Wellbore Stability Analysis Research Articles

Wellbore Stability in Layered Rocks: A Comparative Study of Strength Criteria

AbstractWellbore instability is a critical issue restricting efficient well drilling and successful development of oil and gas field. Most instability problems originate from shale formations because of their distinct laminated structures that cause significant anisotropy and moderate to high clay contents that are prone to shrinkage and swelling. To account for these influences on the mechanical responses of shales, this study aims to identify an appropriate strength criterion for stability analyses. Two anisotropic criteria including single plane of weakness and the modified Hoek–Brown criteria were compared to evaluate their suitability in characterizing the anisotropic strength of layered rocks including shale, schist, and slate under different confining pressures. Comparative case studies indicated that the single plane of weakness criterion overestimates the strength of layered rocks at some orientation angles. The modified Hoek–Brown criterion can fit well with the experimental data of layered rocks. Moreover, wellbore stability analysis models for shale gas wells were built, respectively, for each criterion and applied to in situ scenarios. The single plane of weakness and modified Hoek–Brown criteria provide similar results of collapse pressure, and the shale failure is mainly determined by the bedding plane. This further validates that the modified Hoek–Brown criterion is a good choice for wellbore stability analysis in shale formations with bedding planes. This study shows the potential of using the modified Hoek–Brown criterion to enhance the safety and efficiency of well drilling and operation in shale formations.

Wellbore Trajectory Optimization of an Iranian Oilfield Based on Mud Pressure and Failure Zone

Determination of the borehole and fracture initiation positions is the main aim of a borehole stability analysis. A wellbore trajectory optimization with the help of the mud pressure may be unreasonable since the mud pressure can only reflect the degree of difficulty for the initial damage to occur at the wellbore rather than the extent of the wellbore damage. In this work, we investigate the failure extension in different arbitrary inclination boreholes under different in-situ stress regimes. Assuming the plane strain condition, the Mohr-Coulomb, Mogi-Coulomb, and Modified Lade rock failure criteria are utilized. We present an analytical equation to determine the optimum drilling trajectory of an Iranian oilfield. In order to predict the degree of wellbore damage, the initial shear failure location, failure width, and failure depth of arbitrary wellbores are determined. Then a new model is derived to calculate the initial failure area of a directional wellbore because it is more efficient in a wellbore stability analysis. The results obtained show that in the target oilfield, the vertical and low-deviated direction is the optimum drilling path. According to the results of this work, optimization of the wellbore trajectory based on the estimated failure zone is a reasonable method if a considerable failure zone takes place around the borehole wall.

Geomechanical wellbore stability analysis for the wells drilled in the shallow sediments of the Mediterranean Sea

Gas hydrates are considered as near-future energy resources in the world. Due to high gas hydrate potential of the Mediterranean Sea, gas hydrate exploration wells are expected to be drilled. In this study, for the first time, the caliper logs of the wells drilled during ODP (Ocean Drilling Program) Leg 160 and ODP Leg 161 in the Eastern and Western Mediterranean Sea were evaluated together with the python codes predicting wellbore stresses, unconfined compressive strength (Co) and lower-hemisphere projection. The required Co in the Eastern Mediterranean Sea ranges from 60 psia to 2250 psia from seafloor to target depth. However, the estimated Co of the sediments in the Eastern Mediterranean in this study by using the python codes developed in this study and the caliper logs of ODP Leg 160 ranges from 15 psia to 1250 psia with depth. Similarly, the required Co to keep the wells in gauge and the estimated Co of the sediments in ODP Leg 161 range between 100-12000 psi and 30-11000 psia, respectively. Mostly, Co values of the sediments were less than the required Co values for stable wellbore so extensive washouts were detected both in the wells of ODP Leg 160 and Leg 161. This is because the sediments within gas hydrate stability zone in the Mediterranean Sea are weak and almost unconsolidated. Moreover, in this study, it was shown that it is possible to reduce the degree of washouts by increasing the density of drilling fluid until fracture pressure. According to the estimations in this study, the wellbore enlargements for the wells in ODP Leg 160 and ODP Leg 161 were mostly reduced below 2 inches, below which better wellbore stability conditions, better well log data and better casing cement quality might be obtained.

Wellbore stability analysis of layered shale based on the modified Mogi–Coulomb criterion

Borehole instability was frequently encountered during shale gas drilling. Most conventional models are not applicable to layered formation's wellbore stability analysis on account of anisotropic strength characteristic. In this study, an empirical equation for predicting anisotropic strength was implemented in the Mogi–Coulomb criterion to describe variations of cohesive strength and friction angle of shale formations. A collapse pressure model and its appropriate solution method for layered shale formations were proposed. The impact of different strength criteria and rock anisotropy type on rock strength and collapse pressure was investigated. The analysis indicated that the predicted strength of our modified criterion was usually higher than the weak plane failure criteria. The collapse pressure calculated by the modified Mogi–Coulomb criterion was lower than the weak-plane failure criteria. Furthermore, it was more consistent with real mud weight. Additionally, the anisotropy type of rock notably influences wellbore stability. More significant anisotropy coefficients correspond to higher strengths, which results in smaller collapse pressure values. Improper anisotropy coefficients can over- or under-predict the collapse pressure. Reasonable estimates of collapse pressure of anisotropic rocks can be made through the modified Mogi–Coulomb criterion using limited experimental data and the anisotropy rock type.

New Interface for Assessing Wellbore Stability at Critical Mud Pressures and Various Failure Criteria: Including Stress Trajectories and Deviatoric Stress Distributions

This study presents a new interface for wellbore stability analysis, which visualizes and quantifies the stress condition around a wellbore at shear and tensile failure. In the first part of this study, the Mohr–Coulomb, Mogi–Coulomb, modified Lade and Drucker–Prager shear failure criteria, and a tensile failure criterion, are applied to compare the differences in the critical wellbore pressure for three basin types with Andersonian stress states. Using traditional wellbore stability window plots, the Mohr–Coulomb criterion consistently gives the narrowest safe mud weight window, while the Drucker–Prager criterion yields the widest window. In the second part of this study, a new type of plot is introduced where the safe drilling window specifies the local magnitude and trajectories of the principal deviatoric stresses for the shear and tensile wellbore failure bounds, as determined by dimensionless variables, the Frac number (F) and the Bi-axial Stress scalar (χ), in combination with failure criteria. The influence of both stress and fracture cages increases with the magnitude of the F values, but reduces with depth. The extensional basin case is more prone to potential wellbore instability induced by circumferential fracture propagation, because fracture cages persists at greater depths than for the compressional and strike-slip basin cases.

Wellbore stability analysis to determine the safe mud weight window for sandstone layers

The wellbore stability of a vertical well through the sandstone reservoir layers of the Asmari oil-bearing formation in south-west Iran is investigated. The safe drilling-fluid density range for maintaining wellbore stability is determined and simulated using FLAC3D software and a finite volume model established with drilled strata geomechanical features. The initiation of plastic condition is used to determine the safe mud weight window (SMWW) in specific sandstone layers. The effects of rock strength parameters, major stresses around the wellbore and pore pressure on the SMWW are investigated for this wellbore. Sensitivity analysis reveals that a reduction in cohesion and internal friction angle values leads to a significant narrowing of the SMWW. On the other hand, the reduction of pore pressure and the ratio between maximum and minimum horizontal stresses causes the SMWW to widen significantly. The ability to readily quantify changes in SMWW indicates that the developed model is suitable as a well planning and monitoring tool.

Influence of relation between stress field and bedding space on wellbore stability in shale formation

The shale gas reservoir in Jiaoshiba area is well developed with bedding and shows strong anisotropy, which leads to the problems of wellbore instability in the drilling process. At present, for the shale rich in bedding, the conventional calculation model of shale wellbore collapse pressure cannot meet the needs of shale wellbore stability analysis. In this paper, the analytical method considering the influence of bedding is more accurate. Therefore, in this paper direct shear tests and tri-axial mechanic experiments were conducted on the down-hole core rocks of LongMaxi reservoir shale formation, and the obtained experimental results were combined with single discontinuity theory to establish the characterization method of shale strength anisotropy. Then, combining the characterization method of shale strength anisotropy with the sidewall stress distribution model, the calculation method of borehole collapse pressure is established. Finally, the JY1HF well is taken as an example for calculation and comparative analysis. The results show that the LongMaxi reservoir shale strength deteriorating after drilling fluid immersion is not the main control factor of borehole instability; the shear slip along the weak bedding plane is the main mechanical mechanism of borehole instability. The collapse pressure calculated by the conventional model decreases with the increase of the well inclination, and the collapse pressure calculated by the model in this paper shows a trend of increasing first and then decreasing with the increase of the well inclination, and the results calculated by the model in this paper are in good agreement with the actual working conditions. The research results of this paper can provide theoretical basis and reference for drilling design and borehole trajectory optimization of shale gas wells in Jiaoshiba area or other areas.

Natural fracture characterization and wellbore stability analysis of a highly fractured southwestern Iranian oilfield

Wellbore stability analysis is an essential part of well planning where mechanical characteristics of rocks and the state of in-situ stresses are determined to evaluate the likelihood of borehole failure during drilling. In this study, natural and drilling induced fractures were characterized using image logs for a fractured reservoir located in Southern Iran. Mechanical Earth Modeling (MEM) and wellbore stability analysis were also carried out by the combination of wireline log data and in-situ measurements for a vertical well drilled in this field. The obtained results indicated the presence of several major/minor fractures in the studied interval but majority of them were found to be closed fractures. According to the MEM, formations would suffer from breakout (shear failure) in several intervals which might be linked to their low strengths, huge stress anisotropy and inappropriate selection of the mud weight. Give then fact that the break-out failure obtained from the Mogi-Coulomb criterion had a match with the borehole failure captured by the caliper log, the geomechanical parameters estimated through the methodology proposed should have sufficient accuracy and can be used for the stability analysis of other wells drilled in the same field.

Dynamic analysis of wellbore stress field and wellbore stability in carbonate reservoir production process

Carbonate rocks usually need to be acidified to achieve transformation and increase production. However, the deep carbonate formation is complex, and the change law of mechanical parameters is not easy to grasp. After acidification, accidents such as borehole instability and sand production occur occasionally in the production process, thus damaging the reservoir and reducing the production. In this article, the downhole core of carbonate reservoir of Qixia formation in Sichuan province is taken as the research object, and rock samples with different time of acid soaking were tested by field acid solution. Based on the experimental data, software simulation is used to analyze the dynamic variation law of wellbore stress field and wellbore stability in the process of reservoir production after acidification. It also provides a theoretical basis for selecting a reasonable completion method. The experimental data show that the effect of acid erosion on porosity and permeability of carbonate rocks is more obvious than that of original rock samples, and the overall trend is increasing. Acidification has a significant impact on the rocks’ mechanical strength and elastic modulus which was reduced to the maximum extent after 4 h of acid immersion. Subsequently, the reduction of compressive strength and elastic modulus became smaller, and the overall change law of Poisson’s ratio was not obvious. The simulation analysis shows that during acidizing production, the pore pressure of wellbore is relieved, which is beneficial to the stability of wellbore, and the overall stability of the sidewall after acidizing corrosion showed an improving trend; it is considered that the borehole stability of Qixia formation meets the open-hole completion conditions of acidizing fracturing. However, after acidification of fractured rock samples in Qixia formation, the acidification further reduces rock mechanical properties and wellbore stability, which may lead to wellbore instability and sand production. The experimental data and numerical simulation obtained in this paper provide a certain theoretical basis for the design, simulation, and optimization of acid fracturing of Qixia formation reservoir.

Wellbore Stability Analysis and Drilling Strategy in Depleted Reservoir

Wellbore Stability Analysis and Drilling Strategy in Depleted Reservoir

Wellbore-Stability Analysis by Integrating a Modified Hoek-Brown Failure Criterion With Dual-Porochemoelectroelastic Theory (includes associated erratum)

Summary When drilling through naturally fractured formations, the existence of natural fractures affects the fluid diffusion and stress distribution around the wellbore and induces degradation of rock strength. For chemically active formations, such as shale, the chemical–potential difference between the drilling mud and the shale–clay matrix further complicates the nonmonotonic coupled pore–pressure processes in and around the wellbore. In this work, we apply a recently formulated theory of dual–porosity/permeability porochemoelectroelasticity to predict the time evolution of mud–weight windows, while calculating stresses and pore pressure around an inclined wellbore drilled in a fractured shale formation. The effects of natural–fracture geometric and spatial distributions coupled with the chemical activity are considered in the wellbore–stability analysis. To account for the degrading effect of the fractured shale matrix on the bulk rock strength, a modified Hoek–Brown (MHB) criterion is developed to more closely describe the in–situ state of stress effects on the compressive shearing strength at great depth. Compared with the original Hoek–Brown (HB) failure criterion, the MHB criterion considers the influence of the intermediate principal stress and thus shows better agreement with true–triaxial data for various rocks at varying stress levels. The MHB criterion converges to the original HB criterion when the confining in–situ stresses are equal. Two field case studies indicate that this novel integrative methodology is capable of predicting the operational drilling–mud–weight windows used in these two cases. Another advantage of this newly developed technique is that it can be used as a back–analysis tool to estimate the fracture–matrix permeability from field operational data.

Statistic evaluation of failure criteria in wellbore stability with temperature effects

Geothermal energy is increasingly recognized as a potential of replenishing fossil fuels and meeting decarbonization obligations. HP/HT drilling is also increasingly recognized as a tool to provide an adequate supply of oil/gas. In these two types of activity, the high temperature is the common challenge for drilling. Under high temperature, rocks usually behave differently compared to the ambient condition. Whether the popular failure criteria can be used to evaluate wellbore stability under geothermal conditions is a question which has not been answered yet. In this paper, the triaxial compression tests under different temperature for six types of rock (Strathbogie granite, Tak granite, Tournemire shale, Slate, Carthage marble, and Crab Orchard sandstone) have been used to evaluate six different failure criteria. Two methods have been implemented to obtain rock cohesion and internal friction angle, which are grid search algorithm and Mohr failure line method. Both methods show that Modified Lade is the failure criterion that provides the best performance in predicting rock failure under geothermal drilling. The grid search algorithm guarantees the best performance of the failure criteria, providing with the best-fit rock parameters. The more practical method of Mohr failure line can still generate engineering reasonable predictions if a safety factor of 1.1 is considered, but only the Modified Lade should be used. The thermal influence on rock parameters is also analyzed and the cohesion is found to be decreased with the improvement of temperature, whereas the internal friction angle does not have a consistent relationship with temperature. Overall, considering the considerable variance of depth (several kilometers) and temperature (several hundred degree Celsius) of geothermal and deep oil/gas resources, the Modified Lade has proven to be the best choice to perform wellbore stability analysis.

Meso-fracture mechanism and its fracture toughness analysis of Longmaxi shale including different angles by means of M-SENB tests

Researches on fracture toughness and crack initiation, propagation influenced by bedding at mesoscale are quite beneficial to the understanding of hydraulic fracturing and wellbore stability analyses in shale gas exploitation. Three-point bending tests with Scanning Electron Microscopy (SEM) were applied on 29 Longmaxi shale samples with different pre-notch angle at mesoscale. Peak load, failure mechanism and fracture toughness of shale were analyzed and discussed in detail. Experimental results show that the peak load is directly proportional to the angle β between the bedding plane and the pre-notch. In addition, the crack propagation path is very zigzag with the increase of β. It is obvious that the bedding weak surface seriously affects the crack initiation and propagation under the action of the principal stress field. Mode I crack is normally initiated at a relatively large curvature radius of notch edge. However, mixed mode crack I/II is easily initiated at the location where the principal stress is severely concentrated. Two typical facture modes have been found in Longmaxi shale. At the beginning of the expansion, mode I crack perpendicularly passes through the weak bedding plane and then extends into the weak layer. Mode I/II mixed slant-crack directly passes through the bedding plane or goes zigzag on the bedding plane or become zigzag before reached to weak bedding plane, and then passes through the weak plane perpendicularly or just deflects perpendicularly. The branch crack occurs near the main crack propagates along or through the mineral particles. Meso-cracks are mainly propagated around the crack tip, natural fissures, crack turning point, and large mineral particles. Cracking direction changes during the propagation process and eventually parallel to the main crack and then stops, or it intersects with the main crack and forms exfoliated particles. By utilizing both FEM software and test results, the I/II mixed mode fracture toughness of shale has been calculated, and the pure mode I fracture toughness has been obtained as 0.7359 MPa m0.5 approximately. Furthermore, both stress intensity factor and the T stress change law of I/II mixed mode cracks were also obtained and discussed in detail.

Wellbore stability analysis and application to optimize high-angle wells design in Rumaila oil field, Iraq

Mechanical wellbore instability is one of the main issues during the drilling operations, this problem causes around 14% of drilling nonproductive time (NPT) for new wells in Rumaila oil field; these breakouts were commonly observed in the shale and weak shaly sandstones along Rumaila geological column. In this study, a quantitative wellbore failure analysis was carried out through applying modified Lade criteria on five wells in Rumaila oil field (A, B, C, E and F). The results gave a prediction of the appropriate mud density that can control the wellbore shear failure for any well type in order to eliminate or minimize wellbore instability and indicated that the preferred direction to drill deviated and horizontal wells should be parallel to maximum horizontal stress (NE–SW). The study also introduced the recommended drilling mud weight for vertical and deviated wells in Rumaila oil field for the sake of gaining wellbore stability which requires raising the drilling mud weight to more than 1.27 gm/cm3 starting from bottom of Sadi Formation to the well total depth. The results also showed that wells with low inclination (less than 30°) are less stable than highly inclination wells (more than 30°).

Uncertainty Evaluation of Safe Mud Weight Window Utilizing the Reliability Assessment Method

Due to the uncertainty of formation properties and improper wellbore stability analysis methods, the input parameters are often uncertain and the required mud weight to prevent wellbore collapse is too large, which might cause an incorrect result. However, the uncertainty evaluation of input parameters and their influence on safe mud weight window (SMWW) is seldom investigated. Therefore, the present paper aims to propose an uncertain evaluation method to evaluate the uncertainty of SMWW. The reliability assessment theory was introduced, and the uncertain SMWW model was proposed by involving the tolerable breakout, the Mogi-Coulomb (MG-C) criterion and the reliability assessment theory. The influence of uncertain parameters on wellbore collapse, wellbore fracture and SMWW were systematically simulated and investigated by utilizing Monte Carlo simulation. Finally, the field observation of well SC-101X was reported and discussed. The results indicated that the MG-C criterion and tolerable breakout is recommended for wellbore stability analysis. The higher the coefficient of variance is, the higher the level of uncertainty is, the larger the impact on SMWW will be, and the higher the risk of well kick, wellbore collapse and fracture will be. The uncertainty of basic parameters has a very significant impact on SMWW, and it cannot be ignored. For well SC-101X, the SMWW predicted by analytical solution is 0.9921–1.6020 g/cm3, compared to the SMWW estimated by the reliability assessment method, the reliability assessment method tends to give a very narrow SMWW of 1.0756–1.0935 g/cm3 and its probability is only 80%, and the field observation for well kick and wellbore fracture verified the analysis results. For narrow SMWW formation drilling, some kinds of advanced technology, such as the underbalanced drilling (UBD), managed pressure drilling (MPD), micro-flow drilling (MFD) and wider the SMWW, can be utilized to maintain drilling safety.

Sensitivity analysis of geomechanical parameters affecting a wellbore stability

Wellbore stability analysis is a growing concern in oil industries. There are many parameters affecting the stability of a wellbore including geomechanical properties (e.g., elastic modulus, uni-axial compressive strength (UCS) and cohesion) and acting forces (e.g., field stresses and mud pressure). Accurate determination of these parameters is time-consuming, expensive and sometimes even impossible. This work offers a systematic sensitivity analysis to quantify the amount of each parameter’s effect on the stability of a wellbore. Maximum wellbore wall displacement is used as a stability factor to study the stability of a wellbore. A 3D finite difference method with Mohr model is used for the numerical modeling. The numerical model is verified against an analytical solution. A dimensionless sensitivity factor is developed in order to compare the results of various parameters in the sensitivity analysis. The results show a different order of importance of parameters based on rock strength. The most sensitive properties for a weak rock are the maximum horizontal stress, internal friction angle and formation pressure, respectively, while for a strong rock, the most sensitive parameters are the maximum horizontal stress, mud pressure and pore pressure, respectively. The amount of error in wellbore stability analysis inflicted by the error in estimation of each parameter was also derived.

Stability Analysis of Highly Deviated Boreholes to Minimize Drilling Risks and Nonproductive Time

The Lower Cretaceous Zubair Formation is a regionally extended gas- and oil-producing sandstone sequence in Southern Iraq. Due to the weak nature of the Zubair Formation, the lack of wellbore stability is one of the most critical challenges that continuously appears during the drilling development operations. Problems associated with lack of wellbore stability, such as the tight hole, shale caving, stuck pipe, and sidetracking, are both time-consuming and expensive. This study aimed to construct a geotechnical model based on offset well data, including rock mechanical properties, in situ stresses, and formation pore pressure, coupled with suitable rock failure criteria. Mohr–Coulomb and Mogi–Coulomb failure criteria were used to predict the potential rock failure around the wellbore. The effect of the inclination and azimuth of the deviated wells on the shear failure and tensile failure mud weights was investigated to optimize the wellbore trajectory. The results show that the best orientation to drill highly deviated wells (i.e., inclinations higher than 60 deg) is along to the minimum horizontal stress (140 deg). The recommended mud weight for this selected well trajectory ranges from 1.45 to 1.5 g/cc. This study emphasizes that a wellbore stability analysis can be applied as a cost-effective tool to guide future highly deviated boreholes for better drilling performance by reducing the nonproductive time.

Geomechanical study of gas reservoir rock using vertical seismic profile and petrophysical data (continental shelf in southern Iran)

ABSTRACTGeomechanics is a science dealing with the study of the behaviour of rocks affected by stress. It has various applications in utilisation from oil and gas reservoirs including of the wellbore stability analysis and determination of safe mud window. The main aim of this paper is geomechanical study of Kangan–Dalan reservoir in South Pars gas field in Persian Gulf in south Iran. Seismic waves are affected by physical properties of rocks when passing underground formations; thus, the velocity of these waves is a desired parameter for estimation of geomechanical properties. The velocity of compressional and shear waves has been determined with processing seismic data resulting from vertical seismic profile. In this paper, after calculation of elastic modules of reservoir rock, the imposed stress field was determined and these concepts were used for engineering calculations such as safe mud window, wellbore stability analysis and sand production potential. For well drilling in Kangan-Dalan reservoir, the minimum and maximum mud weights were proposed in average as 1.093 and 2.011 gr/cc and average critical mud weight as 2.48 gr/cc such that if the weight of mud increases, the tensile fractures will be created on the formation and complete loss of mud will happen.

Quantitative risk assessment for Optimum Mud weight window design: A case study

Deterministic approach in selecting mud weight window at pre-drill condition has uncertainties for wellbore stability analysis. Quantitative Risk Assessment (QRA) technique has been successfully used in pre-drill condition addressing the variability of input parameters like, principal stresses and rock properties. The output of this process recommends the mud weight window with probability of drilling success which can avoid the wellbore collapse and lost circulation events. This kind of stochastic approach to predict safe mud weight window can assure stable wellbore with significant cost effectiveness related to drilling success.A workflow to perform QRAis demonstrated which starts with derivation of stresses in deviated wellbore. Seismic velocity data has been used to estimate pore pressure and validated with formation pressure obtained from reservoir characterization instrument data for a study well located in east coast of India. Mogi-Coulomb failure criteria have been used to estimate required mud weights to avoid shear failure. Mud weights required to avoid lost circulation has also been calculated. Then QRA has been accomplished to predict safe mud weight window sat specific depth within 14 ¾” hole and borehole section (12 ¼”). The results have been validated with real data set and based on that exercise, mechanical earth model and uncertainty associated with input parameters for study well has been established. A feasibility study has been performed to check the viability of drilling a near horizontal (75ο inclination) well along the reservoir unit with azimuth of 250ο. The wellbore collapse and lost circulation curves intersect near 35% probability of success point. The results of QRA indicate that the inclination and azimuth would not be favorable for successful drilling in this scenario. Based on these QRA, recommendations have been made in data acquisition plan for future drilling to reduce the variability in input parameters and successful completion of well.

A workflow for infill well design: Wellbore stability analysis through a coupled geomechanics and reservoir simulator

The traditional way of analyzing wellbore stability in depleted reservoirs is by assuming homogenous depletion and uniaxial strain. Then the analytical solution of stress path is obtained. Pore pressure depletion, however, is location and time dependent. The objective of this study is to develop a workflow for infill well design. More specifically, a wellbore stability analysis is developed for infill wells using a coupled geomechanics and reservoir simulator (CGRS) which considers lateral displacement and inhomogeneous depletion. Different shear failure criteria are utilized in a new CGRS wellbore stability model. The upper bounds of shear failure are given by Drucker-Prager Inscribes and Griffith Theory, while the lower bound is given by Drucker-Prager Circumscribe. CGRS wellbore stability model is compared with conventional wellbore stability model (based on the analytical solution of stress path). CGRS wellbore stability can quantify the influence of azimuth on minimum and maximum mud weight during the depletion when initial maximum horizontal stress equals minimum horizontal stress. In addition, CGRS can give the output of a location-dependent mud weight map for the entire reservoir. Neither of the above two functions can be realized by conventional wellbore stability model. Therefore, CGRS can provide dynamic information concerning where to drill, when to drill, and how to drill for infill wells.