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FracPaQ: A MATLAB™ toolbox for the quantification of fracture patterns
Abstract
The patterns of fractures in deformed rocks are rarely uniform or random. Fracture orientations, sizes, and spatial distributions often exhibit some kind of order. In detail, relationships may exist among the different fracture attributes, e.g. small fractures dominated by one orientation, larger fractures by another. These relationships are important because the mechanical (e.g. strength, anisotropy) and transport (e.g. fluids, heat) properties of rock depend on these fracture attributes and patterns. This paper describes FracPaQ, a new open source, cross-platform toolbox to quantify fracture patterns, including distributions in fracture attributes and their spatial variation.
... These are predominantly fracture frequency (P10), fracture intensity (P21) and fracture density (P32) (Dershowitz and Herda, 1992;Mauldon, 1994Mauldon, , 1998. Several methods have also been developed to define fracture properties in 2D (Hardebol and Bertotti, 2013;Zeeb et al., 2013;Healy et al., 2017). ...
... In contrast to 2D methods (e.g. Healy et al., 2017), our 3D approach efficiently visualizes fractures along their full visible extents and results in a robust quantifi- Fig. 3. General workflow that integrates 3D DOM analysis for the study of fracture networks. The workflow focuses on three objectives: (i) to describe the main fracture sets observed in the area, (ii) to quantify the fracture intensity on the outcrop, and (iii) to describe the lateral and vertical variability of fracture intensity using a scan radius of 1 and 0.2 m respectively. ...
... Tradition 2D fracture mapping methods considered fracture traces lying on a 2D surface, consequentially fracture traces are not related with outcrop geometry. Such simplification on outcrop geometry can be acceptable in case of sub-horizontal and flat geometry, however it can introduce relevant errors in case of outcrops with complex geometry (Healy et al., 2017). For example, fractures parallel to the 2D sampling surface are less visible and are consequentially underestimated (Sturzenegger et al., 2011). ...
The presence of hydraulically conductive fracture networks fundamentally affects subsurface fluid flow, the recovery factor and productivity in hydrocarbon carbonate reservoirs. However, methods to directly detect and map the 3D distribution and intensity of fracture networks in the subsurface is difficult. We present a new work-flow and methodologies to overcome these limitations utilizing 3D digital photogrammetry on a prominent out-crop of the Late Jurassic (Kimmeridgian) Jubaila Formation near Wadi Laban (Riyadh, Saudi Arabia) as an example. Subsurface equivalents of the Upper Jubaila Formation host major oil and gas accumulations on the Arabian platform. The workflow includes mapping of fracture intensity, distribution and orientation along a 750 m long exposure transect using a high-resolution 3D Digital Outcrop Model (DOM). Results show two main fracture trends, which were E-W and NNE-SSW oriented. The techniques enable computation of fracture intensity directly on the 3D DOM, and quantification of lateral and vertical fracture intensity variability and fracture corridors. The new methodology of 3D DOM analysis, provides a reliable characterization of fractures and fracture intensity that are critical for enhancing 3D reservoir models.
... Two-dimensional (2D) discrete fracture models were generated from the Niah's regional lineaments and limestone outcrops in the northern area of the Subis build-up. The traced DFNs were then utilised to quantitatively model the fracture distribution using FracPaQ, an open-source MATLAB toolbox [62]. ...
... The work contained in this paper is part of a PhD research project supported by the Centre for Subsurface Imaging (CSI), Universiti Teknologi PETRONAS, which provided research facilities and supported during fieldwork in Niah, Sarawak on February, 2023, with a research fund under cost centres of 015LC0-466 (YUTP-FRG) and 015PBC-021 (YUTP-PRG). The authors [62] also acknowledge the open-source tool FracPaQ version 2.8 for its use in fracture topology analysis. ...
Understanding complex carbonate fracture networks and karstification at various geological scales is challenging, especially with limited multi-scale datasets. This paper aims to reduce uncertainty in the fracture architecture of Central Luconia karstified reservoirs by narrowing observational gaps between seismic and well data by using the discrete fracture models of exposed limestone outcrops as analogues for the subsurface carbonate reservoir. An outcrop-based fracture network characterisation of a near-surface paleo-karst at Subis Limestone combined with lineament analysis was conducted to extract fracture parameters. The karst structure was first delineated using a digital elevation map and outcrop examination. Then, topology analysis was performed, following the creation of two-dimensional discrete fracture models. Two main fracture sets oriented northeast–southwest and northwest–southeast and 79 potential dolines were identified. Fracture intersections, northeast–southwest major orientations, and drainage systems highly influenced the karst features. The Subis Limestone fracture model revealed that the highest number of fractures and total length of fractures were concentrated in the northern part of the Subis Limestone build-up (X: 250–350, Y: 150–250) and became denser towards the northwest direction of the outcrop (X: 600–800). The fractures in the Subis paleo-karsts appear isolated, with I-nodes ranging from 0.74 to 0.94. Hence, it is crucial to incorporate matrix porosity into multiple scales of fracture network modelling to improve upscaling and the modelling of fracture–vug networks, as well as to minimise the underestimation of discrete fracture networks in fractured and karstified limestone.
... Pavement locations were based on quality of exposure along the fold and spatial variability from the main structure. The imaging process involved mounting a camera onto an extendable stick to capture multiple images which are combined using the photogrammetric principle of triangulation to create a set of orthorectified images from which 2D fracture analysis can be undertaken using FracPaq (Healy et al., 2017). This analysis included determining fracture intensity, an important component to geometrical predictions and one which is difficult to measure from subsurface data (i.e., wells only provide 1D information, P10, number of fractures per unit length of borehole; Dershowitz & Herda, 1992). ...
... This analysis included determining fracture intensity, an important component to geometrical predictions and one which is difficult to measure from subsurface data (i.e., wells only provide 1D information, P10, number of fractures per unit length of borehole; Dershowitz & Herda, 1992). Correlating these measurements to a P32 intensity value (area of fracture per unit volume) can be problematic across dimensions and thus the use of surface pavement analogues to calculate P21 (length of fracture per unit area) provides an intermediate fracture dimensioning allowing for a more accurate estimate of P32 intensity (Bisdom et al., 2014;Healy et al., 2017). Fracture connectivity is an additional important controlling parameter to fluid flow that can be determined from pavement analysis using topological relationships. ...
Fracture networks play a critical role in fluid flow within reservoirs, and it is therefore important to understand the interactions and influences these networks have. Our study focusses on the Southern Chotts-Jeffara basin which hosts reservoirs within the Triassic, Permian and Ordovician units containing significant hydrocarbon accumulations. Recent developments on the structural understanding of the basin have proven a regional shortening phase occurring between the Permian and Jurassic forming open folds and a distributed fracture network. Analysis of late Palaeozoic and Mesozoic outcrops within the basin identify several sets of fractures (150/80; 212/86) and compressional structural features which support this shortening hypothesis. We integrate fracture data from surface analogues and subsurface analysis of advanced seismic attributes and well data through structural linking to form a 2D hybrid fracture model of the reservoirs in the region. Through analytical aperture modelling and numerical simulation, we find that the fractures orientated 212° in combination with large-scale fractures contribute significantly to the fluid flow orientation and potential reservoir permeability. Our presented fracture workflow and framework provides an insight in network characterisation within naturally fractured reservoirs of Tunisia and how certain structures form fluid pathways influence flow and production.
Supplementary material: https://doi.org/10.6084/m9.figshare.c.6904499
... (Leapfrog Geo, 2022) and used it to produce meshes for individual vein sets, respecting vein lengths and orientations. 3. We then extracted a slice through these meshes perpendicular to the foliation and parallel to the stretching lineation to capture a 2D snapshot of the vein distribution in this plane (Figure 2a). 4. The resulting 2D vein array was then processed using FracPaq software (see Healy et al., 2017;Suzuki et al., 1998 for details on permeability tensor calculations), which takes as inputs a vectorized version of the vein array and a value for the average opening increment of the veins (W f ) and outputs estimates of permeability as a function of orientation in that plane. ...
Slow slip and tremor is observed along many subduction margins and is commonly linked to fluid pressure variations and migration. Accurate estimates of porosity and permeability around subduction megathrust shear zones are vital for understanding fluid-seismicity interactions. We use high-resolution digital outcrop data and (micro)structural analysis to assess transient permeability and porosity of a deep-seated subduction interface exposed on Syros Island, Greece. We document the orientations, relative timing, and opening aperture (based on crack-seal textures) of veins that were emplaced synkinematically with ductile deformation during early exhumation within the subduction channel. Our findings indicate high permeability
through vein-filled fractures amidst a lower permeability matrix, with transient, fracture-controlled permeabilities ranging from 10 −14 to 10 −15 m 2 and fracture porosities of 1%–10%. These estimates align with low-end values from seismological/geodetic observations in active subduction zones, and are also consistent with fault-valve-like numerical models that suggest high background-to-transient permeability contrasts favor unstable slip.
... Although identifying discontinuities from the images was somewhat subjective, the same person did all these analyses, and so they are likely to be internally consistent. We used FraqPaQ (Healy et al., 2017), a MATLAB software suite, to determine the discontinuity in- tensity, which is the length of all traced discontinuities divided by the area examined in each reach. The discontinuity intensity is reported in units of meters. ...
We explore how rock properties and channel morphology vary with rock type in Last Chance Canyon, Guadalupe Mountains, New Mexico, USA. The rocks here are composed of horizontally to near-horizontally interbedded carbonate and sandstone. This study focuses on first- and second-order channel sections, where the streams have a lower channel steepness index (ksn) upstream and transition to higher ksn values downstream. We hypothesize that differences in bed thickness and rock strength influence ksn values, both locally by influencing bulk bedrock strength and also nonlocally through the production of coarse sediment. We collected discontinuity intensity data (the length of bedding planes and fractures per unit area), Schmidt hammer rebound measurements, and measured the largest boulder at every 12.2 m elevation contour to test this hypothesis. Bedrock and boulder mineralogy were determined using a lab-based carbonate dissolution method. High-resolution orthomosaics and digital surface models (DSMs) were generated from drone and ground-based photogrammetry. The orthomosaics were used to map channel sections with exposed bedrock. The United States Geological Survey (USGS) 10 m digital elevation models (DEMs) were used to measure channel slope and hillslope relief. We find that discontinuity intensity is negatively correlated with Schmidt hammer rebound values in sandstone bedrock. Channel steepness tends to be higher where reaches are primarily incising through more thickly bedded carbonate bedrock and lower where more thinly bedded sandstone is exposed. Bedrock properties also influence channel morphology indirectly, through coarse sediment input from adjacent hillslopes. Thickly bedded rock layers on hillslopes erode to contribute larger colluvial sediment to adjacent channels, and these reaches have higher ksn values. Larger and more competent carbonate sediment armors both the carbonate and the more erodible sandstone and reduces steepness contrasts across rock types. We interpret that in the relatively steep, high-level ksn downstream channel sections, the slope is primarily controlled by the coarse alluvial cover. We further posit that the upstream low-level ksn reaches have a base level that is fixed by the steep downstream reaches, resulting in a stable configuration, where channel slopes have adjusted to lithologic differences and/or sediment armor.
... The manual digitization of fracture traces in 2D (satellite and ground-based images) and 3D (via digital outcrop) was performed in Move 2016.1 software (formerly Midland Valley; now Petroleum Experts). The orientations of digitized fracture traces were extracted using FracPaQ 2.3 (Healy et al., 2017) and Move 2016.1 software. The 2D fracture intensity was calculated from digitized fracture traces by calculating total fracture length per unit area in 2D (m m −2 ). ...
Subsurface datasets typically lack the resolution or coverage to adequately sample fracture networks in 3D, and fracture properties are typically extrapolated from available data (e.g. seismic data or wellbore image logs). Here we assess the applicability of extrapolating fracture properties (orientation, length, and intensity) across observation scales in deformed, mechanically layered carbonate rocks. Data derived from high-resolution field images, medium-resolution digital outcrop data, and relatively low-resolution satellite imagery at Swift Reservoir anticline, NW Montana are leveraged to (i) assess interacting structural and stratigraphic controls on fracture development, and (ii) compare estimated fracture properties derived from multiple observation scales. We show that hinge-parallel and hinge-perpendicular fractures (i) make up the majority of fractures at the site; (ii) are consistently oriented with respect to the fold hinge, despite along-strike variability in the fold hinge orientation; and (iii) exhibit systematic increases in intensity towards the anticline hinge. These fractures are interpreted as having formed during folding. Other fractures recorded at the site exhibit inconsistent orientations, show no systematic trends in fracture intensity, and are interpreted as being unrelated to fold formation. Fracture orientation data exhibit the greatest agreement across observation scales at hinge and forelimb positions, where hinge-parallel and hinge-perpendicular fracture sets are well developed, and little agreement on the anticline backlimb, where fracture orientations are less predictable and more dispersed. This indicates that the scaling of fracture properties at Swift Reservoir anticline is spatially variable and partly dependent on structural position. Our results suggest that accurate prediction and extrapolation of natural fracture properties in contractional settings requires the assessment of structural position, lithologic variability, and spatially variable fracture scaling relationships, as well as consideration of the deformation history before and after folding.
... Rock outcrops are extremely reliable data sources on fracture systems because they contain continuous 2D to 3D data on their geometry and distribution, depending on some controlling factors such as the thickness of layers and the geological structure in which the observed outcrop is located [32,40]. However, three-dimensional porosity data are rarely available, and their acquisition is often very expensive [41]. Two-dimensional (outcrops, quarries, orthophotos, geophysical data, microscopic preparations, etc.) and one-dimensional data (borehole data, "scanlines", etc.) are much more common. ...
Fractured aquifers, especially dolomites, are important hydrocarbon reservoirs and sources of thermal and groundwater in many parts of the world, especially in the Alpine-Dinaric-Carpathian region. The most dominant porosity type is fracture porosity, which acts as the preferential fluid pathway in the subsurface, thus strongly controlling fluid flow. Outcrops provide valuable information for the characterization of fracture networks. Dolomite rock properties and structural and diagenetic processes result in fractured systems that can be considered fractals. The fracture network was analyzed on 14 vertical outcrops in 35 digitized photographs. The values of the fractal dimensions varied slightly by the software and method used, but the trends were consistent, which confirms that all methods are valid. Small values of fractal dimension indicate the dominance of a few small or large fractures, and high values of fractal dimension result from a combination of large numbers of small fractures accompanied by a few large fractures. The mean value of the fractal dimension for analyzed fracture networks was 1.648. The results indicate that the fracture network of the Upper Triassic dolomites can be approximated by fractal distribution and can be considered a natural fractal, and values can be extrapolated to higher and lower scales (1D and 3D).
Recent trends and techniques in the study of brittle deformation: Implications in upper crustal fluid flow
The high-angle faults bounding the High Agri Valley Basin eastwards are well-exposed at the Viggiano Mt., southern Italy. There, these faults crosscut Mesozoic shallow-water carbonates and permit the multiscale analyses of fault and fracture geometry and distribution. Aiming at assessing the control exerted by carbonate lithofacies and scales of observation, we combine field and digital structural analyses to compute the values of 1D and 2D fracture density and intensity, and the dimensional properties of single fault and fracture sets. As a result, from outcrop to reservoir scales we document a scale-variant geometry of faults and fractures, and the great variation of both fracture density and intensity among the studied carbonate lithofacies. Furthermore, we compute a P21 dimensional scaling factor of ca. 20 for over 3 order magnitude, whereas inconsistent results are achieved for the P20 values. Not considering the control exerted by the single carbonate lithofacies, at a reservoir scale we document that the small-scale faults greatly impact the computed P21 values.
This research focuses on how “static” properties of fracture networks can be studied by considering “dynamic” flow simulation. While static properties like, clustering, connectivity, variation in aperture and of course, anisotropy of fracture networks can be quantified using different geostatistical/data analysis techniques. The flow responses through such networks can be simulated, to check if flow simulation can be used as a tool for evaluating its geometry. In order to achieve this, outcrop analogs of fractured reservoirs are converted into permeability structured grids implementing the fracture continuum (FC) concept. These FC models are flow simulated in a streamline simulator, TRACE3D. Results of the first experiment show that rather than the “fractal dimension”, the “lacunarity parameter” which quantifies scale-dependent clustering of fractures is a unique identifier of network geometry and acts as a proxy for fracture connectivity and an indicator of flow behavior. The FC model further accommodates variability in fracture apertures and thus, in a second experiment, a set of models with hierarchical aperture distribution were built and were tested for their TOFs and recovery curves which show that smaller fractures with narrow apertures do not significantly contribute to flow. In a third experiment considering anisotropy, it is observed that tightly clustered fractures along preferential directions can be identified from anisotropy in flow patterns. The results from these three experiments show that flow patterns in fracture networks can indicate the overall scale-dependent clustering, the anisotropy that arises from such clustering and that narrower fractures do not significantly alter overall flow behavior.
Thematic collection: This article is part of the Digitally enabled geoscience workflows: unlocking the power of our data collection available at: https://www.lyellcollection.org/topic/collections/digitally-enabled-geoscience-workflows
The success of any predictive model is largely dependent on the accuracy with which its parameters are known. When characterising fracture networks in rocks, one of the main issues is accurately scaling the parameters governing the dis- tribution of fracture attributes. Optimal characterisation and analysis of fracture lengths and apertures are fundamental to estimate bulk permeability and therefore fluid flow, especially for rocks with low primary porosity where most of the flow takes place within fractures. We collected outcrop data from a fractured upper Miocene biosiliceous mudstone for- mation (California, USA), which exhibits seepage of bitumen-rich fluids through the fractures. The dataset was analysed using Maximum Likelihood Estimators to extract the underlying scaling parameters, and we found a log-normal distribu- tion to be the best representative statistic for both fracture lengths and apertures in the study area. By applying Maximum Likelihood Estimators on outcrop fracture data, we generate fracture network models with the same statistical attributes to the ones observed on outcrop, from which we can achieve more robust predictions of bulk permeability.
In fold-and-thrust belts rocks undergo deformation as fold geometries evolve. Deformation may be accommodated by brittle fracturing, which can vary depending on structural position. We use 2D forward modelling and 3D restorations to determine strain distributions throughout folds of the Achnashellach Culmination, Moine Thrust Belt, NW Scotland. Fracture data is taken from the Torridon Group; a thick, coarse grained fluviatile sandstone deposited during the Proterozoic. Modelling infers a correlation between strain and simple curvature; we use simple curvature to infer how structural position and strain control fracture attribute variations in a fold and thrust belt.
Characterising fractures at outcrop for use as analogues to fractured reservoirs can use several methods. Four important fracture data collection methods are linear scanline sampling, areal sampling, window sampling and circular scanline sampling. In regions of homogeneous fracture networks these methods are adequate to characterise fracture patterns for use as outcrop analogues, however where fractures are heterogeneous, it is more difficult to characterise fracture networks and a different approach is needed.
We develop a workflow for fracture data collection in a region of heterogeneous fractures in a fold and thrust belt, which we believe has applicability to a wide variety of fracture networks in different tectonic settings. We use an augmented circular scanline method, along with areal sampling to collect a range of fracture attribute data, including orientation, length, aperture, spatial distribution and intensity. This augmented circular scanline method more than halves the time taken for data collection, provides accurate, unbiased data that is representative of local fracture network attributes and involves data collection of a wider range of fracture attributes than other sampling techniques alone.
Studies of fault rock permeabilities advance the understanding of fluid migration patterns around faults and contribute to predictions of fault stability. In this study a new model is proposed combining brittle deformation structures formed during faulting, with fluid flow through pores. It assesses the impact of faulting on the permeability anisotropy of porous sandstone, hypothesising that the formation of fault related micro-scale deformation structures will alter the host rock porosity organisation and create new permeability pathways. Core plugs and thin sections were sampled around a normal fault and oriented with respect to the fault plane. Anisotropy of permeability was determined in three orientations to the fault plane at ambient and confining pressures. Results show that permeabilities measured parallel to fault dip were up to 10 times higher than along fault strike permeability. Analysis of corresponding thin sections shows elongate pores oriented at a low angle to the maximum principal palaeo-stress (sigma(1)) and parallel to fault dip, indicating that permeability anisotropy is produced by grain scale deformation mechanisms associated with faulting. Using a soil mechanics 'void cell model' this study shows how elongate pores could be produced in faulted porous sandstone by compaction and reorganisation of grains through shearing and cataclasis. (c) 2014 The Authors. Published by Elsevier Ltd.
Current hydrogeological research in NE Brazil aims to better understand factors controlling storage and percolation of ground water in crystalline terrains, due to the fact that a large proportion (about 60%) of this region is formed by igneous and metamorphic rocks. As part of this effort, we present an integrated geophysical geological study of a fractured bedrock aquifer located in the Caiçara farm, near Equador city, Rio Grande do Norte State, NE Brazil. Pumped wells in this site present very different yields, in spite of short distance of only 20 m. The main lithologies of the site are quartzite and micaschist. Combined interpretation of geophysical images (Resistivity and Ground Penetrating Radar---GPR) and field structural geologic data revealed that the fractured bedrock aquifer is composed of three sets of discontinuities: SW-dipping subhorizontal joints, parallel to the ductile fabric of the country rocks which are probably release joints; NE-dipping subhorizontal fractures, oblique to ductile trend; and subvertical fractures which are orthogonally distributed along N S and E W directions. Important subvertical fracture zones occur at a regular spacing of about tens of meters. Fractures in the E W direction are relatively open, as compared to fractures in the N S direction. Probably, E W fractures were opened by the current neotectonic stress regime in NE Brazil, which is controlled by E W compression and N S extension. It was possible to rank the hydrogeologic potentiality of water well siting as a function of fracture density and proximity to recharge zones, in this way explaining why wells could present very different yield results on the site. The most favorable places for well siting are intersections of subvertical fracture zones located near drainage because highly fractured zones could be, in this way, connected with recharge zones mainly in alluvial deposits. On the other hand, the least favorable places for well siting lie outside subvertical fractures and far from recharge zones. Intermediate hydrogeological potentialities occur in two cases: at a point near the recharge zone but where there is a low density of fractures, or at point far from the recharge zone but where there is a high density of fractures.
The recent economic collapse of the opencast surface coal mining industry in the Midland Valley of Scotland has left behind a number of abandoned voids that reveal stunning 3D exposures in strategically important strata. A strong case can be made for retaining these large-scale, often continuous and very detailed geological sections that might otherwise have been lost to us, along with their relevant digital geological and geotechnical datasets and other social and industrial records. In particular, Spireslack and Mainshill Wood opencast workings might be seen by some as industrial scars on the Scottish landscape, but they can instead both be turned into national assets for multidisciplinary Geoscience research and learning in general, supporting high-quality international research into Carboniferous geology. Such opencast sites can also provide a rich visitor and/or learning experience for the wider public in coal geology, illuminating aspects of a former way of life of previous generations.
Fracture characterization protocols that reduce sampling bias are likely to yield higher quality input for exploration and development decisions when dealing with naturally fractured reservoirs. A new set of estimators for fracture density, intensity, and mean trace length corrects for sampling biases and provides a useful integrated description for bulk aspects of a fracture network. These estimators are based on counts of intersections between fracture traces and circular scan lines and of trace terminations in circular windows. Application to synthetic fracture patterns with known parameters validates the use of the new estimators, which are then applied to natural fault trace maps from seismic volumes and joint trace maps from rock pavements. The new estimators are distribution independent and eliminate the effects of orientation, censoring, and length biases, which limit the effectiveness of other sampling techniques. Estimator accuracy improves as sample size increases, particularly for larger circles that exceed a fracture-defined block size. Estimator accuracy for mean trace length improves when the sample exceeds threshold count values for fracture terminations based on guidance from the analysis of similar synthetic patterns. These new estimators also provide both inputs and independent checks of predictions for fracture-generator programs used to model fracture populations in a rock volume.
In two-dimensions, a fracture network consist of a system of branches and nodes that can be used to define both geometrical features, such as length and orientation, and the relationship between elements of the network – topology. Branch lengths are preferred to trace lengths as they can be uniquely defined, have less censoring and are more clustered around a mean value. Many important properties of networks are more related to topology than geometry.
The proportions of isolated (I), abutting (Y) and crossing (X) nodes provide a basis for describing the topology that can be easily applied, even with limited access to the network as a whole. Node counting also provides an unbiased estimate of frequency and can be used in conjunction with fracture intensity to estimate the characteristic length and dimensionless intensity of the fractures. The nodes can be used to classify branches into three types – those with two I-nodes, one I-node and no I-nodes (or two connected nodes). The average number of connections per branch provides a measure of connectivity that is almost completely independent of the topology. We briefly discuss the extension of topological concepts to 3-dimensions.