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Evaluation of Optimum Mud Weight Window for Prevention of Wellbore Instability in Niger Delta Wells

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A Borehole instability problem contributes significantly to increase in non-productive time (NPT) and overall cost of the drilling. These problems can occur in a variety of forms including stuck pipe, loss circulation, hole enlargement, breakout. Borehole problems such as drill pipe sticking, hole collapse, breakout, ,caving and tight holes have been experienced during drilling of wells in an oil field in the Niger Delta. This work analyzed the major cause of wellbore instability in a field in the Niger Delta by using data from two wells drilled and proposes an optimum mud window for cost-effective and safe drilling operations. Geomechanical model was used to evaluate the in-situ stress and induced stresses with Mogi Coulomb and Mohr Coulomb failure criteria to predict the breakout profile and estimate the optimum mud weight to avoid sticking of the drill string. The result shows that the Mogi failure criterion was 1.46sg and 1.39sg while Mohr failure criteria were 1.48sg and 1.42sg for well 1 and well 2 respectively. Based on the results obtained, Mogi coloumb is preferable to Mohr coloumb criteria. Mogi failure criteria give more accurate result to obtain optimum mud weight window for the field considered in the Niger Delta. Therefore, in the prediction of an optimum mud weight window for any well in the Niger Delta, mohr failure criteria should be employed and adopted.
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IOSR Journal of Engineering (IOSRJEN) www.iosrjen.org
ISSN (e): 2250-3021, ISSN (p): 2278-8719
Vol. 10, Issue 10, October 2020, ||Series -I|| PP 61-66
International organization of Scientific Research 61 | Page
Evaluation of Optimum Mud Weight Window for Prevention of
Wellbore Instability in Niger Delta Wells
Nmegbu Godwin Chukwuma, Bright Bariakpoa Kinate and Samuel Nmezi
Frances
Department of Petroleum Engineering, Rivers State University, Nigeria
Received 10 October 2020; Accepted 26 October 2020
Abstract: A Borehole instability problem contributes significantly to increase in non-productive time (NPT)
and overall cost of the drilling. These problems can occur in a variety of forms including stuck pipe, loss
circulation, hole enlargement, breakout. Borehole problems such as drill pipe sticking, hole collapse, breakout,
,caving and tight holes have been experienced during drilling of wells in an oil field in the Niger Delta. This
work analyzed the major cause of wellbore instability in a field in the Niger Delta by using data from two wells
drilled and proposes an optimum mud window for cost-effective and safe drilling operations. Geomechanical
model was used to evaluate the in-situ stress and induced stresses with Mogi Coulomb and Mohr Coulomb
failure criteria to predict the breakout profile and estimate the optimum mud weight to avoid sticking of the drill
string. The result shows that the Mogi failure criterion was 1.46sg and 1.39sg while Mohr failure criteria were
1.48sg and 1.42sg for well 1 and well 2 respectively. Based on the results obtained, Mogi coloumb is preferable
to Mohr coloumb criteria. Mogi failure criteria give more accurate result to obtain optimum mud weight window
for the field considered in the Niger Delta. Therefore, in the prediction of an optimum mud weight window for
any well in the Niger Delta, mohr failure criteria should be employed and adopted.
Keywords: wellbore instability, Mohr Criterion, Mogi Criterion, Geochemical Property, mud weight window
I. INTRODUCTION
Wellbore instability problems are very common in the drilling of oil and gas wells. During drilling
process, the wellbore may collapse or cave-in. Formation rock may fracture resulting to loss of substantial
volume of drilling fluid. This can lead to a number of severe problems during drilling operation like; stuck drill
string and consequent fishing, sidetracking and reaming operations, reduction in fluid hydrostatic pressure due
to decrease in mud column height, drilling string twisting or parting due to excessive torque and drag, or even a
complete loss of wellbore. Generally, the issue of wellbore instability and other similar challenges significantly
contribute to the already high cost of well construction. Wellbore instability is more common and severe in
shale rock formations. Wellbore instability issues in wells drilling results to lose of budget due to unexpected
and unplanned events caused, which translates to huge amount of money wasted during the well delivery
process.
In this research project, wellbore instability issues in an oilfield in the Niger Delta are investigated.
The oilfield, which is designated “S” Field in this research work, contains oil and gas reservoirs and is located in
the Niger Delta sedimentary basin, Nigeria. The field was first explored in 1993 but has not been producing due
to the fact that no well has been successfully drilled and put to production in the field. The non-production has
resulted to loss of the huge revenue that would have been accruing to the economy, as well as loss of jobs. For
almost 25 years no drilling campaign has been carried out since the two wells drilled in the field experienced
instability issues during drilling, including stuck pipe and loss circulation incidents. These problems led to the
plugging and abandonment of the wells and field after costing the company a huge fortune.
Wellbore instability is a function of formation pore pressure, in-situ stresses, and rock strength properties.
Before a well is drilled, the formation is in equilibrium. Once drilling starts, formation rock is removed. The
wellbore becomes subjected to the stresses surrounding it due to removal of the rock. This changes the in-situ
stresses near the wellbore wall, resulting to stress concentration. This stress concentration will lead to a failure
in the wellbore wall. The basic problem therefore, is the accurate prediction of the rock reaction to mechanical
loading during drilling of a borehole.
In order to avoid wellbore failure an appropriate wellbore pressure (mud pressure) need to be altered to
redistribute the stress concentration near the wellbore. Moreover, wellbore orientation with respect to the in-situ
stresses needs to be taken into consideration to prevent the wellbore failure. The easily controllable parameter in
any drilling operation is the drilling mud pressure. The drilling mud pressure can prevent wellbore failure if it
lies between the collapse pressure and formation fracture gradients. It has the added advantage of controlling or
reducing the effect of mechanical wellbore failure (Bourgoyne et al., 1986). Drilling mud performs many
Evaluation of Optimum Mud Weight Window for Prevention of Wellbore Instability in ..
International organization of Scientific Research 62 | Page
functions, such as cooling and lubrication of the drill bit and drill string, cleaning of the drill bit, transport of
drill cuttings to the surface, transmission of hydraulic energy to mud motors and bit through the drill string, and
control of formation pressure. Traditionally, the drilling mud pressure is designed to control the influx of flow of
the formation fluid into the wellbore regardless of the field stresses and the rock strength effects. Normally, in
practice the mud pressure is designed in excess of the formation fluid pressure by 100 to 200 psi (0.3 to 0.5
lb/gal), the exact value depending on the well operator (French & McLean, 1992; Awal et al., 2001).This creates
a positive overbalance between the mud pressure and formation pore pressure which helps to prevent the flow of
the formation fluid into the wellbore. Thus, due to the in-situ stresses, the mud pressure required to sustain the
wellbore should be greater than the pressure required to balance. Therefore, better approaches are sought to
obtain optimum mud pressure based on the accurate evaluation of rock properties, stresses around the wellbore,
and wellbore trajectory to safe and cost-effective delivery of a well.
II. METHODOLOGY
Data used for this investigation were obtained from daily mud reports for determining daily losses and
cutting size, mud logging report for formation lithology, final well report for productive and NPT, well logging
data for borehole breakout zone and stress orientation from two wells drilled in the oil field.
2.1 Geomechanical models for “S” field
From theoretical to experimental aspects, a comprehensive model on the mechanical effects on
wellbore stability in the “S” field was considered in this study. The process of building a geomechanical model
implies the prediction of the elastic and mechanical properties of the formation rock from physical equations
and correlations (Zoback, 2007; Aadnoy,& Looyeh 2019). Then, the magnitudes of three principle stresses
(vertical stress, minimum horizontal stress, and maximum horizontal stress) and pore pressure are calculated.
Hence, pore pressure, rock mechanical properties, and in-situ stresses are considered among the main factors for
building a geomechanical model. In this study, the two wells so far drilled, located 9 km apart are examined.
2.2 Mohr Coulomb Failure Criteria.
The Mohr-Coulomb model was considered in the analysis of the failure criteria and is presented below
(1)
Where is the shear stress, c is the rock cohesion is the normal stress, and φ is the internal friction angle.
The coefficient of internal friction angle can be formulated as:
(2)
Mohr failure criteria can also be expressed by the maximum and minimum principle stresses, as follows:
(3)
Where are the maximum and minimum principle stresses, respectively. is the unconfined
compressive strength, which is a function of cohesion and internal friction angle and q is the flow factor, which
is related to internal friction angle and can be obtained by:
(4)
(5)
2.3 Mogi-Coulomb criteria
For Proper analysis and comparison, Mogi Coulomb model was also considered and is presented below
(6)
(7)
Mogi also suggested new criteria, defined as:
(8)
Where the octahedral shear stress is expressed as:
(9)
Evaluation of Optimum Mud Weight Window for Prevention of Wellbore Instability in ..
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After this observation, it became obvious the importance of . Therefore, many 3D failure criteria have been
developed. After performing extensive reviews of rock failure models, Al-Ajmi and Zimmerman (2005)
introduced a 3D failure criterion called the Mogi-Coulomb criterion. This criterion can be formulated as a linear
relation in a similar format to the Mohr-Coulomb criterion as follows:
(10)
Where a and b are material constant and are related to c and φ as follow:
(11)
(12)
III. RESULTS
The minimum mud density is estimated by using a constitutive geomechanical model connected with
two failure criteria (Mohr and Mogi). The input parameters for the geomechanical model are summarized in
Table 1. The recommended mud weight express as specific gravity by using Mohr-Coulomb and Mogi-
Coulomb are listed in Table .2.
Table 1: Input for the geomechanical model (in-situ stresses, pore pressure, and mechanical properties), Case
Study 1.
Young’s
modulus
25.2
Gpa
UCS
22
Mpa
Lal (1999)
Friction angle
22.6
Plumb (1994)
Table 2. The Output of the Geomechanical Models, Case Study 1.
Used
Mohr-
Coulomb
Mogi-
Coulomb
1.44
1.48
1.46
Well-2 is the second well in the “S” field. Its objective was to test the sands penetrated in Well-1,
particularly the zone at which Well-1 failed due to instability. It was also designed to appraise all reservoir sands
indicated in Well-1. The well is located 9.3 kilometers away from Well-1. Instability problems such as caving
and tight spots were observed during drilling the well-2 formation. The lithology description in the Well-2
shows the same description as Well-1. Case 1 procedure was followed to construct the stress profile, predict the
rock strength properties, and optimum mud density for Well-2. Figure 1 shows the in-situ stress and pore
pressure. Rock strength properties are displayed in Figure 2. The input (in-situ stresses, rock strength properties,
and pore pressure) and output parameters are listed in Table .3 and Table 4., respectively.
Parameters
Value
Source
Depth
2975
S
v-
58.21
SH
56.2
Peng (2007)
S
h
47.5
Holbrook et al.
(1993)
P
p
33
RFT
Poisson’s ratio
0.21
Evaluation of Optimum Mud Weight Window for Prevention of Wellbore Instability in ..
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Figure 1. In-situ stresses and pore pressure profile through “S” field Case Study Well-1.
Figure 2.Rock strength parameters, Case Study Well-2
Table3. The Input of the Geomechanical Model, Case Study 2.
Parameters
Value
Unit
Source
Depth
2705
m
Sv
65.72
Mpa
SH
59
Mpa
Peng (2007)
S
h
44
Mpa
Holbrook et al. (1993)
P
p
31.7
Mpa
RFT
Poisson’s ratio
0.26
Evaluation of Optimum Mud Weight Window for Prevention of Wellbore Instability in ..
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Static Young’s modulus
17.8
Gpa
UCS
29
Mpa
Lal (1999)
Friction angle
22.3
Plumb (1994)
Table 4. The Output of the Geomechanical Model, Case Study 2.
Used
Mohr-Coulomb
Mogi-Coulomb
MW (SG)
1.35
1.42
1.39
Table 5: Input and Output of the Geomechanical Model for the Two Cases.
Parameters
Well-1
Well-2
Depth
2975
2705
Sv
58.61
65.72
S
H
56.2
59
Sh
47.5
44
SH Orientation
45°-50°
45°-50°
Pp
33
31.7
UCS
20
29
Friction angle
22.6
22.3
Poisson’s ratio
0.21
0.26
Young’s Modulus
25.2
17.8
Related problems
Breakout, pack-off and stuck pipe
Tight spots, breakout, pack-off and
stuck pipe
Mogi-Coulomb
1.46
1.39
Mohr-Coulomb
1.48
1.42
Two wells were drilled with mud weights of 1.44sg and 1.35sg, respectively; the results show that the
used mud weights are less than that required to sustain the borehole wall. The Mohr-Coulomb failure criterion
predicts the optimum mud weights to be 1.48sg for well-1 and 1.42sg for well-2, while Mogi-Coulomb predicts
the optimum mud weighs of 1.46sg for well-1 and 1.39sg for well-2. Vernik and Zoback (1992) pointed out that
Mohr-Coulomb failure criterion did not provide realistic results. Recently, Rahimi and Nygaard (2015) stated
that the Mohr-Coulomb prediction showed overestimated value, while the Mogi-Coulomb prediction was more
reliable. In addition, comparing the mud weights of 1.44sg and 1.35sg used in drilling the two wells and the
geomechanical model outputs show that Mogi-Coulomb gives reasonable predictions of 1.46sg for Well-1 and
1.39sg, for Well-2, which are in close agreement with field observation.
IV. CONCLUSION
This research presents a case study in the “S” oil field in Niger Delta. From the results of the geomechanical
analysis of wellbore instability in the two wells drilled in the field, the following conclusions were drawn;
A geomechanical model was developed using two failure criteria. The analysis of the output of the
geomechanical model shows that the Mogi-Coulomb criterion gives more appropriate results than the
Mohr-Coulomb criterion. Mogi-Coulomb analysis results are in close agreement with field observation.
This is because the Mohr-Coulomb criterion underestimates the rock strength by disregarding the effect of
intermediate principle stress. In contrast, the Mogi-Coulomb criterion gives a more realistic model by
considering the effect of intermediate stress on rock strength.
Several wellbore collapses, stuck pipe, and shale caving (wellbore failure) were observed in the “S” field.
This wellbore failure was due to the use insufficient mud weight without recourse to appropriate
geomechanical analysis.
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Engineering, 2.SPE, Richardson, TX.
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[2]. French, F. R., McLean, M. R. (1992).Development drilling problems in high-pressure reservoirs. In: Proc
SPE Int Meeting Petrol Eng, Beijing, SPE 22385.
[3]. Awal, M. R., Khan, M. S., Mohiuddin, M. A., & Abdulraheem, A. (2001). A new approach to borehole
trajectory optimisation for increased hole stability. In: Proc SPE Middle East Oil Show, Bahrain, 17-20
March. SPE 68092.
[4]. Zoback, M. D. (2007). Reservoir Geomechanics.doi:10.1017/cbo9780511586477
[5]. Aadnoy, B., &Looyeh, R. (2019).Drilling operations and well design: in Petroleum Rock Mechanics, 2nd
Edition, Gulf Professional Publishing
[6]. Al-Ajmi, A. M., & Zimmerman, R. W. (2005).Relation between the Mogi and the Coulomb failure
criteria. International Journal of Rock Mechanics and Mining Sciences, 42(3), 431-439.
doi:10.1016/j.ijrmms.2004.11.004.
[7]. Holbrook, P.W., & Maggiori, D. A. (1993). Real-time pore pressure and fracture gradient evaluation in
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[8]. Vernik, L., & Zoback, M. D. (1992). Estimation of maximum horizontal principal stress magnitude from
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analysis.Master's Degree Thesis.
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.
Nmegbu Godwin Chukwuma, et. al. "Evaluation of Optimum Mud Weight Window for
Prevention of Wellbore Instability in Niger Delta Wells." IOSR Journal of Engineering
(IOSRJEN), 10(10), 2020, pp. 61-66.
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Article
Full-text available
In this paper we utilize the mechanical properties of core samples, detailed observations of stress-induced well bore breakouts and estimates of the magnitude of S(hmin) from hydraulic fracturing experiments to construct a vertical profile of the maximum horizontal principal stress S(Hmax) to 3.5 km depth in the Cajon Pass borehole. As in essentially all other boreholes, the hydraulic fracturing stress measurements in the Cajon Pass borehole yielded appreciably more (and more reliable) data on the magnitude of the least principal horizontal stress, S(hmin), than the maximum principal horizontal stress, S(Hmax). To utilize the breakout observations to constrain the magnitude of S(Hmax), a brittle failure criterion was used that is based on the effective strain energy concept. This failure criterion also allows us to account for the polyaxial stress around the borehole and to incorporate the role of pore fluid pressure on rock strength in various ways. A comparison of the S(Hmax) profile estimated from the breakouts with estimates of S(Hmax) obtained from the hydraulic fracturing tests indicates that they compare fairly well for the case when the effect of pore pressure on hydraulic fracture initiation is negligible. The overall stress state to a depth of 3.5 km in the well corresponds to a normal/strike-slip faulting regime. The S(Hmax) profile indicates several marked decreases in stress magnitude associated with major fault zones, an observation consistent with the S(hmin) profile obtained from hydraulic fracturing tests.
Article
Borehole instability due to stuck pipe, hole pack-off, and lost hole during drilling has been a major problem, which causes the industry to lose millions of dollars. In the last two decades, various rock mechanics principles have been tried, and experience has been shared in the literature as to the efficacy of these principles to minimize the problem. The commercial software, rich in such analytical and numerical models, however, have not mitigated the problem. We have found that this failure is attributable to:lack of full utilization of the classical Kirsch equations that represent the state of stresses around a borehole;negligence/ignorance of the complete in-situ stress field and the Biot's constant; and improper calibration of the rock mechanics parameters; andlack of translating the findings of mathematical models in an appropriate form that can be readily used for field deployment. We have addressed these shortcomings by utilizing more relevant solutions to the Kirsch equations by incorporating poro-elastic behavior of rock. Further, we have utilized hitherto neglected aspects of the geomechanical stability equations, and presented graphical solutions that can be used by field engineers having a minimal knowledge of rock mechanics. One of the important findings of these computer generated, graphical solutions is that the optimized trajectory can be vertical, inclined, or horizontal. This depends on whether the region is tectonically quiet or active. It also depends on whether or not, the prevailing insitu stress regime is normal, overthrust/reverse, or strike-slip type. Thus, the principal insitu stresses, in both magnitude and directions, are a fundamental data set that is needed in designing the safe borehole trajectory. For the deviated hole, further optimization is obtained by determining the best azimuthal direction to reach the target zone. What is more interesting, the optimized trajectory requires a much less mud weight to drill a stable borehole, thereby reducing drilling cost significantly. We have applied our method in a case study in an offshore field, where frequent borehole collapse has been reported in drilling through shale/sand stringers. The successful application of the proposed method would not only save millions by preventing borehole instabilities, but also cut drilling cost significantly by replacing the expensive and environmentally unsafe oil-based mud. Introduction Review of published literature in the last two decades (McLean & Addis, 1990; McLellan & Hawkes, 1998; Svennekjaer & Bratli, 1998; Guo et al., 2000; Onaisi, et al., 2000) shows that borehole instability due to stuck pipe, hole pack-off, and lost hole during drilling are still common, although various rock mechanics principles have been tried, and experience has been shared in order to minimize these major problems.
Book
This book discusses petroleum engineering. Engineering science fundamentals and engineering applications involving these fundamentals are presented. Subjects covered include rotary drilling, drilling fluids, cements, drilling hydraulics, rotary drilling bits, formation pore pressure and fracture resistance, casing design, directional drilling and deviation control, plus two appendices and numerous examples.
Chapter
This interdisciplinary book encompasses the fields of rock mechanics, structural geology and petroleum engineering to address a wide range of geomechanical problems that arise during the exploitation of oil and gas reservoirs. It considers key practical issues such as prediction of pore pressure, estimation of hydrocarbon column heights and fault seal potential, determination of optimally stable well trajectories, casing set points and mud weights, changes in reservoir performance during depletion, and production-induced faulting and subsidence. The book establishes the basic principles involved before introducing practical measurement and experimental techniques to improve recovery and reduce exploitation costs. It illustrates their successful application through case studies taken from oil and gas fields around the world. This book is a practical reference for geoscientists and engineers in the petroleum and geothermal industries, and for research scientists interested in stress measurements and thei
Article
We have shown that linear Mogi criterion does a good job in representing rock failure under polyaxial stress states. When σ2 = σ3 the linear version of Mogi's triaxial failure criterion reduces exactly to the Coulomb criterion. Hence, the linear Mogi criterion can be thought of as a natural extension of the Coulomb criterion into three dimensions (i.e., polyaxial stress space). As Mohr's extension of the Coulomb criterion into three dimensions is often referred to as the Mohr-Coulomb criterion, we propose that the linear version of the Mogi criterion be known as the "Mogi-Coulomb" failure criterion. The classical Coulomb failure criterion can therefore be thought of as a special case, which applies only when σ2 = σ3 of the more general linear Mogi failure criterion. Furthermore, we found that the numerical values of the parameters that appear in the Mogi-Coulomb criterion can be estimated from conventional triaxial test data. Thus, this polyaxial failure criterion can be applied even in the absence of polyaxial (true triaxial) data. This offers a great advantage, as most laboratories are equipped to conduct only traditional σ2 = σ3 tests. Finally, we showed that if the linear form of the Mogi criterion is used, the parameters that appear in it can be unambiguously related to the traditional parameters appearing in the Coulomb failure law. The lack of such a relationship for the parameters appearing in the power-law Mogi criterion has been cited in [8] as a major drawback to the use of that model.
Development drilling problems in high-pressure reservoirs
  • F R French
  • M R Mclean
French, F. R., McLean, M. R. (1992).Development drilling problems in high-pressure reservoirs. In: Proc SPE Int Meeting Petrol Eng, Beijing, SPE 22385.
Real-time pore pressure and fracture gradient evaluation in all sedimentary lithologies
  • P W Holbrook
  • D A Maggiori
Holbrook, P.W., & Maggiori, D. A. (1993). Real-time pore pressure and fracture gradient evaluation in all sedimentary lithologies, SPE 26791.Offshore European Conference, Aberdeen, Scotland, Society of Petroleum Engineers.
The effect of using different rock failure criteria in wellbore stability analysis
  • R Rahimi
  • Nygaard
Rahimi, R. & Nygaard (2015). The effect of using different rock failure criteria in wellbore stability analysis.Master's Degree Thesis.