Rock Failure Criteria Research Articles

Experimental study on dynamic response of hard rock blasting under in-situ stress

Deep mine rock mass is in high static stress and dynamic disturbance coupling conditions, its mechanical properties and failure mode is different from the shallow rock mass, which leads to low rock blasting efficiency and engineering geology hazards. In-depth research on the dynamic response of rock blasting under in-situ stress will help to optimize the blasting design, improve the blasting efficiency and safety of blasting operations, and provide theoretical support for rock blasting in deep mines. In this study, the blasting experiment was conducted on granite specimens under different biaxial static stress conditions. Meanwhile, the dynamic response of rock blasting was monitored, collected, and analyzed using a high-speed digital image correlation (DIC) measurement system, a strain wave acquisition system, and an acoustic emission (AE) system. The results show that small and medium pre-static loads inhibit blast crack propagation, at which time the cumulative AE hits from dynamic loads (CAECd) are more than those from pre-static loads (CAECs), but large pre-static loads promote crack propagation, at which time CAECs are more than CAECd. Secondly, as pre-static load increases, the specimen's maximum strain (εmax) decreases first and then increases, but as lateral pressure coefficient (K) increases, the εmax in the direction of lower static stress decreases gradually and the εmax in the direction of higher static stress remains constant. In addition, the confining pressure magnitude and K affect the area and shape of the failure zone of the specimen, as well as the size and propagation direction of the radial crack. Especially when the confining pressure is high, the specimen will undergo shear failure, and the smaller K is the more serious the shear failure. Finally, the failure criterion of rock under dynamic-static coupling conditions is proposed based on the energy index, and different failure types of rock are discussed.

A Failure Criterion of Weak and Crushable Limestone Rock in Mining Field containing Karstic Cavities

A Failure Criterion of Weak and Crushable Limestone Rock in Mining Field containing Karstic Cavities

Tectonic fractures induced by strike-slip faulting in intracratonic ultradeep carbonate rocks: Insights from the finite element method and self-adaptive constraints computational model for boundary conditions

Numerical simulations of the paleostress field during a period of tectonic fracture formation and rock failure criteria are used to quantitatively predict the development and occurrence of tectonic fractures induced by the formation of the SB18 fault zone in the Middle Ordovician Yijianfang Formation of the Shunnan area, Tarim Basin, China. The results of acoustic emission experiments, mechanical property measurements, and tectonic fracture occurrence observations obtained from core descriptions and fullbore formation microimager logs are combined with the Andersonian model of faulting and the finite element method, which is widely used for the numerical simulation of stress fields, to investigate the paleotectonic and in situ stress fields via numerical simulation. The quantitative prediction of the opening pressure and opening sequence of tectonic fractures is based on the occurrence of tectonic fracture, numerical simulation of in situ stresses, and coordinate system conversion. The results show that the width of the fracture zone induced by strike-slip faulting is ∼310 m. The degree of fracture development is significantly increased when the Young’s modulus, paleostress difference, and paleostress difference coefficient of the rock are elevated. The current horizontal principal stress is positively correlated with the distance from the fault, and the elevated areal density of the secondary faults causes a clockwise deflection of the horizontal stress direction. SSE-striking shear fractures with orientations ranging from 140° to 150° and two sets of tensional fractures with orientations ranging from −40° to −35° and 55° to 60° are preferentially opened in the water injection development stage of the reservoir. As the horizontal stress difference, horizontal stress difference coefficient, and angle between the maximum horizontal principal stress and a fracture decrease, the fracture opening pressure decreases. At the structural highs (burial depths <6225 m) and lows (burial depths >6225 m), the fracture burial depth is positively and negatively correlated with the opening pressure, respectively. Quantitative prediction of tectonic fracture developmental characteristics, opening pressure, and the opening sequence and investigation of the main factors that control their development can help to identify and support opportunities for hydrocarbon exploration and development of fractured carbonate reservoirs.

Experimental and numerical simulation research on hot dry rock wellbore stability under different cooling methods

The exploitation of geothermal energy commonly occurs at depths ranging from 3000 to 10000 m. At these depths, the surrounding rocks experience substantial temperature and pressure, leading to alterations in their physical and mechanical properties. The infiltration of drilling fluid into the wellbore result in thermal exchange with the surrounding rocks, causing a decrease in their strength and stability. Consequently, the wellbore becomes more susceptible to instabilities. This objective of this study was to investigate the physical and mechanical properties of heated granite under three distinct cooling methods: liquid nitrogen cooling, water cooling, and natural cooling. Based on experimental findings, this study enhanced the failure criteria for high-temperature granite and established a thermal-solid coupling model to assess the stability of wellbores in high-temperature granite formations. To examine the impacts of temperature variations between the wellbore and drilling fluid, analyze the stress distribution around the wellbore at various depths, and evaluate the consequences of high-temperature fluid intrusion on wellbore stability. The research indicates that: (1) Under identical high-temperature conditions, it has been observed that liquid nitrogen cooling causes the most significant damage to granite, followed by water cooling. On the other hand, natural cooling has the least detrimental effect on the rock. (2) With increasing temperature, the physical and mechanical properties of the rock undergo two distinct stages of change. The first stage is a gradual variation that occurs between 25 and 400 °C, while the second stage is a rapid variation observed between 400 and 800 °C. Notably, the temperature of 400 °C serves as a critical threshold for the observed property changes in this granite. (3) The rock failure criteria have been modified based on the thermal damage characteristics of cold fluid intrusion into granite and a thermal-hydraulic-mechanical coupled wellbore model has been established. (4) The numerical simulation results revealed that liquid nitrogen cooling had the greatest impact on wellbore temperature, stress, and damage zone. At 500 s, the temperature on the wellbore surface decreased by approximately 700 °C due to liquid nitrogen cooling. The circumferential stress and radial stress decreased by 180 MPa and 35 MPa, respectively. (5) After the intrusion of cold fluid into high-temperature granite, the stress around the wellbore decreases and the rate of decrease in circumferential stress is significantly greater than that of radial stress. This will increase the likelihood of wellbore instability.

Energy evolution analysis and related failure criterion for layered rocks

Energy evolution analysis and related failure criterion for layered rocks

Prediction Unconfined Compressive Strength for Different Lithology Using Various Wireline Type and Core Data for Southern Iraqi Field

Unconfined Compressive Strength is considered the most important parameter of rock strength properties affecting the rock failure criteria. Various research have developed rock strength for specific lithology to estimate high-accuracy value without a core. Previous analyses did not account for the formation's numerous lithologies and interbedded layers. The main aim of the present study is to select the suitable correlation to predict the UCS for hole depth of formation without separating the lithology. Furthermore, the second aim is to detect an adequate input parameter among set wireline to determine the UCS by using data of three wells along ten formations (Tanuma, Khasib, Mishrif, Rumaila, Ahmady, Maudud, Nahr Umr, Shuaiba and Zubair). After calibration with core test, the results revealed that Young’s Modulus correlations are the best to predict UCS with RMSE (53.23 psi). Furthermore, the result showed that using the static Young Modulus as an input parameter in predicting UCS gives a closer result to the laboratory test than using a sonic log. This study found that many previous equations were developed for only one type of rock and tended to generalize poorly to the broader database. This study also provided more accurate rock strength estimation, leading to better prognosis in operational strategies and hydraulic fracturing location planning in oil well development when geomechanical analysis needs to be addressed where no core is available. Finally, the expected continuous rock mechanical profile indicates the formation's strength and stability around the wellbore.

Analysis of seepage and temperature fields and their influence on collapse pressure while drilling in shale oil reservoirs

Shale wellbore instability poses a considerable challenge in terms of drilling efficiency. However, previous studies typically assumed single-phase flow in the seepage field, neglecting the influences of water saturation and temperature on rock mechanical properties. Furthermore, there has been a lack of quantitative analysis on the degree of correlation between various factors and collapse pressure. To address these gaps, this research introduces an oil–water two-phase seepage model to describe the water saturation distribution surrounding the wellbore,and also constructed a wellbore temperature field model that accounts for mechanical and hydraulic heat sources and validated its accuracy. Furthermore, this paper examines the effects of temperature and seepage on shale mechanical parameters. By incorporating a rock failure criterion that accounts for water saturation, a shale collapse model in thermal-seepage coupling is established, which has also been validated. Finally, this paper utilises the grey relational analysis to quantitatively assess the correlation between different variables and collapse pressure. The results indicate that the temperature field and collapse models are consistent with field data. As drilling time increases, water saturation and invasion distance grow, while the seepage radius hardly increases anymore. When porosity exceeds 0.05, its influence on water saturation weakens. Under unbalanced conditions, capillary force drives drilling fluid into the formation. The maximum wellbore temperature is found near the drill bit and exceeds the formation temperature. The bottom-hole temperature difference increases with the increase in well depth and drilling time, sometimes exceeding 20 °C. Among drilling engineering parameters, pump rate has greater influence on bottomhole temperature than WOB, RPM and ROP. Regarding drilling fluid property parameters, density, flow index and consistency coefficient considerably influence bottom-hole temperature. The influence of seepage on collapse pressure is greater than that of the temperature field. Grey relational analysis reveals that pump rate and water saturation have the highest correlation with collapse pressure. As drilling time increases, the correlation degree of pressure difference, horizontal section length and weight on bit gradually increase, while that of the mud behaviour index, consistency coefficient, rock thermal conductivity and mud yield stress slightly decreases.

Study on Rock Failure Criterion Based on Elastic Strain Energy Density

Uniaxial and five conventional triaxial compression tests were conducted on sandstone to obtain the evolution laws of the input energy density, elastic strain energy density, and dissipative energy density. The input and dissipative energy densities increased with increasing axial strain; the elastic strain energy density increased with increasing axial strain at the pre-peak stage and decreased after the peak. According to the linear change rule between the peak elastic strain energy density and confining pressure, the energy density failure criterion of sandstone was established, and the criterion has high precision and few parameters, and the parameters have clear physical meaning. Moreover, the expression of the energy density failure criterion was similar to the classical Hoek-Brown criterion, but its adaptability was more extensive. The strength calculation results for seven different rocks under different confining pressures calculated using the energy density failure criterion were consistent with the experimental values, and the calculation error was smaller than that of the Mohr–Coulomb criterion and Drucker–Prager criterion, verifying the accuracy and applicability of the criterion.

A micromechanical-based failure criterion for rocks after high-temperature treatment

The strength properties of rocks after high-temperature treatment are substantially impacted by thermally-induced cracks. In this work, a new parameter of temperature-dependent initial damage or crack density is proposed and a micromechanical-based failure criterion incorporating it is developed to characterize the strength properties of rocks after high-temperature treatment under compressive loading. The mathematical expression of the suggested nonlinear failure criterion coincides with the Hoek-Brown failure criterion for intact rocks. The micromechanical-based failure criterion is then verified using the experimental results of five sets of rocks (Macheng granite, Tournemire shale, red sandstone, coarse marble and veined marble) after high-temperature treatment. The results shown that the strength properties of rocks after high-temperature treatment can be well captured using the failure criterion incorporating temperature-dependent initial damage. Three evaluation indicators (root mean square error, correlation coefficient and mean absolute error) are indicated that the failure criterion incorporating temperature-dependent initial damage has a strong predictive capability. The parameter of temperature-dependent initial damage provides a great description of the effect of thermally-induced damage on the strength evolution of rocks after high-temperature treatment. The proposed failure criterion in this work will become an effective alternative for predicting the nonlinear strength evolution of pre-heated rocks under compressive loading due to its high reliability and easy-to-use.

Quantitative prediction of ultra-deep tight sandstone fractures based on the theory of minimum energy dissipation

Structural fractures are key factors that influence natural gas migration and accumulation within ultra-deep tight reservoirs. To obtain a quantitative forecast of the development and distribution of reservoir fractures in the Keshen gas reservoir, we analyzed the characteristics of the region's structural evolution and paleo-tectonic stress field settings. A reasonable geological model of the research area was built based on an interpretation of the geological structure, a test for rock mechanics, and an experiment on acoustic emission. Thereafter, a 3-D paleo-tectonic stress field during the mid-late Himalayan movement was simulated using the finite element method. By introducing the principle of minimum energy dissipation, which to our knowledge, was done for the first time in the field, mechanical constitutive models, rock failure criteria, and fracture parameter calculation models were derived and used to determine the quantitative development of fractures and predict zones prone to fracture development. The fracture distribution in the ultra-deep Keshen gas reservoir is mainly controlled by folds, buried depth, faults, and lithology. Interestingly, even in the strong tectonic compression movement, the stress field was not continuously transmitted forward but showed fluctuation or cyclicity, indicating that fractures might have occurred at each time stage. The predicted areas with developed fractures were consistent with wells having high fracture linear density (i.e., measured results) and in locations with high-producing gas wells. We believe that the novel method in this study can be of great significance to fracture research in other ultra-deep petroliferous basins.

A new critical strain energy release rate failure criterion for anisotropic rock under high confining pressure

To research the influences of confining pressure on critical strain energy release rate (Gc) and energy transformation of anisotropic intact rocks, by combining a new modified Hoek–Brown failure criterion for anisotropic rock under high confining pressure (M-HB-HC) with the energy release rate failure criterion, the new critical strain energy release rate failure criterion is established. Triaxial compression tests for marble and sandstone samples with different bedding plane dip angles under different confining pressures have been performed. The study shows that Gc depends on both the bedding plane dip angle α and confining pressures, and the variation of Gc with the dip angle under low confining pressure is not evident, but is significant at the high confining pressure. Under the same confining pressure, the value of bedding marble’s Gc is greater than that of bedding sandstone. By comparing prediction results of several failure criteria with that of the proposed failure criterion and the experimental data, we found that the prediction of the new critical strain energy release rate failure criterion has better accuracy. The proposed failure criterion is applicable for different type of anisotropic intact rocks. This paper only researches the transverse isotropic case of the anisotropic rock. The other anisotropic will be studied in the future.

A benchmark study of different numerical methods for predicting rock failure

Nowadays, with many available numerical methods developed in rock mechanics, researchers have always focused on the parameter calibration of the numerical model but ignored the predictive capability of these methods. A comparative study of nine commonly used numerical methods was performed for predicting rock failure through international cooperation organized by the Discontinuous Deformation Analysis (DDA) commission of the International Society for Rock Mechanics (ISRM). Two steps of numerical modelling were conducted including a calibration procedure from given experimental results and a numerical prediction for benchmark tests with these calibrated parameters for three types of rocks. Through the comparison between different numerical and experimental results, the inherent weaknesses and strengths of different numerical methods in terms of predicting rock failure were identified and analysed. The influence of human intervention in terms of parameter selection is even more significant than the choice of different numerical methods. Some potential factors (i.e., different boundary conditions, heterogeneity of rock material, strength parameters, particle packing, and failure criteria) that may occur in numerical and physical tests were further discussed. Through the comparison of different failure criteria, we found the selection of rock failure criterion might be the major factor that affected the predictive capability of numerical methods, and the nonlinear failure model (the Hoek–Brown criterion) showed the superiority in the prediction of rock fracturing subjected to complex stress conditions. This comparative work also enlightens the significance of a high-quality calibration process and the future advancement of rock failure criteria.

Numerical study of rock-breaking mechanism in hard rock with full PDC bit model in compound impact drilling

Geothermal reservoir rocks have the characteristics of high stress, high hardness and high abrasiveness, which makes the drilling rate low and the drilling cost high, and hinders the application of geothermal energy. Improving drilling efficiency in hard rock strata is the prerequisite for large-scale application of geothermal energy. Impact drilling is considered to be the most effective drilling method in hard formations. PDC (Polycrystalline diamond Compact) bit compound impact drilling is one of the impact drilling methods, which has been widely concerned in recent years. In order to further improve the drilling efficiency of this method, the mechanism of PDC bit compound impact rock breaking is studied. Drucker–Prager criterion was used as rock constitutive relation, equivalent plastic strain was used as rock failure criterion, MSE (mechanical special energy) was used as evaluation method of rock-breaking efficiency, and a three-dimensional simulation model of full-size PDC bit for compound impact rock-breaking was built. The rock breaking law in PDC bit compound rock breaking process and the impact vector angle and impact duration on rock breaking efficiency are analyzed. The results show that the compound impact drilling can effectively alleviate the stick–slip vibration phenomenon, and improve drilling efficiency. In the compound impact mode, compressive stress failure and tensile stress failure occur at the same time, and the tensile stress failure is the main failure mode. The peak stress of the rock is 5.49% larger, and the residual plastic deformation is about 4.28% smaller than that of non-impact mode. With the increase of impact angle, the MSE decreases first and then increases a little. The optimal impact vector angle is in the range of 30°–50°. With the increase of the impact time, the MSE decreases in a wavy manner. In the range of 0.2 ms–1.0 ms, the optimal impact time is 0.8 ms–1.0 ms.

Well trajectory optimization of ultradeep and high-pressure drilling engineering based on high in situ stress as the main control factor: A case study from the Ordovician carbonate reservoir in the Shunbei area of the Tarim Basin

With increasing oil and gas exploration and development, well trajectory optimization has gradually become the focus of the oil and gas industry. Considering the wellbore instability in the Shunbei area of the Tarim Basin, the well trajectory was scientifically optimized under the guidance of rock mechanics, drilling engineering, and mathematical methods, combined with actual geologic data, and with in situ stress as the main controlling factor. In this paper, the stress state of the wellbore is analyzed by linear elastic theory to establish the stress distribution model of the wellbore. The safety window model of wellbore stability is established using different rock failure criteria to calculate the collapse pressure and fracture pressure of the formation. Based on this, the safe mud density window is defined to achieve wellbore trajectory optimization. Finally, the influence factors of wellbore stability are discussed, and the applicability of different rock failure criteria is evaluated. The results indicate that under the normal faulting stress regime condition in the study area, the direction of horizontal minimum principal stress is the best drilling direction, where the borehole inclination angle of [Formula: see text] > 50° is the optimal well trajectory. The wellbore stabilities of high-angle deviated wells and horizontal wells are better than those of low-angle deviated wells and vertical wells. The calculation results of the Mogi-Coulomb criterion can describe the conditions of the in situ stress field more accurately. The safe windows for different well trajectories are obtained directly by the numerical method, which is very practical for optimizing well trajectories and improving wellbore stability.

Slope Stability Analysis Using Finite Element Method on Tepus-Jerukwudel Road

Tepus-Jerukwudel Road construction is one of the South Coast Java Road sections located in Gunungkidul Regency, Special Region of Yogyakarta. One of the hills with the deepest excavation depth is at STA 14+350. The research location includes the Punung Formation which is dominated by reef limestones. The existing rock lithology is floatstone. The depth of the road excavation is more than 20 meters. The slope design is 3V:1H. This research aimed to analyze the slope stability of the Tepus-Jerukwudel Road and assess the safety factor of the slope design. We used the finite element method (FEM) in the Rocscience Phase2 v8.0 software by applying the Generalized Hoek-Brown method for the rock failure criteria. The loads considered in the slope stability analysis were live loads, dead loads, surcharge loads, and seismic loads. The results of the slope analysis without seismic loads resulted in the safety factor values for the left and right slopes of 4,49 and 3,32, respectively. For seismic loads conditions, the safety factor values for the left and right slopes are 3,74 and 2,66. The results indicated that slope design of the road is in a stable condition in accordance with the estimated static and seismic loads.

Optimization of rock failure criteria under different fault mechanisms and borehole trajectories

Optimization of rock failure criteria under different fault mechanisms and borehole trajectories

Stress state in parts of NE India: borehole collapse modelling with sensitivity analysis

Structural heterogeneities and tectonic forces in NE India give rise to changeable in situ stresses with varying orientations in this part of India. Wells located in the seismic gap in Upper Assam, the Naga Thrust and the Chittagong–Mizoram–Tripura fold belt of Mizoram are used to study stress state and borehole collapse models for the area. The absence of stress studies in the Mizoram area acts as an impetus to undertake stress studies. Poroelastic modelling shows the average ratio of maximum horizontal to vertical stress to be 0.79 for normal-faulted, 1.18 for thrust-faulted and 1.12 for strike-slip-faulted regimes. The S H direction varies from 193°N in Upper Assam to 213°N in Mizoram areas. The image log from a well in the Mizoram area shows the rotation of S H direction ( c . 85°) to vary from 500 to 3707 m due to structural heterogeneity. The thrust and strike-slip regimes in the study area pose the major threat for safe borehole drilling in this complex terrain. To mitigate this issue, Mohr–Coulomb (MC) and Mogi–Coulomb (MG) rock failure criteria are discussed to predict the minimum mud weight required for borehole drilling. MG-predicted mud weight (MW) ensures borehole stability in wells in normal-faulted sediments, while MC-predicted MW prevents shear failure in wells in thrust and strike-slip regimes. A disc plot is used to model a stable wellbore drilling path with a minimum MW. A vertical well is shown to be stable in a normal-faulted regime, whereas horizontal drilling is preferable in the fold–thrust belt. Sensitivity analysis of geomechanical input parameters on MW using Monte Carlo simulation shows that S H has the maximum effect on MW regardless of the faulting regimes.

Successful mechanical earth model construction and wellbore stability analysis using elastic and plasticity solutions, a case study

Geomechanics issues and stability of wells are essential parts of the drilling operation. If wellbore stability is not analyzed comprehensively, instability problems or failures can occur in the wellbore leading to many drilling difficulties. Many parameters such as mud weight, well trajectory, in-situ stresses, etc., could affect the wellbore instability. Wellbore stability analysis can be implemented using an elastic or a plastic solution. This paper uses both solutions to analyze wellbore stability for a well called Well-A in an Iranian southwest oil field. First, a mechanical earth model (MEM) was established and calibrated through the required information. Afterward, regarding the linear elastic solution, four rock failure criteria, including Mohr–Coulomb, Mogi–Coulomb, Hoek–Brown, and Modified Lade, were employed to predict the safe mud weight windows. In the section on plasticity, the stability analysis was executed analytically using an elastic/ideally plastic model. Then the numerical solutions using a finite difference code were applied to determine the optimum mud weight and evaluate the results of analytical computations. From the obtained results, it was found that the Mogi–Coulomb criterion provides the most reliable safe mud weight window for an elastic analysis. Numerical solution results sufficiently confirmed the authenticity of analytic elastic and elastic/perfectly plastic solutions. In addition, the numerical method was capable of determining the proper optimum mud weight using Mohr–Coulomb combined with plasticity. According to the depths of Well-A experiencing wellbore instabilities and associated geomechanical problems, the results of both numerical and linear elastic solutions demonstrated an advantageous agreement with the actual observations. Consequently, the study elucidates that in order to achieve superior drilling operations, the approaches used to analyze wellbore stability could be competently employed to determine the possibility of wellbore instabilities and design an appropriate mud weight concerning Well-A or subsequent similar wells located in the present oil field.

Data-Driven Rock Strength Parameter Identification Using Artificial Bee Colony Algorithm

Rock strength parameters are essential to understanding the rock failure mechanism and safely constructing rock excavation. It is a challenging problem for determining the rock failure criterion and its parameters due to the complexity of rock media. This study adopts an artificial bee colony (ABC) algorithm to determine the Hoek-Brown failure criterion, widely used in rock engineering practice, based on experimental data. The ABC-based approach is presented in detail and applied to a collection of experimental data collected from the literature. The ABC-based approach successfully determines the Hoek-Brown failure criterion, and the determined failure envelope is in excellent agreement with the measured curve. The maximum relative error obtained by ABC is only 2.15% and is far less than the 12.24% obtained by the traditional method. Then, the developed approach is applied to the Goupitan Hydropower Station, China, and determines the rheological parameters of soft rock based on the Burgers model. The deformation of an experiment located in the Goupitan Hydropower Station is evaluated based on obtained parameters by the developed approach. The predicted deformation matches the monitored displacement in the field. The obtained parameters of the failure criterion characterize the mechanical behavior of rock mass well. Thus, the method used provides a reliable and robust approach to determining the mechanical parameters of the failure criterion.

Determining shear failure gradient and optimum drilling mud window in the Ourhoud oil field, Berkine Basin, Algeria

Optimum drilling mud window provides a workable downhole mud pressure range to prevent formation fluid influx, borehole instabilities, and fluid loss into the formation while drilling, and this can be achieved by a comprehensive geomechanical modeling using well data. We have integrated the wireline logs, drilling data, and measured downhole data to assess the vertical stress, pore pressure, minimum horizontal stress, and shear failure (SF) gradient of the 3400 m thick Mesozoic and Paleozoic succession in the Ourhoud field, Berkine Basin. We have interpreted the hydrocarbon pressure gradient in the Triassic Argilo-Greseux Inferieur reservoir as 0.32 psi/ft and found the overburden shales to be hydrostatically pressured (0.46 psi/ft). Poisson’s ratio-based minimum horizontal stress has a 0.73–0.80 psi/ft gradient, whereas frictional faulting theory provides a lower limit of 0.66 psi/ft. We have observed massive wash outs in the caliper logs against the Cretaceous shales, which is more prone to compressive failures. To address this wellbore failure, we have modeled SF gradient by Mohr-Coulomb rock failure criteria and compared the results with the mud pressure used in drilling. We have inferred that at least 10.5 ppg drilling mud weight is required to prevent such wellbore instabilities in the Mesozoic shales, whereas the minimum allowable mud weight for the Carboniferous shale is 10 ppg. Based on the interpreted pressure gradients, we have recommended an optimum downhole drilling window for the Ourhoud field, which will be helpful to deliver stable wellbores in future drilling campaigns.