tingay et al., 2008) in resistivity (electrical) images ( Figure 3). Drilling induced tensile fractures appear as pairs of narrow conductive features (on resistivity images) or low-amplitude features (on acoustic images) that are generally parallel to the borehole axis and separated by approximately 180° (Figure 3; Tingay et al., 2005). ...
... On top of mountains, S Hmax is oriented parallel to the ridge and perpendicular at the foot of the mountain chain. Along passive continental margins, similar effects like for topography are to observe (Bell, 1996b;Bott and Dean, 1972;King et al., 2012;Stein et al., 1989;Tingay et al., 2005;Yassir and Zerwer, 1997). Sonder (1990) investigated the interaction of different regional deviatoric stress regimes (σ D ) with stresses arising from buoyancy forces (σ G ) and observe a rotation of S Hmax of up to 90 • . ...
... In the context of that study the term discontinuity refers to fault planes or fault zones. Similar to the Earth surface, (nearly) frictionless faults act like a free surface in terms of continuum mechanics (Bell et al., 1992;Bell, 1996b;Jaeger et al., 2007). One of the three principal stresses must be oriented perpendicular to the frictionless fault, the two remaining ones are parallel to the discontinuity. ...
... S Hmax will be oriented parallel to the structure for stiff units and perpendicular to weak units, which agrees with the literature (Bell, 1996b;Zhang et al., 1994). The largest stress rotation occurs nearest to the material transition and decreases with distance 400 to the material transition, similar to other models (Spann et al., 1994). ...
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It has been assumed, that the maximum compressive horizontal stress (SHmax) orientation in the upper crust is governed on a regional scale by the same forces that drive plate motion. However, several regions are identified, where stress orientation deviates from the expected orientation due to plate boundary forces (first order stress sources), or the plate wide pattern. In some of this regions a gradual rotation of the SHmax orientation has been observed. Several second and third order stress sources have been identified, which may explain stress rotation in the upper crust. For example lateral heterogeneities in the crust, such as density, petrophysical or petrothermal properties and discontinuities, like faults are identified as potential candidates to cause lateral stress rotations. To investigate several of the candidates, generic geomechanical numerical models are utilized. These models consist of up to five different units, oriented by an angle of 60° to the direction of contraction. These units have variable elastic material properties, such as Young's modulus, Poisson ratio and density. Furthermore, the units can be separated by contact surfaces that allow them so slide along these faults, depending on a selected coefficient of friction. The model results indicate, that a density contrast or the variation of the Poisson's ratio alone sparsely rotates the horizontal stress orientation (≦ 17°). Conversely, a contrast of the Young's modulus allows significant stress rotations in the order of up to 78°; not only areas in the vicinity of the material transition are affected by that stress rotation. Stress rotation clearly decreases for the same stiffness contrast, when the units are separated by low friction discontinuities (19°). Low friction discontinuities in homogeneous models do not change the stress pattern at all, away from the fault; the stress pattern is nearly identical to a model without any active faults. This indicates that material contrasts are capable of producing significant stress rotation for larger areas in the crust. Active faults that separates such material contrasts have the opposite effect, they rather compensate stress rotations.
... Such variations are due to the fact that the stress orientation is not just influenced by the plate boundary forces, but is also sensitive to second-and third-order factors (e.g. topography, density contrast active faults and open fractures) influencing the stress field on much a smaller scale (Bell, 1996;Tingay et al., 2006;Heidbach et al., 2007). Accordingly, one possible, local, cause, for the change in the Onshore Cluster is the proximity between the Apuan Alps and the Viareggio Basin and the concomitant material contrast leading to a contact parallel orientation of the maximum horizontal stress. ...
... Accordingly, one possible, local, cause, for the change in the Onshore Cluster is the proximity between the Apuan Alps and the Viareggio Basin and the concomitant material contrast leading to a contact parallel orientation of the maximum horizontal stress. Also, weak faults and open fractures, acting as a free surface, lead to reorientation of the maximum horizontal stress parallel to the fault plane (Bell, 1996;Tingay et al., 2006). ...
Article
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The Inner Northern Apennines (Italy) are a region with a dominant N-S to NNW-SSE fault system, but dissected and offset by several E-W to NE-SW trending structures and lineaments. The knowledge about the nature of these transverse structures, their origin, activity and role in current tectonic motions is limited and debated. To better establish the location, subsurface shape, and kinematics of faults related to the Livorno-Empoli lineament, one of the major transverse structures in the Northern Apennines, we analysed the seismicity in western Tuscany. In the Viareggio Basin we identified and relocated two distinct earthquake clusters as well as calculated 12 new focal mechanisms. The results show that the clusters consisted of several swarms from the years 2006, 2015, 2016 and 2021. The events had a depth between 2 and 15 km and were located along a NE-SW oriented, SE dipping fault system dissecting the Viareggio Basin. Focal mechanisms show oblique normal slip. We interpret the fault system to form a connection between the Viareggio Basin and the Lucca Basin to the east as well as continuing offshore. The results show that the transversal faults of the Inner Northern Apennines are seismogenic, with the length, position and onshore to offshore nature of the fault suggesting reactivation of pre-existing structures.
... The switch in sense of offset can be explained as a response to movement away from the resistant block of the Khorat Plateau, which would favour dextral motion on NW-SE trending fault systems and sinistral motion on NE-SW to N-S trending fault systems (Fig. 32). As a large, relatively stiff region embedded in softer crust, following Bell (1996), the stress trajectories would have swung from a more E-W direction to a NW-SE to N-S direction approaching the Khorat Plateau (Fig. 32), favouring the observed change in sense of motion on the strike-slip faults (Fig. 25). ...
... C) Sketch for the Late Oligocene period showing how the NW-SE trending faults of Province 4 could exhibit dextral motion, while the Ailao Shan-Red River Shear Zone underwent sinistral motion. D) Sketch redrawn from Bell (1996) illustrating how a hard body embedded within a softer matrix can deflect stresses, this model is applied to B, and C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) appropriate regional location to have concentrated a single, major, highdisplacement strike-slip fault, although notably the splaying, region of the Mae Ping Fault does skirt the south side of the plateau (Fig. 30). ...
Article
Cenozoic strike-slip faults in Indochina were initially interpreted as related to SE expulsion of rigid blocks of continental crust between narrow, high-displacement strike-slip faults away from the Himalayan Orogen. Revision of this view indicates four main stages of development are present. The main ‘rigid’ block of Indochina actually underwent extensive, widespread Paleogene transpressional deformation during stage 1, involving pervasive strike-slip faulting, folding, structural inversion, sub-horizontal mid-lower crustal shearing, and widespread exhumation of several kilometres. This stage is associated with ∼E-W oriented maximum horizontal stress (SHmax) (modern orientation). The three subsequent stages are: 2) 30–27 Ma transition, with retreat of transpressional deformation to the SE Tibetan Plateau, 3) 27–17 Ma onset of major sinistral displacement on ASRRSZ, 4) <17 Ma-present, diachronous reversal of motion on many faults, and cessation of extension in Thailand rifts. SHmax orientations evolved dominantly to between NE-SW and NW-SE orientations. Stage 4 is strongly linked with growth of the Tibetan Plateau, the modern GPS field, stress state, and geophysically-defined orientations of crustal and mantle flow. Five different strike-slip provinces have been identified for the post 30 Ma period, based on variations in structural style and kinematics caused by interacting factors including: 1) distance from the East Himalayan Syntaxis, 2) strength variations within SE Asian lithosphere, 3) pre-existing crustal fabrics 4) proximity to key regions contributing to the stress-state of SE Asia, 5) crustal and/or mantle flow. Whether the strike-slip faults are a product of a top- or bottom- driven system remains uncertain. Probably, the system is a hybrid.
... Bulk modulus is one of the mechanical properties used in numerous geoengineering issues [7][8][9][10]. The bulk modulus of rocks could be determined by either the stress-strain relation (i.e., static measurement) or the propagating elastic wave velocities (i.e., dynamic measurement) [11]. ...
... The relationship between static and dynamic bulk moduli is essential for modeling the formation deformation and predicting the in situ effective stress [4,5,7,8]. The Increasing pressure c = 0.36 Figure 11: Modified Hashin-Shtrikman lower bounds for six differential pressures under both room-dry (dashed lines) and brine-saturated (solid lines) conditions. ...
Article
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The static bulk modulus of unconsolidated sands is essential for predicting the in situ effective pressure to reduce drilling risks in deepwater reservoirs; however, the dynamic bulk modulus is often more broadly available from seismic or well logging data. Therefore, it is tempting to investigate the relationship between static and dynamic bulk moduli. We perform a series of ultrasonic velocity measurements on 21 deepwater reservoir sands from the Gulf of Mexico (GoM) to study the static and dynamic bulk moduli simultaneously. Both room-dry and brine-saturated ultrasonic velocities are measured under hydrostatic stress conditions to derive the dynamic bulk moduli. Under brine-saturated conditions, if the pore pressure is kept constant, the pore volume change with the confining pressure can be monitored accurately by a digital pump, which is subsequently used to estimate the static bulk modulus. The experimental results suggest that both the static and dynamic bulk moduli decrease upon pressure unloading. The pressure-dependent bulk moduli are modeled using the Hertz-Mindlin contact theory at the critical porosity and combined with the modified Hashin-Shtrikman lower bound for other porosities. The results suggest that the theoretical estimates can serve as the lower bound for the dynamic bulk modulus and the upper bound for the static bulk modulus. Under room-dry conditions, the static-to-dynamic modulus ratio decreases from a value approaching 0.8 to approximately 0.25 with decreasing differential pressure. Moreover, the effects of brine saturation on the relationship between the static and dynamic moduli are investigated using Gassmann’s equation. The brine saturation substantially reduces the difference between the static and dynamic bulk moduli, making the static-to-dynamic modulus ratio approach unity, which may be relevant to the in situ reservoir rock properties.
... Hydraulic fracturing is an important approach for shale oil development. During hydraulic fracturing operations in unconventional hydrocarbon reservoirs, the present-day in situ stress state is a critical factor that should be taken into account (Bell 1996;Tingay et al. 2009;Schmitt et al. 2012;). In addition, knowledge of present-day stress field indicates important effects on wellbore stability and reservoir management (Zoback et al. 2003;Binh et al. 2007;Tingay et al. 2009;Rajabi et al. 2016;Ju et al. 2017Ju et al. , 2019. ...
... The present-day in situ stress state can be described by the stress tensor, which includes the orientation and magnitudes of three orthogonal principal stresses (Engelder 1993). In general, the stress tensor may be reduced to four components, namely the magnitudes of horizontal maximum principal stress (S Hmax ), horizontal minimum principal stress (S hmin ) and vertical stress (S v ), and the orientation of horizontal stresses (Bell 1996;Zoback et al. 2003;Ju et al. 2017). ...
Article
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The Yanchang Formation Chang 7 oil-bearing layer of the Ordos Basin is important in China for producing shale oil. The present-day in situ stress state is of practical implications for the exploration and development of shale oil; however, few studies are focused on stress distributions within the Chang 7 reservoir. In this study, the present-day in situ stress distribution within the Chang 7 reservoir was predicted using the combined spring model based on well logs and measured stress data. The results indicate that stress magnitudes increase with burial depth within the Chang 7 reservoir. Overall, the horizontal maximum principal stress (SHmax), horizontal minimum principal stress (Shmin) and vertical stress (Sv) follow the relationship of Sv ≥ SHmax > Shmin, indicating a dominant normal faulting stress regime within the Chang 7 reservoir of Ordos Basin. Laterally, high stress values are mainly distributed in the northwestern parts of the studied region, while low stress values are found in the southeastern parts. Factors influencing stress distributions are also analyzed. Stress magnitudes within the Chang 7 reservoir show a positive linear relationship with burial depth. A larger value of Young’s modulus results in higher stress magnitudes, and the differential horizontal stress becomes higher when the rock Young’s modulus grows larger.
... In-situ stress refers to the internal stress that is present in the Earth's crust and can be divided into self-weight and tectonic stress (Bell, 1996;Kang et al., 2010). Self-weight stress is caused by an overburden, such as an overlying rock mass, and can usually be calculated by the weight of the mass. ...
... In-situ stress also has a significant influence on the stability of a wellbore, the amount of oil recovered (e.g. stimulation), and reservoir management (Rajabi et al., 2017;Bell, 1996;Zoback et al., 2003;Liu et al., 2016). Therefore, an accurate estimation of in-situ stress state can effectively guide the CBM exploration and production. ...
Article
The in-situ stress and its behavior during depletion are critical for investigation of the reservoir permeability, wellbore stability, enhanced oil recovery (e.g. stimulation), fault reactivation, and reservoir management. However, the rules that govern in situ stress variation in coalbed methane (CBM) reservoirs and its influences on reservoir stability are still unclear due to the matrix shrinkage effect. In this study, the distribution characteristics of in-situ stress in the Zhengzhuang (ZZ) region were investigated by multi-loop hydraulic fracturing tests and a theoretical stress-depletion response model was built to reveal the dynamic rules of in-situ stress during CBM depletion. Additionally, considering the redistribution of in-situ stress and the limitation induced by fault friction coefficient, a criterion for stress-based failure was developed to analyze the stability of CBM reservoir. The results suggest that in the ZZ block, the fracturing pressure, closure pressure, and the maximum and minimum horizontal principal pressures are positively correlated with burial depth. During drainage, the horizontal principal stress reduces and the effective horizontal principal stress increases linearly as pore pressure reduction. During gas desorption phase, the horizontal principal stress decreases and the effective horizontal principal stress increases non-linearly under weak desorption, whereas both decline non-linearly in response to strong desorption. In addition, the failure criterion for CBM reservoirs indicates that reservoirs in a normal faulting stress regime are the most easily damaged, which may occur during drainage or desorption. However, reservoir damage can only be induced under strong desorption in a strike-slip faulting stress regime and cannot occur in a reverse faulting stress regime. If faults do develop in reservoirs, the reservoir stability is governed by the fault friction coefficient. Finally, by combining the data, the changes in in-situ stress and the stability of the reservoirs during depletion in the ZZ region were revealed and its implications for CBM development were discussed.
... To characterise the behaviour of rock mass in such a challenging and extreme condition, a reliable and cost-efficient quantitative determination of in situ stresses is critical (Beamish and Crosdale 1998;Brady and Brown 2006;Wagner 2019). Studying the ground stress field is essential for enhancing exploration and sustainable recovery of natural resources, as it provides insights into the deformation behaviour of underground structures under complex in situ loading conditions (Bell 1996;Brooke-Barnett et al. 2015). ...
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Sustainable underground mining operation at deep levels requires a clear understanding of in situ stress conditions to ensure safety of personnel and equipment for continuous exaction of natural resources. Obtaining representative three-dimensional (3D) stress data at depth remains a significant challenge due to the operational complexities, high costs and time demands. Despite various methods proposed, core-based in situ stress estimation stands out as a cost-effective and reliable approach. Yet, these techniques come with inherent complexities within the laboratory environment, introducing considerable uncertainties and subjectivity in reliable stress estimation. The diametrical core deformation analysis (DCDA) was introduced to address these challenges, providing improved measurement repeatability and mitigating uncertainties. However, DCDA is limited to two-dimensional (2D) stress state estimation, leaving the determination of the full 3D stress tensor as an unresolved challenge. Therefore, this study presents a novel integrated methodology that combines DCDA with ultrasonic mapping to determine the full 3D stress state from core samples including the azimuth and dip angle of stress components. Both techniques leverage the expansion of core samples under various directions following the release from in situ stress, with greater expansion expected along the axis with the highest principal stress. Stress magnitudes were then calculated using a new analytical technique and the robustness and reliability of the proposed methodology were validated through analysing eight core samples from two vertical boreholes in an Australian underground metalliferous mine. The results were compared with the on-site overcoring stress measurements, having core-based measurements providing reliable predictions of the three principal stresses’ magnitude, azimuth, and dip angle. The current study contributes to sustainable mining by providing a more accurate and less invasive technique for 3D in situ stress estimation. Such an advancement helps to reduce uncertainties in geotechnical assessments, enabling efficient and sustainable mine planning and operation.
... For example, Rajabi et al. (2014) explained that lack of knowledge about the in situ stresses in the Middle East can be compensated from petroleum wells to feed into the global stress map database. However, regional variations in the state of stress might not be consistent with the global stress field, which necessitates conducting additional studies on the stress pattern on smaller scales, particularly in petroleum basins (Bell, 1996;Tingay et al., 2005;Zoback, 2010). ...
... However, at smaller scales, various depth-wise systematic stress variabilities have been observed, primarily associated with the presence of mechanical detachment zones. [76][77][78] In most basin-wide stress mapping projects, the focus is primarily on examining the spatial pattern of stress orientation and comparing S Hmax orientations between individual boreholes. However, little attention has been given to the detailed analysis of stress orientation pattern from near surface to deeper intervals. ...
Article
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Most of in-situ stress data in the Australian continent comes from wellbore stress analysis in deep hydrocarbon reservoirs, and earthquake focal mechanism solutions near the Australian plate boundaries, where geophysical tools facilitate understanding of the present-day stress patterns. This resulted in a paucity of stress information in many other regions such as the northern Bowen Basin, which is an active mining province, but with low seismicity rates and limited deep petroleum exploration. The mining industry runs several hundred kilometres of image logs annually to characterise geotechnical attributes. These logs provide an image from the borehole wall, which facilitates analysis of stress-related borehole deformations for in-situ stress characterisation. This paper examines the orientation of horizontal in-situ stress using different types of image logs in mine boreholes across the northern Bowen Basin. Analyses of 128 km of image logs in 680 vertical boreholes resulted in the interpretation of 9046 pairs of stress-related indicators including 735 drilling induced fractures and 8311 borehole breakouts. Our comprehensive database comprises 890 quality-ranked data records for the orientation of maximum horizontal stress (SHmax), which makes the Bowen Basin as a basin with the highest data density in the world in terms of quality-ranked stress information according to the World Stress Map. Statistical analysis of SHmax orientation reveals that the mean SHmax orientation in northern Bowen Basin is N018 deg ± 16 deg. The results show that this orientation is consistent over long distances, which is in contrast with several eastern Australian basins. This uniform stress pattern agrees well with plate-scale geomechanical model predictions, which further highlights the impact of plate boundary forces in the contemporary stress pattern of this region. Detailed image log investigation did not show any systematic rotation of stress; however, some small-scale stress perturbations were observed in the vicinity of sharp stiffness contrasts and geological structures.
... Some of the earliest measurements of strain in the crust (with the ultimate objective of understanding the state of stress) date to the 1940's in coal mines in England (Greenwald 1937;Trotter 1948;Potts 1951;Hobbs 1964), with increased scope in the 1960's following the application of strain gauge technology (Higson 1964;Price 1964Price , 1966Leeman 1969). Additional indicators of stress/strain were quickly recognized, such as stress pop-ups, overcoring measurements, recrystalized grains, borehole breakouts, and hydrofracing (Bell and Babcock 1986;Bell 1996). These techniques have developed into the standard method for estimating the state of stress at the borehole-and basin-scale (Gough and Gough 1987;Tingay et al. 2005;Zoback 2010). ...
Article
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A well-recognized characteristic of the World Stress Map (WSM) database is the continued presence of large spatial gaps in the distribution of the data records despite the more than 40 years development history of the database. The current release has more than 30,000 high-quality (A-C) data records (often referred to as “stress indicators”), but while some continental areas (such as Australia) have seen a significant increase in spatial converge with the latest release other continental regions (Africa, central Asia, most of South America) remain markedly sparse. In this contribution we 1) review the current state of the spatial distribution of stress indicators in the continental regions (above sea level); 2) quantify the clustering of the stress indicators in the latest WSM release using the Hopkins statistic as a way to explore the current spatial distribution of the indicators and assess future WSM releases; and 3) present three approaches (joint inversion, seismic anisotropy, and InSAR) that provide a way to fill in the gaps (both in the SHmax orientation and principal stress magnitudes) in regions which lack active seismicity and where borehole drilling is cost prohibitive. These three approaches have the potential to guide procedures for improving apriori estimates of the ambient stress field in the Earth's crust and reduce the uncertainty in predicting both the magnitude and orientation of the principal tectonic stresses.
... Bulk modulus is one of the mechanical properties used in several geoengineering problems (Wang et al., 2022;Bock, 1993;Bell, 1996;Andhumoudine, 2021;Yang and Liu, 2021) and it could be obtained by the stress-strain relation (like static measurement) or the propagating elastic wave velocities (such as dynamic measurement) (Wang et al., 2020). The static bulk modulus symbolizes the mechanical firmness of subsurface basins (Zimmer, 2004;Zoback, 2007). ...
Article
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Modelling of bulk modulus from sand-shale lithology has been researched. This is to come up with a model than relates the bulk modulus with the Lame's first parameter. Data adequate for this finding were obtained from three Niger Delta oil wells A, B and C. Microsoft Excel was used for all stages of analysis. The result indicates the average values of Poisson's ratio and Vp/Vs ratio as 0.5 and 1.637 respectively for all wells. Other results include the depth range of about 5300ft to 6600ft; shear modulus varies from 4.05 x 10 9 to 11 x 10 9 N/m 2 ; the range of result of young modulus varies from 1.
... This large amount of stress data is expected to accurately reflect the stress field distribution trend within the whole mine area in space. Note that some shallow stress measurements derived from the OC and HF methods were carefully analyzed, as they may be influenced by terrain effects [11,29]. To evaluate the data quality and ensure the comparability of the different stress indicators, all the data records were ranked based on the worldwide accepted WSM quality ranking system [30]. ...
Article
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Full knowledge of the current tectonic stress state is crucial for assessing open-pit mine slope stability and regional tectonic evolution and geodynamic processes. Overcoring, hydraulic fracturing, and acoustic emission in situ stress measurement techniques were adopted to determine the 3D stress tensor in an iron mine district, North China, and 25 sets of stress data ranging from 56 to 490 m were measured. Accordingly, the current tectonic stress state and its relationship to regional geological tectonics were investigated. The results indicated that the stress condition seemed to favor thrust and strike-slip faulting, and the stress field was particularly controlled by the horizontal tectonic stress. A high horizontal tectonic stress considerably influenced the stability of high and steep slopes in this mine district, which requires great attention. The stress directions derived from different methods were almost similar, indicating a dominant NEE–SWW stress field direction or near-E–W direction, comparable to the direction revealed by focal mechanism solutions and other stress indicators around the mine district. According to geological structure analysis, the present-day stress field in this district generally inherited the third-stage tectonic stress field while partially retaining the characteristics of the second-stage tectonic stress field, which is the result of dynamic action and tectonic movement during different geological periods, and the maximum principal stress direction of the tectonic stress field that affects the modern tectonic activity in this area is the NEE–EW direction.
... According to Homberg et al. (1997Homberg et al. ( , 2004, stress perturbation near fault tips is significantly influenced by the respective fault and the magnitude of the applied stresses. The coincidence of the orientation of σ2 and the strike of nearby faults indicates that stress deflection due to the existence of structural heterogeneities (sensu Hudson & Cooling, 1988;Casas et al. 1992;Zhang et al. 1994;Bell, 1996;de Joussineau et al. 2003;Yale, 2003) plays a significant role in the Franconian Platform. However, the overall stress field is relatively constant. ...
Article
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The Franconian Platform of SE Germany and the underlying Permian and Triassic rocks that developed from latest Permian to Triassic time were affected by multiple compressional and extensional events that created a complex fracture, fault and stylolite network. We reconstructed the spatio-temporal variations of post-Triassic palaeostress fields in the Franconian Platform and Triassic strata using fault-slip and tectonic stylolite inversion. Our highly resolved stress inversion enables us to demonstrate a cyclic stress evolution from the stress regime of normal faulting to thrusting, strike-slip and back to normal faulting. Five main stress fields correlating with two stress cycles are determined for Late Jurassic to Cenozoic time. The first cycle consists of: (SF1) an initially NE-SW-directed horizontal extension during Late Jurassic to Early Cretaceous time; (SF2) NNE-SSW-directed horizontal compression with an early set of tectonic stylolites prior to the development of reverse and thrust faults; and (SF3) a strike-slip-dominated setting with (N)NE-(S)SW horizontal compression representing a first relaxation. The second cycle comprises (SF4) NW-SE-directed horizontal extension during Oligocene-Miocene time; and (SF5) a second strike-slip-dominated regime with WNW-ESE to NW-SE horizontal compression during the Alpine shortening, creating the youngest set of tectonic stylolites. In addition, we consider the transitional stages between thrusting and a strike-slip regime as a snapshot in the process of intraplate tectonics.
... After five years of fluid injection into the lower reservoir section, fluid migration through the fault zone has also led to an increase in pore pressure in the upper reservoir part ( Considering the internal details of the fault zone the model shows contrasting values for the magnitudes of S1,eff and rotations away from the original vertical direction ( Figure 6) at the stiffness contrast between the fault core lenses (Young's Modulus 30 GPa) and the surrounding weaker fault core material (YM 10 GPa). Softer rock properties in the surrounding fault core material lead to a large stiffness contrast with respect to the fault core lenses, which correlates with previous studies (Zhang et al., 1994;Bell, 1996) on the effects of stiffness contrast on the stress magnitude and orientation, respectively. This also correlates with the results for the elastic and plastic strain pattern inside the fault zone ( Figure 8). ...
Conference Paper
Coupled hydro-mechanical simulations are by now a standard tool for reservoir modeling. However, challenges remain regarding the proper implementation of faults into such reservoir-scale numerical models which result mainly from the scale difference between the element size of the numerical grid and the internal heterogeneity of real fault zones. In this study, we present an AI-based upscaling approach from detailed fault zones to homogenized fault properties in finite element models. The detailed fault zone model contains a fault core with shear bands and host rock lenses as well as accompanying damage zones with a fracture density decreasing with distance from the fault core. This detailed fault zone model is automatically compared by using the Structural Similarity Index and difference images with a database of 432 models describing the fault zone as one uniform material homogenizing the different fault zone units. The goal is to introduce a new upscaling workflow of hydro-mechanical fault zone properties in reservoir-scale simulations through a database driven approach to represent the fault in a numerical simulation with an optimal grid size while maintaining the bulk effect of a detailed fault zone description on fluid flow, stress and deformation and thus, improving the predictive power of reservoir-scale simulations.
... It may cause many drillingrelated issues, such as lost circulation, pack-off, stuck bottom-holeassembly (BHA), or casing (Al-Zankawi et al., 2017;Abdideh and Fathabadi, 2013). Having a complete geomechanical model of downhole formations would help address different issues along the reservoir life ranging from maintaining wellbore stability during drilling to determining formation instability during production, including selecting the appropriate wellbore completion design and even predicting the long-term response of a reservoir to pressure depletion (Bell, 1996;Maleki et al., 2014;Molaghab et al., 2017;Warpinski and Teufel, 1987;Willson et al., 2002;Zoback et al., 2003). Therefore, accurate estimation of horizontal stresses is critical for optimizing the drilling and the completion process of the horizontal wells' and improving the hydraulic fracturing design. ...
Article
Subsurface planning and modeling are highly dependent on the accurate determination of in-situ stresses. For example, well instability, and hydraulic fracture operations highly depend on the values of in-situ stress. In-situ stress setting can be described in terms of three orthogonal components; overburden stress (σv) and minimum (σh) and the maximum (σH) horizontal stresses. σv can be determined using the density logs. σh and σH can be estimated from borehole injection tests or theoretical finite elements methods. However, these methods are complex, expensive, or need unavailable tectonic stress data. Therefore, The objective of this study is to introduce the application of machine learning (ML) techniques to predict σh and σH from the surface drilling data. The drilling data values vary in response to the in-situ stress. This work demonstrates how ML techniques can predict the minimum and maximum horizontal stresses using drilling data while drilling that were collected from a gas reservoir. Random forest (RF), functional network (FN), and adaptive neuro-fuzzy inference system (ANFIS) were implemented to predict σh and σH during drilling using measured surface drilling parameters. A dataset of 2573 points from two wells (Well-1 and Well-2) was used to build the different ML models. The data from Well-1 were used to train and test the model, and Well-2 data were then used to validate the developed models. The three ML models accurately predicted the σh and σH based on the drilling data. The three models showed similar results for σh prediction with a correlation coefficient (R) values greater than 0.96 and an average absolute percentage error (AAPE) less than 0.53%. Comparing the results of RF, ANFIS, and FN models for σH prediction showed that RF outperforms the other two models with R values for the training dataset of 0.99 compared to 0.94 and 0.87 for ANFIS, and FN, respectively. The RF, ANFIS, and FN models were able to capture the σh and σH trends with depth for the validation data. These results show the ML capabilities for real-time predicting σh and σH while drilling from the surface drilling parameters without additional costs.
... Thus, a comprehensive geomechanical model of downhole formations helps address many issues through different stages of reservoir life. An example of these issues are maintaining borehole stability in drilling operations, formation instability within the production operations and sand production, and the applicable wellbore completion design selection [5][6][7][8][9][10] . ...
Article
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Determination of in-situ stresses is essential for subsurface planning and modeling, such as horizontal well planning and hydraulic fracture design. In-situ stresses consist of overburden stress (σv), minimum (σh), and maximum (σH) horizontal stresses. The σh and σH are difficult to determine, whereas the overburden stress can be determined directly from the density logs. The σh and σH can be estimated either from borehole injection tests or theoretical finite elements methods. However, these methods are complex, expensive, or need unavailable tectonic stress data. This study aims to apply different machine learning (ML) techniques, specifically, random forest (RF), functional network (FN), and adaptive neuro-fuzzy inference system (ANFIS), to predict the σh and σH using well-log data. The logging data includes gamma-ray (GR) log, formation bulk density (RHOB) log, compressional (DTC), and shear (DTS) wave transit-time log. A dataset of 2307 points from two wells (Well-1 and Well-2) was used to build the different ML models. The Well-1 data was used in training and testing the models, and the Well-2 data was used to validate the developed models. The obtained results show the capability of the three ML models to predict accurately the σh and σH using the well-log data. Comparing the results of RF, ANFIS, and FN models for minimum horizontal stress prediction showed that ANFIS outperforms the other two models with a correlation coefficient (R) for the validation dataset of 0.96 compared to 0.91 and 0.88 for RF, and FN, respectively. The three models showed similar results for predicting maximum horizontal stress with R values higher than 0.98 and an average absolute percentage error (AAPE) less than 0.3%. a²⁰ index for the actual versus the predicted data showed that the three ML techniques were able to predict the horizontal stresses with a deviation less than 20% from the actual data. For the validation dataset, the RF, ANFIS, and FN models were able to capture all changes in the σh and σH trends with depth and accurately predict the σh and σH values. The outcomes of this study confirm the robust capability of ML to predict σh and σH from readily available logging data with no need for additional costs or site investigation.
... When extensional events are greatly separated in time, newly formed fault populations may be stratigraphically and vertically separated from older, often buried, fault populations, meaning that plan-view relationships may not be informative (Çiftçi & Langhi, 2012;Collanega et al., 2019). Furthermore, the presence of ductile units such as salt or shale may vertically partition strain and further decouple fault populations, thus producing a strong influence over the basin geometry and stratigraphy as well as fluid flow and fracture distribution across and surrounding the ductile, incompetent interval, (Bell, 1996;Coleman et al., 2017;Gartrell et al., 2003Gartrell et al., , 2004Lewis et al., 2013;O'Sullivan et al., 2021;Shipilin et al., 2020). At present, it remains unclear how strain is distributed and partitioned between vertically separated fault networks, and what effect, if any, the deeper fault population may exert over incipient faults in these instances. ...
Article
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When extension events are greatly separated in time, older faults may be buried and stratigraphically separated from newly developing faults at shallower depths. During rifting, the buried structures may reactivate and propagate upwards to be expressed within the shallow system. The degree of linkage between structural levels determines the influence that the deeper structures can exert over the geometry and evolution of the incipient fault system. In this study we use 3D seismic reflection data to examine how a deep fault population across the Laminaria High, NW shelf of Australia influences the development of a younger fault system at shallow depths. These fault populations are non‐parallel and decoupled across a mechanically weak interval. The majority of shallow faults are not linked to those at depth. However the reactivation and upward propagation of some of the deeper faults produces anomalously oriented structures at shallow depths, hard‐linked to the deeper structures. One fault in particular shows along‐strike variability, with the deep segment reactivated and present at shallow depths in the west. To the east, the shallow and deep fault segments become decoupled across a mechanically weak interval, although some soft‐linkage and strain transfer still occurs. We suggest that this switch in the degree of coupling along the fault is due to the geometry of the deeper structure, with the transition corresponding to a prominent relay ramp. We show how the geometry of a deeper fault may affect its propensity to reactivate during subsequent extensional events, ultimately affecting the degree of structural inheritance that is expressed within younger, shallower fault populations.
... The region in north-eastern Germany belongs to the NGB in which there are thick salt deposits. Salt can act as a mechanical decoupling horizon between the layers above and below (Ahlers et al., 2019;Bell, 1996;Cornet and Röckel, 2012;Heidbach et al., 2007;Hillis and Nelson, 2005;Tingay et al., 2011). In such cases, the stress state below represents the regional trend transferred through the crust, while the stress state above is only affected by local sources often controlled by local density and strength contrasts. ...
Article
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The contemporary stress state in the upper crust is of great interest for geotechnical applications and basic research alike. However, our knowledge of the crustal stress field from the data perspective is limited. For Germany basically two datasets are available: orientations of the maximum horizontal stress (SHmax) and the stress regime as part of the World Stress Map (WSM) database as well as a complementary compilation of stress magnitude data of Germany and adjacent regions. However, these datasets only provide pointwise, incomplete and heterogeneous information of the 3D stress tensor. Here, we present a geomechanical–numerical model that provides a continuous description of the contemporary 3D crustal stress state on a regional scale for Germany. The model covers an area of about 1000×1250 km2 and extends to a depth of 100 km containing seven units, with specific material properties (density and elastic rock properties) and laterally varying thicknesses: a sedimentary unit, four different units of the upper crust, the lower crust and the lithospheric mantle. The model is calibrated by the two datasets to achieve a best-fit regarding the SHmax orientations and the minimum horizontal stress magnitudes (Shmin). The modeled orientations of SHmax are almost entirely within the uncertainties of the WSM data used and the Shmin magnitudes fit to various datasets well. Only the SHmax magnitudes show locally significant deviations, primarily indicating values that are too low in the lower part of the model. The model is open for further refinements regarding model geometry, e.g., additional layers with laterally varying material properties, and incorporation of future stress measurements. In addition, it can provide the initial stress state for local geomechanical models with a higher resolution.
... Overall, the S Hmax orientation indicated a dominant NNW-SSE direction (Fig. 5). Additionally, the orientation showed minor variations in different geological locations, which may result from the development of small faults, folds, natural fractures, lithological transitions, or differences (Martin and Chandler 1993;Bell 1996;Ju et al. 2018b). ...
Article
Resources, including petroleum and natural gas, from deep and ultra-deep reservoirs have become an important contributor to global reserve growth and deliverability construction. Knowledge of the present-day in situ stress field has significant applications for efficient exploration and development. The Bozi 3 deep sandstone reservoir is important for increasing natural gas production in the Kuqa Depression. However, little attention has been given to the present-day in situ stress state of the Bozi 3 Block. In this study, the in situ stress orientation and magnitudes were investigated based on stress indicator interpretation, well log calculation, and geomechanical modeling. Drilling-induced tensile fractures and borehole breakouts indicated a dominant NW-SE direction for the horizontal maximum principal stress (SHmax) in the Bozi 3 deep sandstone reservoir. The stress regime for the Bozi 3 Block indicates a dominant strike-slip faulting type (SHmax≥Sv≥Shmin). Numerical simulation of the present-day in situ stresses showed that the SHmax and horizontal minimum principal stress (Shmin) magnitudes were − 150~− 178 MPa and − 121~− 157 MPa in the Cretaceous, respectively. Natural fractures are generally stable in the present-day stress field. When the specific gravity reaches 2.15, approximately 75% of all natural fractures are reactivated. Under the present-day stress state in the Bozi 3 Block, new horizontal wells should be drilled with ENE-WSW-trending. The results for the present-day in situ stress state are expected to provide geological and engineering references for deep gas production in the Bozi 3 Block of Kuqa Depression.
... In the experiments, deformation in the cover was influenced by re-orientation of the stress and strain fields across the ductile basement layer, as there were no weak layers that separated the cover and basement materials (cf. "attached stress regime" in Bell, 1996). As a result, faults in the brittle cover were localized above areas of thinning in the ductile basement (compare Figures 4c and 4d). ...
Article
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During rifting, pre‐existing penetrative basement fabrics can affect new faults in cover rocks by a mechanism that does not appear to involve reactivation. This subtle form of inheritance can significantly impact fault network architecture in rift basins above laterally variable basement domains with geomechanical anisotropies. Here we use multi‐layer, brittle‐ductile, crustal‐scale analogue experiments to study the influence of penetrative basement anisotropies on fault patterns in the overlying cover during a single phase of orthogonal rifting. The experiments were designed to test whether basement anisotropies, oriented 45° to the extension direction, can lead to the formation of rift faults that are oblique to both the imposed extension direction and basement anisotropies. Our experiments show that a penetrative, vertically layered, mm‐wide basement anisotropy creates extension‐oblique faults in the overlying cover. We interpret this to arise when local strike‐slip kinematics along the interfaces of mechanically contrasting materials in the basement combine with the regional imposed orthogonal extension, creating a transtensional regime. The width and spacing of alternating “strong” and “weak” basement zones interact with rift kinematics, impacting the orientation, kinematics and spacing of new faults in the cover. New insights on the influence of penetrative, pre‐existing basement fabrics on localized re‐orientation of 3D strain in the cover have implications for understanding complex fault systems in rift basins and transfer zones.
... In vertical wells, maximum and minimum principal stress orientation are interpreted from the drilling induced tensile fractures (DITF) and borehole breakouts (BO). The wellbore enlargements induced from the in-situ stress (Bell and Gough, 1983;Bell, 1996;Rajabi et al., 2017) are represented by the BO that appear as a pair of broad, poorly defined conductive or low acoustic impedance zones with longer travel time located 180 0 away from each other on the image logs Ranjbar-Karami et al., 2019;Mukherjee et al., 2020). On the contrary, DITFs occur due to tensile failure around the borehole often from the wellbore drilling and are identified as steeply dipping conductive fractures on the image logs (Aadnoy, 1990;Rajabi et al., 2017). ...
Article
The Surat Basin, which is a major eastern Australian Coal Seam Gas (CSG) reservoir, produces gas from its Middle to Late Jurassic sub-bituminous Walloon Coal Measures, and shows variability in initial reservoir permeability across the basin. For a given lithostatic stress, the presence of vitrinite group or telovitrinite macerals within the coal can influence the fracture abundance within the coal reservoirs of similar rank, which in turn potentially impacts the initial reservoir permeability of the CSG reservoirs. The predictive relationship between megascopic banding character or lithotype, microscopic maceral composition and fracture intensity within CSG reservoirs is well explored in bituminous coals, but not so much in sub-bituminous coals. Here, we analyse wellbore image log data along with the conventional geophysical logs, maceral analysis data, core descriptions validated by core photos from the well completion reports and interpreted permeability data from well tests to systematically investigate the relationship between lithotype, maceral composition with a focus on vitrinite/telovitrinite, and fracture intensity in the sub-bituminous rank Walloon coals, to test their influence on permeability trends across the basin. Permeability-depth trends in the Walloon coals vary regionally across the basin, and show different inclinations depending on coal measures stratigraphy, coal lithotype composition, fracture intensity and stress orientation between local anticlines, monoclines and synclines. Depth normalised permeability and thickness-weighted telovitrinite content data from 55 wells were analysed. Furthermore, analysis of 3.8 km of borehole image logs, along with 156 maceral composition data points and thickness-weighted average lithotypes derived from the core description across ten vertical wells, revealed a moderately positive relationship between the coal type and fracture abundance. This indicates some coal compositional control on the fracture abundance at this lower rank. The derived average total fracture intensity (cleat and sinusoidal coal fracture) from borehole image logs and thickness-weighted averaged telovitrinite content from the core analysis also exhibited an exponential increase, with the increasing depth normalised permeability derived from the well test data within the analysed wells. This relationship suggests the influence of the variation in coal composition and fracture intensity on the rheological behaviour of the CSG reservoir within the Walloon Coal Measures. Spatial distribution of depth normalised permeability from the 55 wells across different domains and their relationship with the maceral composition within the study area also suggests the influence of geological structures on the depth normalised permeability. Therefore, though coal composition controls the fracture intensity, under favourable stress and fracture conditions the relationship can be substantially modified in parts of the Surat Basin, and altogether these factors control the rheological behaviour of the Walloon Coal Measures.
... The relative magnitudes of the three principal stresses define a stress regime (Anderson, 1951) (Figure 3); under any given stress regime, the type of failure defined will typically dominate (Heidbach & H€ ohne, 2008;Sibson, 1977). For further information regarding the definition of lithospheric stresses, see Bell (1990), Bell (1996aBell ( , 1996b, Bell (2003), Chan et al. (2014), Couzens-Schultz and Chan (2010), Plumb et al. (2000) and Zoback (2007). ...
Article
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Western Australia’s Canning Basin is an underexplored prospective basin with proven petroleum systems and small-scale production. Recently, several formations within deeper depocentres have been investigated for unconventional hydrocarbon resources. Modern petroleum resource evaluation generally depends on an understanding of local and regional stresses that are a primary control over the formation and propagation of induced fractures. There are significant gaps in our understanding of these factors within the Canning Basin. This study characterises the regional stress regime of the onshore Canning Basin and presents modelled present-day stress, allowing for the identification of significant stress heterogeneity and natural fracture barriers. Interpretation of wireline data reveals a present-day state of stress with variation in magnitude and faulting-type. An approximately northeast–southwest regional present-day maximum horizontal stress orientation is interpreted, which is in broad agreement with the Australian Stress Map and previously published data. One-dimensional mechanical earth models, constructed for intervals from 15 Canning Basin wells, highlight the relationship between lithology and stress. Significant changes in stress within and between lithological units, owing to the existence of discrete mechanical units forming numerous inter- and intra-formational stress boundaries, likely to act as natural barriers to fracture propagation, particularly within those units currently targeted for their unconventional resource potential, are interpreted. A strike-slip faulting stress regime is interpreted broadly through the basin. However, when analysed in detail there are three distinct stress zones identified: (1) a transitional reverse- to strike-slip faulting stress regime in the top ∼1 km, (2) a strike-slip faulting stress regime from ∼1 km to ∼3.0 km and (3) a transitional strike-slip to normal faulting regime at depths greater than ∼3.0 km. This study is a component of the Australian Government’s Exploring for the Future initiative, which focusses on gathering new data and information about northern Australia’s resource potential. • KEY POINTS • New stress data, including 15 mechanical earth models, are presented for Western Australia’s onshore Canning Basin. • Broadly, a strike-slip stress regime with a NE–SW-oriented maximum horizontal orientation is described. • Significant stress heterogeneities and natural fracture barriers are identified.
... Thus, stiffness contrasts between the fault represented as volumetric weak zone and the host rock lead to magnitude changes as well as stress rotations. Softer rock properties in the fault zone lead to a larger stiffness contrast with respect to the surrounding, which results in broader ranges of between maxima and minima for S1,eff magnitudes and stress reorientations, respectively (Zhang et al., 1994;Bell, 1996). Similarly, the spatial extent of the stress perturbations increases with decreasing stiffness of the fault zone and increasing stiffness contrast, respectively. ...
Thesis
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Hydro-mechanical reservoir models are used to obtain quantitative insights into the spatial distribution of stress, strain and pore pressure. Recent studies have shown that different approaches to incorporate faults into such reservoir simulations have a profound impact on the modeling results. Since faults are a key feature in the subsurface affecting both the hydraulic and mechanical behavior of a reservoir, their proper implementation in the numerical model is crucial. Fault representation has to accurately model the effect faults have on (1) fluid flow and (2) the local stress field. However, a fault is not just a discrete geological feature but rather a fault zone with a complex geometry and various rock units with distinct material properties. This small-scale heterogeneity can hardly be represented in reservoir scale finite element models considering the typical grid size used in these simulations. Thus, fault representation in reservoir-scale hydro-mechanical simulations has to be based on simplifications and upscaling techniques. To improve decision making and help in choosing the right fault representation, knowledge about the different effects each simplification and each approach used to incorporate faults has on the modeling results is necessary. This thesis focuses on different approaches of fault representation with a single upscaled set of material properties in reservoir-scale hydro-mechanical finite element models. The main objectives are (1) Implementing the fault geometry with respect to the finite element grid properly (2) Addressing the scale differences between the internal heterogeneity of the fault zone (centimeters to meters) and the typical size of the calculation cells of the numerical grid (meters to tens of meters) accurately (3) Assigning fault material properties to the numerical models, which stem – if available at all – from rock mechanical testing on core samples with a diameter of a few centimeters and therefore require upscaling and merging techniques In order to meet these challenges three research articles were published, each based on simple generic fault zone models. The approaches analyzed to represent faults in reservoir-scale hydro-mechanical include a regular rectangular grid, a grid geometry adapted to the fault geometry as well as fault representation by contact elements. Fault representation as volumetric weak zones is investigated for different grid geometries, fault dip angles as well as different mesh resolutions inside the fault zone. In addition, the impact of different elastic and frictional fault zone properties is assessed. Differences and similarities in the calculated stress and strain patterns as well as the pore pressure field obtained from different fault implementation strategies are discussed and general recommendations concerning the implementation of faults in hydro-mechanical reservoir models are given. Fault representation as either volumetric weak zones or contact elements leads to significant differences in the stress and strain patterns in the vicinity of the fault zone (< 50 m). While fault dip is not of critical importance for fluid flow, it has a significant impact on the stress perturbation induced by the fault. Another important finding is that the mesh resolution has to be considered very carefully as – particularly in combination with a rectangular grid – interlocking effects and serious errors can occur. If, however, the focus of a modeling study is not in the vicinity of the fault zone, a rectangular grid with the appropriate mesh resolution allows for faster and easier model generation in comparison to a curvilinear grid adapted to the fault geometry. Regarding material parameters, Young's modulus and cohesion assigned to the fault zone have the most significant impact on the modeling results, while the internal friction angle and Poisson's ratio play a subordinate role. Overall, this thesis provides recommendations and guidelines to improve fault representation in reservoir simulations. The goal is to gain more realistic simulations and thus, more reliable modeling results to improve forecasts, lower costs and reduce risks during subsurface operations.
... In-situ stress indicators such as borehole breakout are the stress-induced enlargements (Bell, 1996;Bell & Gough, 1983) and generally show a broad, parallel, poorly resolved zone of higher conductivity or low acoustic amplitude, and long travel time on the image logs (Figures 1 and 4). These compressive failure zones of the borehole wall will occur 180˚ apart. ...
Conference Paper
Defining pressure dependent permeability (PDP) behaviour in coalbed methane (CBM) or coal seam gas (CSG) reservoirs using reservoir simulation is non-unique based on the uncertainty in coal properties and input parameters. A diagnostic fracture injection test (DFIT) can be used to investigate bulk permeability at a reservoir level and at lowered net effective stress conditions. As coal has minimal matrix porosity and under DFIT conditions cleat porosity is fluid saturated with reasonably definable total compressibility values, the DFIT data can provide insight into PDP parameters. At pressures above the fissure opening pressure, pressure dependent leak off (PDL) behaviour increases exponentially with increasing pressure. Many authors have noted that with decreasing pressure PDP declines exponentially with increasing net effective stress. Thus, PDP behaviour can be defined by PDL. In this paper, we show how combined analyses, using typically collected field data, can be used to better define and constrain the modelling of PDP. We illustrate this process based on a well case study that includes the following data: fracture fabric and porosity reasonably defined from image log and areal core studies; DFIT data acquired under initial saturation conditions; hydraulic fracturing data; and longer term production data. These analyses will be integrated and used to constrain the parameters required to obtain a rate and pressure history-match from the post-frac well production data. This workflow has application in other coal seam gas cases by identifying key variables where hydraulic fracturing performance has been unable to overcome limitations based on pressure or stress dependent behaviours and often accompanied by low reservoir permeability values. While this is purposely targeting areas where only typically collected field data is available, this workflow can include coal testing data for matrix swelling/shrinkage properties or other production data analysis techniques.
... Thus, stiffness contrasts between the fault represented as volumetric weak zone and the host rock lead to magnitude changes as well as stress rotations. Softer rock properties in the fault zone lead to a larger stiffness contrast with respect to the surrounding, which results in broader ranges of between maxima and minima for S 1,eff magnitudes and stress reorientations, respectively [79,80]. Similarly, the spatial extent of the stress perturbations increases with decreasing stiffness of the fault zone and increasing stiffness contrast, respectively. ...
Article
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The proper representation of faults in coupled hydro-mechanical reservoir models is challenged, among others, by the difference between the small-scale heterogeneity of fault zones observed in nature and the large size of the calculation cells in numerical simulations. In the present study we use a generic finite element (FE) model with a volumetric fault zone description to examine what effect the corresponding upscaled material parameters have on pore pressures, stresses, and deformation within and surrounding the fault zone. Such a sensitivity study is important as the usually poor data base regarding specific hydro-mechanical fault properties as well as the upscaling process introduces uncertainties, whose impact on the modelling results is otherwise difficult to assess. Altogether, 87 scenarios with different elastic and plastic parameter combinations were studied. Numerical modelling results indicate that Young's modulus and cohesion assigned to the fault zone have the strongest influence on the stress and strain perturbations, both in absolute numbers as well as regarding the spatial extent. Angle of internal friction has only a minor and Poisson's ratio of the fault zone a negligible impact. Finally, some general recommendations concerning the choice of mechanical fault zone properties for reservoir-scale hydro-mechanical models are given.
... In general, three orders of stress patterns are present in the Earth's crust: first order (plate tectonic scale), second order (regional scale), and third order (local scale) (Zoback, 1992). The third order (local scale) of stress patterns is controlled by basin geometry, topography, local inclusions, density contrasts, and active faults (Heidbach et al., 2007;Zoback, 1992) and can show significant deviations from regional and plate stress patterns (e.g., Bell, 1996aBell, , 1996bPierdominici & Heidbach, 2012;Tingay et al., 2006). The present-day stress field is described by four main stress indicators: earthquake focal mechanisms, wellbore breakouts and drilling-induced fractures, in situ stress measurements (including over-coring, hydraulic fracturing, and borehole slotter), and young geologic data (from fault-slip analysis and volcanic vent alignments) (e.g., Heidbach et al., 2016). ...
Article
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Knowledge of the in situ stress state of the Earth's crust plays a key role in understanding geological processes including plate tectonics, earthquakes, slope failure, and igneous emplacement. In this paper, we determine the in situ stress orientation from the PTA2 borehole on the island of Hawai'i, drilled into a lava flow dominated sequence between Mauna Kea and Mauna Loa. High‐resolution acoustic images were collected from the open hole interval 886 m to 1,567 m. Based on identification of 371 borehole breakouts for a total length of 310 m, the mean orientation of the minimum horizontal principal stress is N106° and remains constant across different volcanic rock fabrics. Changes in borehole breakout shape are linked to the different strength of volcanic facies and intra‐facies. The orientation of the present‐day stress field at Mauna Kea deviates from the plate forces and regional tectonic stress field. We interpret the compressive stress regime at the PTA2 site as resulting from the competing gravitational fields of the large topographic highs of Mauna Kea and Mauna Loa. Our study reveals that the mass accumulation associated with shield volcano growth imparts significant local variations to the subsurface stress state on volcanic islands consisting of overlapping shield volcanoes. The results have significant implications for stress accumulation leading to brittle failure and flank collapse, along with potentially influencing magma accumulation and ascent pathways during volcanic island evolution. This study provides the first insights into the orientation of the present‐day stress field between the major island forming shield volcanoes of Hawai'i.
... The static elastic properties of the rocks are essential for studying the tectonic deformation of the earth's crust (Kumazawa and Anderson, 1969;Kohlstedt, et al., 1995;Watts and Burov, 2003). They are also important for predicting the in situ stress to reduce drilling risks and for hydraulic fracturing to enhance reservoir productivity (Plumb and Hickman, 1985;Bock, 1993;Bell, 1996;Zoback, 2007). The static properties of the subsurface rocks usually can not be measured in situ, and taking rock samples from thousands of meters below the surface is prohibitively expensive. ...
Article
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The static properties of subsurface rocks are needed for geomechanical applications, but the dynamic properties are usually more extensively available and can be acquired with much less effort. Therefore, it is important to know the relationships between the static and dynamic properties. Studying these relationships based on traditional triaxial testing yields confusing results, partly due to intrinsic deficiencies of the experimental setup and testing procedures. We have developed a new approach to study the relationship under hydrostatic stress conditions. The traditional ultrasonic velocity measurement system is sufficient for all of the testing, and it requires no application of the traditional strain gauge. During the pressure-dependent ultrasonic velocity measurements, the rock sample experiences static deformation when the pressure changes are in the order of megapascals. If the pore pressure is kept constant, the pore volume will vary with the confining pressure. The pore volume change can be monitored accurately by the pore pressure controlling pump, and it can be related to the volume strain of the rock for estimating the static bulk modulus of the rock sample. Based on the laboratory ultrasonic measurements available in the literature, we analyzed the effects of the differential pressure, clay content, crack density, and pore fluid on the relationship between the static and dynamic bulk moduli. From the analyses, we inferred that most of the laboratory-observed differences between the static and dynamic property may not exist for the reservoir rock under in situ conditions.
... In this study, we adhere to the common assumption that one of the principal stresses is vertical, and the other two principal stresses (S Hmax and the minimum horizontal principal stress, S hmin ) therefore lie in the orthogonal horizontal plane (Bell, 1996;Peška & Zoback, 1995;Snee & Zoback, 2018;Zoback, 1992;Zoback et al., 2003). We, however, note that in structurally complex areas such as the western Transverse Ranges where oblique-slip faulting, inclined-axis fault-block rotations, and fault Sankur et al. (1990) showing the NW-trending Sockeye anticline structure, which is cut by NW-striking thrust faults that extend from at least 1.5-2 km depth towards the surface. ...
Article
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The Santa Barbara Channel represents the offshore portion of the Ventura Basin in Southern California. Ongoing transpression related to a regional left step in the San Andreas Fault has led to the formation of E‐W trending en‐echelon fault systems that accommodate localized shortening across the basin. Recent studies have suggested that faults within the channel could be capable of a multisegment rupture and producing a Mw 7.7–8.1 tsunamigenic earthquake. However, dynamic rupture models producing these results do not account for stress heterogeneity. With only sparse information available on the stress field in this region, further borehole‐derived stress constraints are essential for obtaining a more comprehensive understanding of the hazards related to the complex fault systems. We used caliper logs from 19 wells obtained from industry to identify stress‐induced borehole breakouts beneath the Holly and Gail oil platforms in the channel. Our newly developed forward modeling technique provides constraints on the orientations and relative magnitudes of the three principal stresses. At Gail, we determine a reverse faulting stress regime (SHmax = 1.7; Shmin = 1.6; SV = 1.0) and an SHmax azimuth of N45°E. Our results are consistent with local structures, which reflect deeper regional scale trends, and with similar studies onshore nearby. At Holly, an SHmax rotation from ~N36°W to ~N57°E occurs across ~100 m depth in a single well and differs from nearby results, indicating that short‐length scale (<10 km laterally and <1 km in depth) stress heterogeneity is associated with complex changes in fault geometry.
... Hence, borehole breakouts are elongated perpendicular to the present-day SH max direction (Bell and Gough, 1979). Image loges can also be used to interpret DITF (Fig. 11d), which are oriented parallel to the in-situ SH max orientation (Bell, 1996). ...
Article
In this paper, the effect of transverse faults on the orientation, density and fractures apertures and amount of mud loss is investigated in Asmari Formation as a major oil reservoir in Zagros folded belt zone, Iran. This carried out on three selected anticlines as the surface Kuh-e-Kamarab, and subsurface Marun and Aghajari oil fields. Subsurface data on fracture characteristics were obtained from Image logs, mud losses, and reported drilling data. These are compared with outcrop fracture characteristics gathered from fieldwork and satellite image interpretation. The results show that fracture orientations are almost identical in both outcrops and subsurface data, whereas the fractures density and aperture are different along and across the folded layers and are generally governed by their location with respect to the transverse faults. The closest sector to the faults displays the highest fracture density and greater aperture. Quantitative analysis of fractures from the Kuh-e-Kamarab Anticline indicates average fracture density almost two times greater close to the Izeh-Hendijan transverse (IZH) fault than the other parts of the fold. In the Marun and Aghajari oil fields, fracture density shows three to five times greater close to the transverse faults. Analysis of the Breakout (BO) and Drilling Induce Tensile Fractures (DITF) indicate that the directions of maximum horizontal stress (SHmax) in the wells located in the bent part of the Marun field consistent with the stress state of the Mn transverse fault. Moreover, the SHmax orientation of wells in the southern periclinal closure of the Aghajari Field is also affected by the IZH transverse fault. The result of Iso-mud loss maps in the oil fields reveals that the severe mud loss occurred in areas close to the transverse faults.
Article
Knowledge of subsurface stresses is critical to management and prediction of fluid behaviour in the Earth's crust. However, uncertainties associated with the estimation of vertical stress magnitudes are rarely explored. In settings where the principal stresses are close, small changes of only a few MPa can have drastic effects on, stress regime prediction, fault reactivation and expected fracture behaviour. Using petroleum data sets from the Moomba Gas Field in the Cooper Basin, Australia, we assess the effects of interpolation between gaps in density wireline logs that have been filtered to remove sections of poor density tool contact in coal-bearing sequences. In addition, we compare the sonic velocity-density Nafe-Drake and Gardner transforms by comparing estimates of density from sonic velocity against density from wireline logs. Results indicate vertical stress could be underestimated by ≈ 3 MPa at ≈ 3 km in well Moomba 61, using traditional methods. The Gardner transform shows a stronger correlation between sonic and density values for rocks in the Cooper Basin. However, once calibrated to the regional sonic velocity-density trend, the Nafe-Drake transform better matches the density logging tool in Moomba 61. This study delivers a more robust workflow for estimating S v in basins with deep coal-bearing strata.
Article
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The orientation of SHmax is commonly estimated from in-situ borehole breakouts and earthquake focal mechanisms. Borehole measurements are expensive, and therefore sparse, and earthquake measurements can only be made in regions with many well-characterized earthquakes. Here we derive the stress-field orientation using stress-induced anisotropy in nonlinear elasticity. In this method, we measure the strain derivative of velocity as a function of azimuth. We use a natural pump-probe approach which consists of measuring elastic wave speed using empirical Green’s functions (probe) at different points of the earth tidal strain cycle (pump). The approach is validated using a larger data set in the Northern Alpine Foreland region where the orientation of maximum horizontal compressive stress is known from borehole breakouts and drilling-induced fractures. The technique resolves NNW-SSW to N-S directed SHmax which is in good agreement with conventional methods and the recent crustal stress model. We confirm that the natural pump-probe method can be applied to dense large-scale seismic arrays. The technique is then applied to the Southern Alps to understand the contemporary stress pattern associated with the ongoing deformation due to counterclockwise rotation of the Adriatic plate with respect to the European plate. Our results explain why the two major faults in Northeastern Italy, the Giudicarie Fault, and the Periadriatic Line (Pustertal-Gailtal Fault) are currently inactive, while the currently acting stress field allows faults in Slovenia to deform actively. We have demonstrated that the pump-probe method has the potential to fill in the measurement gap left by conventional approaches, both in terms of regional coverage and in depth.
Article
Carbonate reservoir is the focus of oil and gas exploration and also the subject of much research in the world. High-yield industrial oil and gas flow has been found in the fault-controlled carbonate reservoirs of the Middle Lower Ordovician in the Shunbei area. In the process of drilling and development, this type of reservoir faces many problems closely related to the current stress state, which need to be solved urgently. In this paper, the direction of the maximum horizontal principal stress is determined by using the global positioning system (GPS), the azimuth of fast shear wave propagation, and borehole enlargement. In addition, under the calibration of acoustic emission experiment results, cross-dipole logging data are used to evaluate the in situ stress in the study area. According to the results of the rock mechanics experiment and logging interpretation, the conversion relationship between dynamic and static parameters of rock mechanics is established. Finally, according to the fault and horizon interpreted by 3D seismic data, a 3D geological model of the study area is established. Under the constraint of logging interpretation and experimental results, the stress field in the study area is simulated by the iterative boundary element method. The simulation results show that the distribution of the in situ stress field is affected by the arrangement of faults. From the map view, it shows the segmentation along the fault strike; that is, different segments show different characteristics. In the vicinity of strike-slip faults, the principal stress direction is deflected to varying degrees. The relative error between the simulated value and the measured value is less than 10%, which verifies the reliability of the numerical simulation results. The research results can provide a reference for the determination of optimal mud weights, the optimization of wellbore trajectory, and the selection of acid fracturing operation parameters.
Article
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Geological body in ultra-deep is easy to deformation and instability with high pressure, high temperature, high stress accumulation of high strength energy, leading to high risk of major disasters and accidents. Special geological attributes in deep drilling are of great significance for engineering practice. Tarim Basin is taken as an example, systematically combing the geological attributes of the whole stratum series during the drilling process, especially taking the non target stratum as the target stratum,analyzing the background and occurrence environment of various geological attributes affecting ultra-deep oil and gas drilling engineering, the internal interaction between drilling engineering and geological attributes is revealed from "sedimentary construction, structural transformation and sedimentary structural coupling", and the research and description methods of special geological attributes are emphatically discussed. The results show that: (1) compared with medium and shallow oil and gas reservoirs, 95% of the footage of ultra-deep well drilling is in the non-target layer, and the drilling of non-target layer directly affects the exploration and development process; (2) Compared with medium and shallow oil and gas reservoirs, surface conditions, complex lithology, geological trace, formation fluid and ultra-deep and extremely high temperature and pressure field are great challenges for ultra-deep well drilling; (3) The systematic and scientific analysis of the special geological factors of ultra-deep drilling engineering improves the comprehensiveness of oil and gas reservoir understanding and the evaluation of well control safety risk, which is conducive to risk prevention and well location deployment; (4) The special geological analysis of the whole stratum further constructs the bridge between geology and drilling engineering, and promotes the integration of geological engineering from concept to practice.
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Based on the comprehensive analysis of wellbore characteristics in a deep shale gas field, the in-situ stress state of the shale reservoir was assessed in this study for the Longmaxi formation in the Dingshan area, Southwestern China. The data obtained from hydraulic fracturing, drilling-induced fractures, and in-situ core testing were used to determine the magnitude and direction of the maximum principal horizontal stress in five wells. Besides, hydraulic fracturing and cross-multipole array acoustic logging (XMAC) were employed to determine the vertical variation of the in-situ stress. Based on the logging interpretation and mechanical test results, the spatial distribution of rock mechanical parameters in the Dingshan area was assessed by the amplitude variation versus offset (AVO) seismic inversion. A 3D heterogeneous mechanical inversion model was realized via the ANSYS and CATIA3D finite element software packages, providing the area in-situ stress field simulation. The depth, fault strike, and position change effects on the main stress, horizontal stress difference, and horizontal stress difference coefficient were numerically simulated. The results show that the maximum principal stress azimuth was mainly concentrated in the NE20°-NE80° sector. Moreover, the development zone of natural fractures was related to the area with the highest principal stress differences. The maximum principal stress variation in the study area was mainly in the compressive range from −135 to −45 MPa, gradually increasing from east to west and south to north. The stress type mainly depended on the depth, fault strike, and rock mechanical parameters, while the stress difference and stress difference coefficient near the fault structure were relatively small. This study’s findings are considered instrumental in improving the borehole stability, determining the casing setting point, and optimizing the well location in deep shale reservoirs with similar geological conditions.
Article
Sand production is a serious problem in oil and gas wells, and one of the main concerns of production engineers. This problem can damage downhole equipment and surface production facilities. This study presents a sand production case and quantifies sanding risks for an oil field in Iraq. The study applies an integrated workflow of constructing 1D Mechanical Earth Modeling (MEM) and predicting the sand production with multiple criteria such as shear failure during drilling, B index, and critical bottomhole pressure (CBHP) or critical drawdown pressure (CDDP). Wireline log data were used to estimate the mechanical properties of the formations in the field. The predicted sand production propensity was validated based on the sand production history in the field. The interpretation results of some wells anticipated in this study showed that when a shear failure occurs during drilling, the B index is around 2 × 104 MPa or less and the CBHP is equal to the formation pore pressure. For this case, sand control shall be carried out in the initial stage of production. On the other hand, when the shear failure does not exist, the B index is always greater than 2 × 104 MPa, and the CBHP is mostly less than the formation pore pressure. In this case, implementing sand control methods could be postponed as the reservoir pressure undergoes depletion. However, for the anticipated field, sand control is recommended to be carried out in the initial stage of well production even when the CBHP is less than the formation pore pressure since sanding will be inevitable when the reservoir pressure depletes to values close to the initial reservoir pressure. The tentative evaluation of the stress regime showed that a normal fault could be the stress regime for the formations. For a normal fault stress regime, the study explained that when the reservoir permeability is isotropic, an openhole vertical wellbore has less propensity for sand production than a horizontal wellbore. Moreover, when the wellbore azimuth is in the direction of the minimum horizontal stress, the CBHP will be lower than in any other azimuth, and sanding will take place at higher wellbore inclination angles. For the anticipated field, because of the casedhole well completion and the anisotropic reservoir permeability, a horizontal well drilled in the direction of minimum horizontal stress with oriented perforation in the direction of maximum horizontal stress is an alternative method for controlling sand production.
Chapter
In situ stress distribution and mechanical properties both control the engineering performance of shale, coal, and tight sandstone gas/oil reservoirs. A clear understanding of the above parameters is of significance for engineering design and gas/oil recoveries. The detailed utilization of well logging data combined with basic laboratory tests can provide insight to both mechanical property and in situ stress distributions. The comprehensive combination of in situ stress and mechanical properties is presented here to clarify the understanding of unconventional reservoirs. The acquisition of static and dynamical parameters, brittleness of shale and present in situ stress magnitude is presented with a case study from southern China. The Lower Silurian Longmaxi formation is one of the most promising shale gas reservoirs in China. In this chapter, Longmaxi formation shale samples from the Jiaoshiba area, Sichuan Basin, China, were selected for laboratory experiments, and a series of testing methods such as scanning electron microscope, uniaxial compression, and triaxial compression were carried out. The analysis presented combines logging data with the geomechanics and petrophysical properties of the Longmaxi formation shale. The results reveal the mechanical properties and in-situ stress characteristics of this shale reservoir, and provide the necessary information for field hydraulic fracturing and wellbore stability applications in the Longmaxi shale gas development process.
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The stress field in the Earth's crust plays a central role in the site-selection process for a deep geological repository for high-level nuclear waste. Site selection and construction planning must take into account several factors that are influenced by the stress state. These include the excavation damage zone, the hydraulic permeability of the host rock, the self-sealing capacity, the effects of seismic events and the possible reactivation of faults as migration pathways for fluids and radionuclides. Likewise, the initial stress state is of central importance for the long-term studies to prove site safety over 1 Ma. To obtain a continuous description of the current 3D stress state, 3D geomechanical numerical models are used. These models have to be calibrated with data on stress magnitudes to obtain robust predictions. One of the central goals of the SpannEnD project (Spannungsmodell Endlagerung Deutschland, http://www.spannend-projekt.de, last access: 31 October 2021) was to build the first comprehensive and publicly accessible stress magnitude database for Germany, including a quality ranking of the data compiled from different methods. This database is the logical extension of the database of the World Stress Map project, in which so far only information on stress orientations and the stress regime has been compiled systematically. We present this first compilation of stress magnitude data published and made available by Morawietz et al. (2020). The stress data density is generally low and heterogeneous, so that a model calibration at the scale of a site model is not possible. Therefore, the main objective of the SpannEnD project is to develop a 3D geomechanical numerical model for the whole of Germany. The resulting 3D stress field will provide the basis for regional and local models in a later phase of the site selection process. Details on this are presented in three complementary contributions in this symposium by Reiter et al., Röckel et al. and Ahlers et al. The new Geology Data Act (Geologie-Datengesetz) now allows access to considerably more data, which will be incorporated into an update of the database after assessment according to the defined quality criteria. This database extension will improve the reliability of the predictions of the geomechanical models on different spatial scales.
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Knowledge of the crustal stress state is important for the assessment of subsurface stability. In particular, stress magnitudes are essential for the calibration of geomechanical models that estimate a continuous description of the 3-D stress field from pointwise and incomplete stress data. Well established is the World Stress Map Project, a global and publicly available database for stress orientations, but for stress magnitude data only local data collections are available. Herein, we present the first comprehensive and open-access stress magnitude database for Germany and adjacent regions, consisting of 568 data records. In addition, we introduce a quality ranking scheme for stress magnitude data for the first time.
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The Upper Shihezi sedimentary rocks in the Linxing region has been estimated with a significant volume of tight sandstone gas. However, lateral distribution of the present‐day stress magnitude is poorly understood, which limits further gas production. Hence, a one‐dimensional mechanical earth model and a three‐dimensional heterogeneous geomechanical model are built to address this issue. The results indicate that the strike‐slip stress regime is dominant in the Upper Shihezi Formation. Relatively low stresses are mainly located around wells L‐60, L‐22, L‐40, L‐90, etc, and stress distributions exhibit the similarity in the Members H2 and H4. The differential stresses are relatively low in the Upper Shihezi Formation, suggesting that complex hydraulic fracture networks may be produced. Natural fractures in the Upper Shihezi Formation contribute little to the overall gas production in the Linxing region. In addition, the minimum principal stress gradient increases with Young’s modulus, suggesting that the stiffer rocks commonly convey higher stress magnitudes. There is a strong interplay between stress distribution and heterogeneity in rock mechanics. Overall, the relative error between the predicted and measured results is less than 10%, implying that the predicted stress distribution is reliable and can be used for subsequent analysis in the Linxing region.
Article
Research on in situ stress has important theoretical and practical significance for the exploration and development of oil and gas reservoirs. The orientation and magnitude of in situ stress in the Gaoshangpu Oilfield northern area (GO-NA) were analyzed using borehole breakout data and acoustic emission measurements. Mechanical experiments, logging interpretation, and seismic data enabled spatial characterization of rock mechanics parameters. A 3D geological model and 3D heterogeneous rock mechanics field of the GO-NA were constructed. Petrel and ANSYS modeling provided detailed prediction of the 3D stress field in the GO-NA. The results indicate that the maximum horizontal stress orientation in the GO-NA is generally ENE–WSW-trending, with significant changes in in situ stress orientation within and between fault blocks. Along surfaces and profiles, stress magnitudes are discrete and in situ stress is of the Ia-type. Observed inter-strata differences were characterized by five different types of in situ stress profile. Faults are the most important factor in the large distributional differences in the stress field of reservoirs observed within the complex fault blocks, significantly affecting magnitudes and orientations in the stress field. The next most important influence on the stress field is the reservoir’s rock mechanics parameters, which affect in situ stress magnitudes. A strong linear correlation exists between reservoir depth and in situ stress magnitude. This technique provides a theoretical basis for more efficient exploration and development of low-permeability reservoirs. It also serves as a reference for the detailed prediction of inter-well in situ stress in regions with similarly complex fault blocks.
Article
The present-day in-situ stress field has many important and practical applications during the exploration and development of hydrocarbon resources. The Ordos Basin is a significant petroliferous basin in China, within which, the Yanchang Formation serves as the most significant tight oil reservoir. However, an overview of previous studies indicates that little was known as to the present-day stress state in the Yanchang Formation. Therefore, the present-day stress field in the Yanchang Formation tight oil reservoir of the Ordos Basin is systematically analyzed in this study. The orientation of horizontal maximum principal stress (SHmax) within the Yanchang Formation of Ordos Basin ranges between north-northeast-south-southwest-trending (NNE-SSW) and east-northeast-west-southwest-trending (ENE-WSW) based on interpretations of borehole breakouts and drilling-induced fractures from borehole imaging logs. The SHmax orientation shows various tendencies in different wells, which may result from differential development in lithology, natural fracture and bedding plane over various regions. The estimated magnitudes of vertical stress (Sv), SHmax and horizontal minimum principal stress (Shmin) in the studied wells indicate that the normal faulting stress regime is dominant within the Yanchang Formation. In addition, the geomechanical assessments of wellbore stability and natural fracture reactivation are also performed. Natural fractures in the Yanchang Formation Chang 7 tight oil reservoir of Ordos Basin are developed striking mainly in the northeast-southwest-trending (NE-SW) and west-northwest-east-southeast-trending (WNW-ESE). Among them, those fractures with NE-SW-trending strikes are more conductive with reference to the present-day stress state, contributing more to tight oil production in the Ordos Basin. In addition, within the Yanchang Formation, if horizontal wells are drilled towards the NNE-SSW-trending to ENE-WSW-trending, they are more likely to have wellbore instability issues. The results in this study provide important geological information for tight oil production in the Yanchang Formation of Ordos Basin.
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Wellbore instability can lead to expensive operational problems during the drilling, completion and production of horizontal and inclined wells. This paper reviews the direct and indirect symptoms of wellbore instability, its root causes, and various empirical and deterministic modelling approaches to predicting the risk of hole collapse or convergence. In general, linear elastic models that are only concerned with stability at the wellbore wall often give overly pessimistic predictions. An alternative approach, using the extent of the "yielded" zone around an unstable wellbore and the kinematics of rock detachment, is proposed for practical risk assessments. A case history for an open hole completed horizontal well in a limestone reservoir under high drawdown is described. General guidelines for conducting field-oriented stability assessments conclude the paper. Introduction Wellbore instability during the drilling, evaluation, completion and production phases of a well has become an increasingly important concern for many operators applying horizontal well technology. Traditional conservative completion methods for vertical wells are being challenged as operators attempt to reduce well costs and still derive the improved productivity and access to hydrocarbon reserves offered by horizontal wells. More recent horizontal well innovations include the use of underbalanced drilling techniques(l), slimhole completions, side track or re-entry wells with open hole build sections(2,3), and multiple laterals from a single vertical or horizontal wellbore(4). In applying these new technologies, there are often issues posed during the well planning stage where the risk of hole collapse in the short or long term must be addressed. In many cases, the selection of an optimal strategy to prevent or mitigate the risk of wellbore collapse might compromise one or more of the following other elements of the overall well design: the rate of penetration; the risk of differential sticking; drilled cuttings and mud disposal options; hole cleaning abilities; hole size, and consequently the completion and stimulation options available; formation damage risk; stimulation requirements; the ability to log the hole; and the selection of surface sand handling facilities (where sand production is anticipated). In many cases there may be insufficient experience with a given reservoir and the desired completion, hence the prior performance of vertical wells cannot be used, by itself, to guide the well design. This paper reviews the symptoms of wellbore instability and its fundamental causes. Published approaches to wellbore stability prediction will be described, particularly those which address the most common problems faced in developing normally to slightly under pressured oil and gas fields. Emphasis is placed on techniques and conditions applicable to Western Canada where there has been a rapid pace of horizontal well development, particularly with re-entry wells. Predictive techniques applicable to build, inclined or horizontal sections of a well, during the drilling, completion and subsequent production phases, will then be described. Selected case histories from the literature are cited and an example of a wellbore stability prediction for Shell Canada's first horizontal open hole completion is described. This paper will not review all the wellbore stability models which have been developed for vertical wells.
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Stresses in rock are a three-dimensional quantity defined by direction and magnitude (tensor). They are the result of many processes (tectonic, gravity, thermal, etc.) which are again modified by the physical properties of the rock mass. This chapter addresses the problems of obtaining representative ground stress values and the distribution of ground stresses in the Canadian Shield, and describes recently developed instrumentation for monitoring the effects of load redistribution or stress changes during underground excavation activity. -from Author
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Gas and condensate are pooled in organically immature sandstone beds at Alma F-67. The reservoir is connected to underlying overpressured, mature shale by normal faults. A comparison between leak-off pressure and pore pressures in the geopressured sequence shows that little increase in fluid pressure would be required to open vertical fractures and initiate fluid migration. The fluid migration is believed to have occurred along faults identified as present day "free surfaces' by breakouts. -Author
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In-situ stress orientation was measured in 17 wells throughout the Scott field in order to predict the orientation of waterflood induced fractures. The Scott field is heavily faulted and significant variations in in-situ stress orientation were found between different fault blocks. The in-situ stress orientations appear to be strongly controlled by the orientation of the local faults and tectonics. The direction of maximum horizontal stress appears rotated 30° to 50° on either side of the NNW-SSE regional compression trend generally found in the North Sea. In most cases, the maximum horizontal stress is parallel to the orientation of nearby normal and wrench faults, as would be predicted by a tectonic model of such faulting. However, the tectonics of the region suggest that the rotation of the regional stress field from fault block to fault block is due to the presence of the faults rather than active faulting. The orientation of the in-situ stress was determined from shear acoustic anisotropy measurements on cores from 5 wells and from wellbore elongation measurements in 13 wells. The two methods show very similar orientations within the same fault block. No statistical variability in stress orientation was noted between different formations or with depth. Shear acoustic anisotropy utilizes the polarization of shear acoustic waves propagating through oriented core samples and has proven to be very reliable in determining stress orientation. The observed wellbore elongations in the field do not appear due to breakouts. The character, magnitude, placement, and orientation of the wellbore elongations strongly support the premise that the measured elongations were due to drilling or coring induced wellbore fractures. Previous experience by the authors and other recently published observations support this conclusion and the reliability of these elongations for the determination of stress orientation.
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Hydraulic fracturing is a means of developing low-permeability gas sands. The most significant parameter controlling created fracture geometry is in-situ stress slate. Measurements in the Deep Basin have demonstrated differences of 6 MPa in the minimum horizontal in-situ stress over 15 – 30 m with distributions which are both favourable and unfavourable to fracture containment within a producing zone. Conversely, stress variations less than 2 MPa have been observed over a 90 m depth range which contained both a producing interval and thick bounding shale beds. The degree of hydraulic fracture containment has been observed to be related to the vertical profile of the minimum horizontal Stress. In this region the maximum horizontal principal stress is estimated to be nearly equal to the vertical stress at depths below 2 km. It is about 1.4 times the minimum principal stress. This large contrast suggests that fracture azimuth should be controlled completely by the in-situ stress state in the Deep Basin. Introduction Low-permeability reservoirs such as tight gas sands in the Deep Basin of Western Canada are generally uneconomic because of low production rates, not insufficient hydrocarbon reserves. Current technology for increasing these rates relies on hydraulic fracturing. Effective hydraulic fracturing creates a highly conductive channel contained primarily within the productive zone and extending as far as possible away from the wellbore. When a fracture is not contained primarily within the productive zone, an excessive amount of fracturing fluid is used. This has some potentially serious consequences. Increased formation damage may result from excessive fluid leak-off(1), improper proppant placement, such as settling below the zone is possible, and excessive fracture conductivity damage can occur from having to recover extra fracturing fluid(2,3). The in-situ stress state is the most significant parameter controlling fracture containment. An induced fracture can be contained within a zone having a lower stress than that in the bounding layers. Laboratory experiments have shown that under appropriate conditions stress differences of 2–3 MPa are sufficient to contain a fracture(4,5). Field observations have shown stress variations of 6–8 MPa over short" vertical distances(6,7). Fracture containment was observed under appropriate conditions. This paper presents data pertinent to the in-situ stress state and its influence on hydraulic fracturing in the Deep Basin of Alberta. As pan of an extensive research program conducted by Esso Resources Canada Limited in support of its investment in tight gas sands, the vertical distribution of the minimum principal stress was determined over three intervals in two wells in the Deep Basin. Two of these intervals contained zones with sufficient productive promise to warrant hydraulic fracture treatments. These were monitored to a sufficient extent to relate the measured vertical profiles of minimum horizontal stress to the behaviour of the induced hydraulic fractures. Stress Determinations Although many methods exist for determining in-situ stresses, the use of hydraulic fracturing is the only practical technique for deep wellbores. During the hydraulic fracturing process a fracture is initiated or reopened by increasing wellbore pressure as fluid is pumped into the well and then closes after pumping stops.
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The Weyburn Field is located at the western periphery of the prolific Midale-Steelman oilfield trend along the northeastern flank of the Williston Basin. Hydrocarbon production is from the Mississippian Midale Beds. These beds constitute a small portion of a thick succession of Mississippian cyclic sediments. The Midale Beds dip southward toward the centre of the Basin. Post-Mississippian erosion has truncated the entire Midale section immediately north of the field. Oil is trapped by a combination of structural, diagenetic and stratigraphic elements. Production data indicate a preferential fluid migration that is caused by the presence of a northeast- to southwest-trending fracture system. Variation in the types of diagenesis affecting lithostratigraphic units has led to the formation of different porosity types. As a result of the varying types and areal distribution of the porosity, a significant difference in reservoir characteristics between zones is apparent. Consequently an understanding of reservoir quality and distribution is essential to optimize oil recovery, whether through recompletion, infill drilling or enhanced recovery. Introduction The Weyburn Field of southeastern Saskatchewan, as outlined by Saskatchewan Energy and Mines, covers portions of Twps. 05 to 07, Rges. 11 to 14, W2M. The field is found near the present northwestern edge of the Midale-Steelman trend (Fig. 1)Oil accumulation is in the Midale Beds of the lowermost Mississippian Charles Formation (Madison Group). The discovery well, Central Del Rio No. 14-06-007-13 W2M, was drilled to a dep[h of 1562 m and completed as a Midale Beds oil producer in 1955. Within the designated field boundary, approximately 950 wells have been drilled to date. The greater part of the pool was unitized in 1963 and an inverted nine-spot waterflood scheme was implemented in 1964. At the end of 1986, the Weyburn Unit included 547 producing wells, 117 injectors, 42 miscellaneous wells (water source, pressure observation, temporarily shut in, etc.) and 2B abandoned wells. Geology Regional Setting The Mississippian Madison Group of the Williston Basin contains a 400 m to 700 m thick section of carbonate and evaporite sediments. The sedimentation pattern is typical or a major upward shoaling sequence, deposited as a thick basin ward migrating wedge of sediments. The Madison Group in southeast Saskatchewan has been subdivided into a series of marker-defined stratigraphic units termed "Beds ", as illustrated in figure 2(2,3). Continued subsidence resulted in the southward dipping strata observed at the present. Subsequent post-Mississippian erosion truncated progressively older Mississippian strata northward across southern Saskatchewan, creating the present-day Mississippian subcrop pattern. Weyburn Area The Upper Madison Group stratigraphy and typical log response in the Weyburn area is shown in Figure 3. The Midale Beds comprise the Frobisher Evaporite and the Midale Carbonate. Within the Weyburn Field boundary, the depositional edge of the Frobisher Evaporite is indicated by a lateral change from massive dense anhydrite in the northeast to dolomitic, often argillaceous lime mudstones and wackestones in the southwest(4). The Midale Carbonate in the Weyburn area is subdivided into two distinct lithostratigraphic units, referred to as Vuggy (lower) and Marly (upper) (Fig. 3).
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The relationships between overpressures and porosity, and overpressures and stress regime, are found to differ significantly according to the geological setting and overpressure generating mechanism. These factors must be taken into account in pore pressure prediction techniques and in correlations between pore pressure and minimum horizontal stress. Two case histories from overpressured Canadian basins are presented to illustrate the above. Introduction "Sonic and density logs…provide a means of determining pore pressure and the overburden stress which are needed to calculate the fracture pressure" (Anderson et al. 1972). "Pressure generation in the Gulf of Alaska is somehow related to the high lateral stresses. For this reason it is not surprising that the traditional methods of abnormal pressure prediction developed in basins such as the Gulf of Mexico do not work in the Gulf of Alaska" (Hottman et al. 1979). Many sedimentary basins are overpressured at depth, sometimes so severely that the overburden rocks arc practically floating on top of the overpressured formation. This poses severe problems in assessing the safety window for mudweights, which have to counterbalance the overpressure to prevent blowouts without exceeding the minimum stress and fracturing the formation. It also exerts a significant influence on seal integrity. There has therefore been a great deal of research on correlating pore pressure and the minimum (usually horizontal) stress. Mudweight balancing is at its most sensitive at the onset of overpressuring because, often, severe overpressures are suddenly encountered over depth intervals in the order of tens of metres. Accurate pore pressure prediction is therefore crucial. The main assumption made in pressure prediction techniques is that the overpressure will be associated with a density anomaly that will be detected using geophysical techniques. This paper will demonstrate that the overpressure generating mechanisms strongly affect pore pressure-porosity and pore pressure- stress relationships in sedimentary basins. 2 ORIGIN OF OVERPRESSURING Overpressured rocks can be observed worldwide in many types of sedimentary and tectonic environments. Overpressures are usually associated with thick sedimentary sequences of ages varying from Jurassic to Tertiary. A seal is required to preserve overpressuring, and its effectiveness controls the shape of the "transition zone" from normal, hydrostatic pressures to maximum overpressures. The narrower the transition zone, the more difficult it is to counterbalance the overpressures. Another control on the shape of this zone is the timing of overpressuring. It is therefore important to identify the possible mechanisms of overpressuring within a sedimentary basin, as well as their timing. A number of different factors may contribute to abnormally high fluid pressures (reviewed by Fertl 1976, Rieke and Chilingarian 1974, Mouchet and Mitchell 1989, Yassir 1989).Overpressuring by rapid loading and undercompaction is the most widely accepted mechanism (Rieke and Chilingarian 1974). If a sediment is allowed to dissipate its excess pore fluids as it is buried, it compacts and the pore pressure gradient remains hydrostatic. When complete pore pressure dissipation is impeded by a seal or low permeability, however, the sediment becomes underconsolidated, with an abnormally high porosity for its depth of burial.
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The Midale Unit is a naturally fractured carbonate reservoir in the Williston basin of southeastern Saskatchewan. Recently, the Midale working-interest owners initiated a CO2-flood pilot project with several closely spaced wells. A comprehensive interwell pressure-transient test program was conducted for reservoir-characterization purposes before pilot program was conducted for reservoir-characterization purposes before pilot operations began. The combination of conventional, pulse, and interference tests resulted in a detailed local description of this naturally fractured reservoir. Selected pressure-transient data from the reservoir-characterization program are presented. The data dramatically illustrate the program are presented. The data dramatically illustrate the pressure-transient characteristics of an anisotropic, naturally fractured pressure-transient characteristics of an anisotropic, naturally fractured reservoir. Analytical techniques are reviewed briefly, and an interpretation technique for multiwell pressure-transient tests in wells with negative skins is included. A discussion of the pressure-transient behavior that is observed in wells with typical field spacing is also included. Introduction A significant proportion of the world's oil reserves exists in naturally fractured rocks. As a result, the pressure-transient behavior of naturally fractured reservoirs has been studied widely. These reservoirs, referred to as dual-porosity or dual-permeability systems, can be modeled with matrix blocks of various shapes separated by continuous fracture networks. Typically, the system permeability is largely associated with the fractures, while the tighter matrix contains most of the reservoir-storage volume and acts as a fluid source for the fracture network. Initial production from dual-porosity reservoirs is from the more permeable fracture network, which results permeable fracture network, which results in pressure decline in the fractures. Depletion of the fracture system causes pressure gradients to develop between the matrix blocks and the fractures, which results in fluid flow from the blocks into the fractures. This transitional flow period controls the reservoir-pressure behavior until the matrix fracture pressure regime equilibrates, after which the reservoir acts as a uniform system with composite properties. Pressure-transient testing is an important technique used to describe naturally fractured reservoirs. Each flow period (fractures, transitional, and composite) has distinctive pressure-transient characteristics. Pressure-transient behavior in real Pressure-transient behavior in real dual-porosity reservoirs, however, can be subtle and ambiguous, especially in normal situations with typical well spacing. For example, pressure transients controlled by depletion of the fracture network are rarely seen in the field because of the fast onset of the transitional flow period. Transitional- and composite-system pressure transients can be attributed to such other causes as layers, skin, faults, or boundaries. Wellbore storage can obscure key parts of the pressure-transient curve. For these and other pressure-transient curve. For these and other reasons, few comprehensive pressure-transient data bases exist for naturally fractured reservoirs. The scarcity of good data impedes refinement and validation of precise pressure-transient models. The Midale field (Fig. 1) produces oil from relatively tight, naturally fractured carbonates. The field was discovered in 1953 and produced competitively under primary depletion until unitization for waterflooding in 1962. Secondary performance is dominated fieldwide by the oriented-fracture system, which causes a permeability anisotropy of about 25:1. Recent implementation of a tertiary miscible CO2-flood pilot project in the Midale Unit, with 10 wells on 4.4 acres [1.8 ha], has allowed for very detailed reservoir characterization. Conditions in the Midale CO2-flood pilot are amenable to both single- and multiwell pressure-transient programs. An extensive, precise pressure programs. An extensive, precise pressure transient data base was acquired during pilot-testing operations. The reservoir pilot-testing operations. The reservoir model that was obtained from pressure-transient analyses is consistent with geological, petrophysical, and operational data, and it petrophysical, and operational data, and it provides significant refinement over earlier provides significant refinement over earlier reservoir models.
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A method of gas permeability for coal and sedimentary rocks under triaxial stress conditions is outlined. The transmissibility of compressed air through these rocks is shown to vary markedly, depending on both the rock lithology and containment pressure.
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If enough stress-generated breakouts have been identified on four-arm dipmeter logs to map regional horizontal stress trajectories, it may be possible to use anomalous orientations to identify open fractures and non-sealing faults. These act as free surfaces and, if they were oriented obliquely to regional stresses, deflect them locally. This type of analysis is simple to apply and appears to offer a novel way to obtain key information required for optimizing the production of oil and gas. In offshore eastern Canada, the regional stress regime is well established and local deflections appear to be caused by near-by open fractures and non-sealing faults. In the Aquitaine Basin in France the regional stress signature is not homogeneous. Though some anomalous orientations appear to be caused by open fractures and faults, it is clear that many are not. hence the method may not be applicable everywhere. Possible reasons include differences in the σ H:σ h ratios of the two areas and the geotectonic settings. -Authors
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Hydraulic fracture stress measurements have been performed through perforations at depths from 1310 to 2470 m at the U.S. Department of Energy's Multiwell Experiment site. The results of over 60 stress tests conducted through perforations have shown that small-volume hydraulic fractures generally provide an accurate, reproducible measurement of the minimum in situ stress. However, unusual behaviour can occur in some tests and techniques to evaluate the behaviour are suggested. Stress results show that the stress distribution is dependent on lithology at this site; mudstones, shales and other non-reservoir rocks generally have a near-lithostatic stress, while sandstones have a considerable lower minimum stress value.
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Knowledge of the stress field in the Earth's crust is crucial for the understanding of processes such as faulting and fracturing. It is also of fundamental importance for the design and construction of underground structures. The two most common methods used for in situ stress determinations for engineering geology purposes are (1) the stress relief method, in which the differences in strain resulting from induced stress relaxation are studied, and (2) the hydraulic fracturing method, in which one of the presumed principal stresses is replaced by a hydraulic pressure. Of these, the stress relief method is more applicable to design and construction problems as it enables the three-dimensional stress tensor to be measured. The in situ rock stress measurements reported here were carried out in a 500-m deep borehole in Precambrian bedrocks in Sweden and show high stress levels throughout the rock mass. A comparison between the actual recorded stresses and the theoretical stress field from a 2,000 m-thick land ice may provide a possible explanation for the measured stress situation.
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Stress provinces, characterized by nearly constant directions and relative magnitudes of horizontal principal stresses, are potentially informative of the forces acting on a lithosphere plate. This applies most strongly to stress provinces which occupy a large part of a plate, such as the Mid-Continent Stress Province of North America identified by Zoback and Zoback1. We have used fractures known as breakouts in oil-wells, which are controlled by the in situ stress, to show that this stress province extends through the western Canadian sedimentary basin to the arctic coast. There is a strong possibility that a uniform stress orientation, with greatest compression NE–SW, is co-extensive with the North American continent east of the Rocky Mountains excluding the Appalachian and Gulf of Mexico stress provinces. We further report new breakout orientations in the far northwest of Canada, which locate the boundary between the Mid-Continent Stress Province and an Alaskan stress province, with largest compression NNW–SSE, probably controlled by the subduction of the Pacific plate under Alaska.
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Provinces of regionally consistent horizontal stress orientations persist throughout the upper crust, and these may contain local areas where horizontal stress (σH) orientations seem to rotate relative to the regional stress orientations in a consistent manner. We suggest that the concept of aninclusion with anisotropy and heterogeneity of mechanical properties may be useful to explain some cases of σH rotations. σH rotations up to 50° and magnitude changes of 40% could reasonably be expected, based on simulations. Patterns correlate well with crustal observations; rotation of σH orientation reaches 25° to 58° in the different cases studied. We also model a strike-slip fault zone as a soft inclusion, showing reorientations of σHmax which agree with observed rotations near the Murre Fault in the Jeanne d'Arc Basin, Canada. Results imply that understanding the behaviour of soft and stiff inclusions, or expanding and contracting thermal inclusions for that matter, can help explain a number of geophysical and geological phenomena related to stress patterns.
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The regional stress field and its local variation were determined for the northern part of central Switzerland (Fig. 1) by using overcoring techniques (doorstopper, triaxial strain cell) and observations of breakouts in deep boreholes. The results are compared with fault plane solutions of earthquakes and with the orientation of horizontal stylolites.In the northern part of central Switzerland the NW-SE-orientation of the maximum horizontal stress (SH) which is characteristic for Central Europe was observed only in the crystalline basement. In the Folded Jura and south of it in one well the greatest principal horizontal stress above the Triassic decollement horizon is oriented approximately in a N-S to NNE-SSW direction.This direction persists into the western Tabular Jura and the southernmost Rhine Graben. Only in the eastern part of the Tabular Jura the greatest principal horizontal stress shows a NNW-SSE to NW-SE orientation. Comparison of the near surface stress field as determined by in situ stress measurements and borehole breakouts with the directions of horizontal stylolites generated during the evolution of the Folded Jura, indicates that the orientation of the recent stress field near the earth's surface is the same as that which prevailed during the Upper Miocene to Lower Pliocene.The central part of northern Switzerland is therefore the first area in Central Europe where it is possible to demonstrate that the near surface stress field is decoupled from that in the crystalline basement. The difference in the orientation of the greatest principal horizontal stress is about 50°.
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Analysis of post-Laramide (Tertiary) uplift and erosion in the southern Alberta Plains indicates that the greatest erosion occurred in regions where the youngest stratigraphic units are preserved. This is consistent with geological structure and the few preserved stratigraphic constraints on erosional history. Throughout this region, erosion is pervasive and the uplift history is commonly constrained by coalification data. The analysis of coalification, erosion and paleogeothermics are interrelated and knowledge of them is important to understand hydrocarbon generation and tectonic history. Several independent attempts to analyse coalification, erosion and paleogeothermics in this area have given different results. The most striking differences are those between estimates derived from the analysis of well profile coalification gradient and those determined using a commonly accepted relationship between the amount of eroded section and the equilibrium moisture content of near-surface coals. The match between present and paleogeothermal gradient fields is better when the latter technique is used. Correlation is expected between the late orogenic and present geothermal gradient fields because the hydrodynamically controlled advective heat flow pattern that now dominates geothermal gradients in the Mesozoic and Tertiary Foreland succession was initiated by the growth of topography in the west throughout Cordilleran orogeny. Well profile coalification gradients have significant, though commonly unacknowledged, uncertainties. These uncertainties are propagated in erosion and paleogeothermal gradient calculations. Ideally the relationship between erosion estimates using well profile coalification gradients and near surface coal properties can be compared. Unfortunately, the uncertainties in well profile coalification gradients prevent this.