Mogi-Coulomb Failure Criterion Research Articles

Time-dependent reliability analysis of wellbore stability during drilling in hydrate exploitation

Methane hydrates is regarded as an important alternative energy in the future. Due to the complexity of marine environment, the mechanical parameters and initial stress state of formation have great uncertainties which can affect the wellbore stability in hydrate exploitation. In this study, a new time-dependent analytical model is proposed, considering the influence of hydrate dissociation on the mechanical properties and the partial coupling among pore pressure/temperature/mechanical field. The analytical solutions are verified and validated by numerical solutions. Based on the analytical solution and Mogi-Coulomb failure criterion, the reliability of wellbore stability during drilling in hydrate exploitation is analyzed by the Response Surface Method. Through the reliability analysis, the probability of wellbore instability increases and the safe drilling fluid pressure window range is narrowed with time, and finally tended to be stable at t = 12 h. Hydrate dissociation would lead to a more narrowed pressure window range, about by −38.75%. The most significant parameter whose uncertainty affects the instability probability is initial in-situ stress, followed by the initial internal friction angle and Biot’s coefficient.

Integrated 3D Mechanical Earth Modelling to Intensively Investigate the Wellbore Instability of Zubair Oil Field, Southern Iraq

Wellbore instability in the Zubair oil field is the main problem in drilling operations. This instability of the wellbore causes several problems including poor hole cleaning, tight hole, stuck pipe, lost circulation, bad cementing, and well kick or blowout that lead to an increase in the nonproductive time. This article aims to set up an appropriate drilling plan that mitigates these instability issues for further well drilling. Field data from six wells (logs, drilling and geological reports as well as offset well tests) were used to build a one-dimensional mechanical earth modeling for each well. The constructed model of selected wells was combined to construct the three-dimensional mechanical earth modeling, which can enable the distribution of all estimated geomechanical parameters along the field of interest. The results revealed that the strip slip and normal faults are the common fault regimes in Zubair oil field located in carbonate rocks and clastic rocks, respectively. The Mogi-Coulomb failure criterion is conservative in determining the minimum and maximum mud weight, and it agrees with the determination of real failure from the image/and caliper logs. The best direction to drill the deviated and horizontal wells was towards the minimum horizontal stress with 140o azimuth from the north. The recommended ranges of mud weight along the sections of 12.25" and 8.5" of the highly deviated and horizontal wells were (between 1.46 and 1.58 gm/cm3) without any expected wellbore instability problems. Based on the outcomes of 3D model, it is expected that the wellbore instability issues are most likely to occur in the domes of; Shauiba and Hamma and in formations; Tanuma, Khasib and Nahr-Umr. In contrast the less wellbore instability problems are expected to expose in Rafidya dome. The study presents an appropriate mud window that can be used to design to minimize the wellbore stability problems when new wells will be drilled in Zubair oil field.

A combined approach for estimating horizontal principal stress magnitudes from borehole breakout data via artificial neural network and rock failure criterion

In this paper, a newly proposed approach on horizontal stress estimation from borehole breakout data is presented. In a previous study, a machine learning model was developed, capable of estimating maximum horizontal stress (σH) from breakout data accurately. However, due to the limitation in experimental data, it was difficult to obtain the minimum horizontal stress (σh) reliably. In this study, a series of breakout tests on Hydrostone-TB specimens was carried out to investigate the influence of σh and vertical stress (σv) on breakout geometries, as these two parameters were rarely studied previously. Results revealed that both breakout angular span and depth decrease with increasing σh or σv, although the influence of σh is more significant. Based on experimental results from this paper as well as the literature, nine failure criteria were examined on the prediction accuracy of σh providing the magnitude of σH. Except for one model, all the other eight failure criteria consider the influence of σv, as indicated in experimental findings. However, none of the failure criteria yielded reasonable σh estimations. To overcome this problem, an Artificial Neural Network (ANN) model was developed from the experimental dataset. Once the model was constructed, it was examined against twenty-three field data, and yielded an acceptable average error rate of 15.88% on σh considering the easily accessible breakout data. Then a comparative analysis on σH estimation was performed via a number of approaches, namely, Kriging, ANN, and constitutive modeling. Results revealed that the use of the Mogi-Coulomb failure criterion is the most reliable approach for σH estimation, with an average error rate of 6.82%. Overall, this newly presented ‘ANN’-‘Mogi-Coulomb’ approach to horizontal stress estimation shows reasonable prediction results, which is expected to be improved in future studies by including additional data.

Comprehensive mechanical earth modeling using well data

Mechanical earth model is a numerical representation of the stress state, pore pressure and rock mechanical properties. While the different parameters in the MEM are interconnected in various ways, they fundamentally stem from the same basic set of log measurements. This paper demonstrates a scientific workflow to build a comprehensive well-centric MEM for a well in a carbonate reservoir in Persian Gulf and calibrate it via utilizing the existing data including LOT, MDT, drilling incidents, etc., to minimize the uncertainties related to data limitations. Finally, the calibrated MEM was used for sensitivity analysis of various wellbore trajectories and mud weight/pressure window for planning future wells and safe drilling in this underdevelopment field. Poroelastic modeling along with rigorous calibration indicates strike slip toward normal faulting stress regime. Also, shear wave anisotropy analysis results in N40E orientation for the maximum horizontal stress. The optimum mud weight for safe drilling was found to be 120 pcf for Gachsaran Formation, 105 pcf for all the formations from Asmari to Kazhdumi limestone and 85 pcf for all the formations from Yamama to total depth. On this basis, it is recommended to put the casing shoes at the base of Gachsaran, Kazhdumi limestone and final depth. The results recommend planning the future wells horizontally in the NE–SW direction for formations having SS stress regime and wells near vertically in formations having NF stress regime. Besides, the drilling safe mud window resulting from Mogi–Coulomb failure criterion is broader, while the Mohr–Coulomb is more limited.

Determination of low and high mud weight shear failures at borehole walls using the modified 3-D Mohr–Coulomb failure criterion

An important step in borehole stability analysis is to estimate a safe and stable mud weight window. A rock failure criterion is required for this purpose. Mud weight windows define the mud program and the number of casing set points thus improving well control and the economy of the drilling project. The aim of this paper is to derive analytical equations for estimating the values of mud pressure corresponding to low and high mud weight shear failures for various modes of stress state using a modified polyaxial Mohr–Coulomb failure criterion. The results are then compared with those of the Mohr–Coulomb and Mogi–Coulomb failure criteria as well as modified 3D Mohr–Coulomb B failure criterion. Also through some case studies, this paper investigates the applicability and efficiency of this failure criterion in predicting values of mud pressure corresponding to shear failure at the borehole walls.

A cost-effective chemo-thermo-poroelastic wellbore stability model for mud weight design during drilling through shale formations

Drilling through chemically-active shale formations is of special importance due to time-dependent drilling fluid–shale interactions. The physical models presented so far include sophisticated input parameters, requiring advanced experimental facilities, which are costly and in most cases unavailable. In this paper, sufficiently-accurate, yet highly practical, models are presented containing parameters easily-derived from well-known data sources. For ion diffusivity coefficient, the chemical potential was formulated based on the functionality of water activity to solute concentration for common solute species in field. The reflection coefficient and solute diffusion coefficient within shale membrane were predicted and compared with experimental measurements. For thermally-induced fluid flow, a model was utilized to predict thermo-osmosis coefficient based on the energy of hydrogen-bond that attained a reasonably-accurate estimation from petrophysical data, e.g. porosity, specific surface area (SSA), and cation exchange capacity (CEC). The coupled chemo-thermo-poroelastic governing equations were developed and solved using an implicit finite difference scheme. Mogi-Coulomb failure criterion was adopted for mud weight required to avoid compressive shear failure and a tensile cut-off failure index for mud weight required to prevent tensile fracturing. Results showed a close agreement between the suggested model and experimental data from pressure transmission tests. Results from a numerical example for a vertical wellbore indicated that failure in shale formations was time-dependent and a failure at wellbore wall after 85 min of mud–shale interactions was predicted. It was concluded that instability might not firstly occur at wellbore wall as most of the conventional elastic models predict; perhaps it occurs at other points inside the formation. The effect of the temperature gradient between wellbore and formation on limits of mud window confirmed that the upper limit was more sensitive to the temperature gradient than the lower limit.

Analytical modelling of coal failure in coal seam gas reservoirs in different stress regimes

Coal seam gas (CSG) reservoirs typically have low rock strength. During gas production, pressure depletion and matrix shrinkage may cause the differential stress around the wellbore to exceed the coal mechanical strength and result in rock failure. Coal failure has several detrimental consequences including coal fines production, permeability reduction, wellbore filling and damage to pumps and compressors. The matrix shrinkage causes a unique stress path in CSG reservoirs. However, the details of how matrix shrinkage affects coal failure still have remained uncertain. This paper presents a new workflow to evaluate stress distribution around CSG wells and predicts coal failure by coupling the effects of pressure depletion, matrix shrinkage and wellbore simultaneously. The model calculates Maximum Coal Free Drawdown Pressure (MCFDP) by considering the effects of all contributing parameters and Mogi-Coulomb failure criterion. Data from a vertical well in the San Juan Basin in USA were used to evaluate the validity of the developed model. The developed model was applied to evaluate coal failure under three different stress regimes. The results indicate that pressure depletion and matrix shrinkage have a significant effect on coal failure in all stress regimes. In the case of a normal stress regime, it is found that vertical wellbores are the most stable during the life of a reservoir. However, in the case of strike-slip and reverse stress regimes, pressure depletion and matrix shrinkage could cause the change of stress regime and therefore, the optimum wellbore trajectory could change. Additionally, it is found that in the normal stress regime the depletion and matrix shrinkage reduces the MCFDP of horizontal wellbores more than the vertical wells. However, in the reverse stress regime, depletion and shrinkage cause more reduction of MCFDP in vertical wellbores compared to horizontal wells.

Strength and failure characteristics of jointed marble under true triaxial compression

The rock structure and three-dimensional stress state play a vital role in the mechanical behaviour of rock masses. Here, a series of true triaxial compression tests (σ1 > σ2 > σ3) are conducted on jointed marble (50 × 50 × 100 mm3) containing a natural stiff joint, taken from the China Jinping Underground Laboratory (CJPL-II) project. The purposes of this study are to investigate the joint effect and estimate the stress dependency of jointed marble. The test results show that jointed marble can fail in four distinct forms, namely, splitting or shearing of intact marble, opening of the joint or sliding along the joint, and these failure modes are influenced by the joint configuration and the minimum and intermediate principal stresses. Generally, jointed marble has more brittle post-peak behaviour than intact marble. The linear Mogi-Coulomb failure criterion can be modified to describe the strength of the jointed marble under true triaxial compression. The jointed marble strength is more sensitive to the minimum principal stress than to the intermediate principal stress. A maximum decline of 25% in strength is observed, which corresponds to a joint dip angle of 60° at σ2 = 60 MPa and σ3 = 30 MPa. The link between the experimental results and in situ fracturing at CJPL-II is also demonstrated.

An analytical solution to wellbore stability using Mogi-Coulomb failure criterion

Abstract Deep wellbores/boreholes are generally drilled into rocks for oil and gas exploration, monitoring of tectonic stresses purposes. Wellbore and tunnel in depth are generally in true triaxial stress state, even if the ground is under axisymmetric loading condition. Stability of such wellbores is very critical and collapse of wellbore must be avoided. Mogi-Coulomb failure criterion is a better representation of rock strength under true triaxial condition. In this paper, an analytical solution is proposed using Mogi-Coulomb failure criterion. The solution is obtained for rock mass exhibiting elastic-perfectly plastic or elastic-brittle-plastic behaviour considering in-plane isotropic stresses. The proposed solution is then compared with exact analytical solution for incompressible material and experimental results of thick-wall cylinder. It is shown that the results obtained by the proposed analytical solution are in good agreement with the experimental results and exact analytical solution. A reduction of about 13%–20% in plastic zone from the proposed closed-form solution is observed, as compared to the results from the finite element method (FEM) based Mohr-Coulomb criterion. Next, the influences of various parameters such as Poisson's ratio, internal pressure (mud weight), dilation angle, and out-of-plane stress are studied in terms of stress and deformation responses of wellbore. The results of the parametric study reveal that variation in the out-of-plane stress has an inverse relation with the radius of plastic zone. Poisson's ratio does not have an appreciable influence on the tangential stress, radial stress and radial deformation. Dilation angle has a direct relation with the deformation. Internal pressure is found to have an inverse relation with the radial deformation and the radius of plastic zone.

Critical Conditions for Coal Wellbore Failure During Primary Coalbed Methane Recovery: A Case Study from the San Juan Basin

Wellbore failure/collapse is a common problem encountered during coalbed methane (CBM) production. The wellbore failures lead to significant production of coal fines, reductions in well productivity, and downhole accidents. These negative outcomes make it essential to predict the conditions responsible for the failure of wellbores and to know when control measures are needed. This study proposed a semi-analytical model of coal wellbore stability during primary CBM recovery. The model incorporates the effects of sorption-induced strain, horizontal stress anisotropy, reservoir depletion, and well trajectory. Plane-strain poroelasticity and the Mogi–Coulomb failure criterion were applied to estimate critical drawdown and depletion for coal wellbore failure. A real field case from the San Juan Basin was simulated to validate the applicability of the proposed model. Wellbore failure-free operating envelopes were then generated for the life span of the field. The effects of sorption-induced strain and well trajectory on wellbore stability were analyzed, and the onset conditions for local wellbore failure and reservoir-scale coal failure were compared. Results revealed that the critical reservoir pressure (CRP_1) for local wellbore failure exceeded the pressure (CRP_2) for field-scale failure, i.e., local wellbore failure occurred earlier than reservoir failure. The CRP_1 value predicted using the model aligned well with actual field observations, thus verifying the model. The desorption-induced shrinkage effect inhibited wellbore failure when radial stress was the minimum principal stress; however, the shrinkage effect enhanced wellbore failure when radial stress was the intermediate principal stress. The optimal well trajectory for the San Juan Basin occurred at the inclination angle of approximately 20° and azimuth of 90°; and the CRP_1 had its minimum value at this trajectory. This study can help optimize field production plans, well trajectories, and well completion to mitigate coal wellbore instability.

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.

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.

New empirical failure criterion for shale

A new failure criterion was presented to predict the ultimate strength of shale under the triaxial and polyaxial state of stress. A database containing 93 datasets was obtained from the results of the uniaxial, triaxial, polyaxial compressive tests, an indirect tensile test was collected from reliable references, and this test was carried out on the shale samples taken from the southwestern oilfields in Iran. The database was used to evaluate the proposed criterion, and its accuracy was compared against the popular failure criteria in rock mechanics, particularly those used for stability analysis such as the Hoek-Brown, Mohr-Coulomb, Drucker-Prager, and Mogi-Coulomb failure criteria. In order to evaluate the model, seven important statistical indices were selected. Subsequently, curves from various failure criteria were fitted to the triaxial and polyaxial data, and the corresponding coefficients and statistical indices were determined. The results obtained indicated that, in all cases, compared to the other failure criteria, the proposed criterion succeeded to predict the ultimate strength at a higher accuracy. Also the proposed criterion was used calculate the uniaxial compressive and tensile strengths with a minimum error. For a further examination of the proposed criterion, a series of results from the triaxial test including the ductile failure data were utilized for evaluation of the applicability of the proposed criterion to the ductile zone. It showed that the criterion could predict the ultimate strength of shale over a wide range of stresses.

Failure Characteristics of Granite Influenced by Sample Height-to-Width Ratios and Intermediate Principal Stress Under True-Triaxial Unloading Conditions

The failure modes and peak unloading strength of a typical hard rock, Miluo granite, with particular attention to the sample height-to-width ratio (between 2 and 0.5), and the intermediate principal stress was investigated using a true-triaxial test system. The experimental results indicate that both sample height-to-width ratios and intermediate principal stress have an impact on the failure modes, peak strength and severity of rockburst in hard rock under true-triaxial unloading conditions. For longer rectangular specimens, the transition of failure mode from shear to slabbing requires higher intermediate principal stress. With the decrease in sample height-to-width ratios, slabbing failure is more likely to occur under the condition of lower intermediate principal stress. For same intermediate principal stress, the peak unloading strength monotonically increases with the decrease in sample height-to-width. However, the peak unloading strength as functions of intermediate principal stress for different types of rock samples (with sample height-to-width ratio of 2, 1 and 0.5) all present the pattern of initial increase, followed by a subsequent decrease. The curves fitted to octahedral shear stress as a function of mean effective stress also validate the applicability of the Mogi–Coulomb failure criterion for all considered rock sizes under true-triaxial unloading conditions, and the corresponding cohesion C and internal friction angle φ are calculated. The severity of strainburst of granite depends on the sample height-to-width ratios and intermediate principal stress. Therefore, different supporting strategies are recommended in deep tunneling projects and mining activities. Moreover, the comparison of test results of different σ2/σ3 also reveals the little influence of minimum principal stress on failure characteristics of granite during the true-triaxial unloading process.

A novel model for wellbore stability analysis during reservoir depletion

It is not common to build a geomechanical model for depleted reservoirs as logging and coring are costly and time consuming in such reservoirs. On the other hand, in regular analysis of wellbore stability, the effect of time is completely ignored. As a result, with time there will be some errors in the evaluation of wellbore stability in depleted reservoirs. In order to determine the optimum wellbore trajectory during the reservoir life, it is necessary to have a model which can estimate the rock properties based on the first full logging suite and also consider the depletion effect. In this study, a novel model is proposed which combines a mechanical earth model, borehole circumferential stresses, Mogi-Coulomb failure criteria and simulation of pore pressure variation near wellbore to determine the optimum well trajectory during drilling and production in a condensate offshore reservoir. As a main output from the application of the new model, it was found that the most stable wellbore trajectory changed after 18–27 years of production. This critical output from this study raises the need to consider the possibility of changing the designed well trajectory over the life of the reservoir to maintain wellbore stability and optimize the drilling and production operations.

Development of a mechanical earth model in an Iranian off-shore gas field

Wellbore instability is a quite common event during drilling, and causes many problems such as stuck pipe and lost circulation. It is primarily due to the inadequate understanding of the rock properties, pore pressure, and earth stress environment prior to well construction. This study aims to use the existing relevant logs, drilling, and other data from offset wells to construct a precise mechanical earth model (MEM) describing the pore pressure, stress magnitudes and orientations, and formation mechanical properties of South Pars Gas field. Since the core test data, MDT/XPT data, and LOT/XLOT data were not available to calibrate the developed model, each component of the model was determined using a range of existing methods and relations, and then the wellbore instability was analyzed based on the developed MEM and the Mogi-Coulomb failure criterion. The predicted incidents such as the lost circulation and tight hole were then compared with the caliper log and reported drilling events to determine the consistency of the model. Since the stability analysis based on the developed MEM had the most agreement with the caliper log and reported drilling events, the equations presented by Eaton and Zoback had good estimations of the pore pressure and rock strengths. Also the estimated horizontal stresses were precise enough to enable the constructed MEM to predict the wellbore instabilities. The stress regime in the field of study was strike-slip, which is frequently specified in the industrial technical reports of the studied field. Finally, it was concluded that the Mogi-Coulomb failure criterion minimized the conservative nature of the mud pressure prediction due to the consideration the strengthening effect of the intermediate stress.

Determination of a safe mud window and analysis of wellbore stability to minimize drilling challenges and non-productive time

The instability of a wellbore is still one of the common problems during drilling. The cause of such a borehole failure can often be mitigated by suitably determining the critical mud pressure as well as the best well trajectory. Therefore, we could save the time and the cost of drilling and production significantly by precluding some drilling problems. The main objective of this paper is to apply a geomechanical model based on well data including the in situ stresses, pore pressure, and rock mechanical properties coupled with suitable rock failure criteria in order to obtain a safe mud window and a safe drilling direction. The Mogi–Coulomb failure criterion was used for deriving the failure equations for tensile and compressive failure modes. For comparison, the analysis was also carried out using the traditional Mohr–Coulomb failure criterion. Variations of wellbore inclination and azimuth were also used to recommend upper and lower mud pressure bounds and the most stable borehole orientations. The best trajectory selection for the inclined borehole was also investigated. Furthermore, the effect of drilling mud pressure and wellbore orientation (θ = 0o) and (θ = 90o) on wellbore stability and stress distribution around the wellbore was assessed. The stability model has been applied to a well located in an oilfield in Iran and showed that the new model is consistent with field experience.

Transient thermo-poroelastic analysis of drilling-induced mechanical damage in nonfractured rocks

Permeability variations in reservoirs and around boreholes are of great interest in petroleum engineering due to the fact that they can significantly affect reserve estimates, reservoir development, well production or injection rate, and the likely success of remedial actions of near-wellbore damage. A fully coupled transient thermo-poroelastic concept with and without rock mechanical damage models is employed to evaluate stress distribution and permeability variation around the boreholes and breakouts. The anisotropy concept is applied to permeability, rock modulus, and uniaxial compressive strength using Weibull distribution. The Mogi–Coulomb failure criterion is employed to model breakout initiation and propagation around the borehole. Strain-based stress–permeability equation and stress–permeability models based on the experimental results and correlations are used to evaluate permeability changes near the borehole. Two cases of overbalanced and underbalanced drilling conditions are employed. The simulations for medium with isotropic and anisotropic physical properties result in varied effective stress distribution, breakout geometries, and consequently different distributions of tangential and radial permeability. Generally, in isotropic medium, permeability reduces to less than 70 % of its original value in the overbalanced condition and 80 % in underbalanced operations; however, there are differences between permeability along x and y directions. For anisotropic medium, permeability increases to more than 150 % all over the borehole periphery in the overbalanced operations; moreover, permeability enhancement is essentially larger along maximum in situ springline. In the underbalanced condition, permeability decreases to less than 70 % along minimum in situ stress direction and increases to more than 200 % in a limited area near the borehole along maximum in situ stress springline.

Using different rock failure criteria in wellbore stability analysis

The minimum and the maximum allowable drilling mud pressures for two wellbores are calculated analytically by Mohr–Coulomb, Hoek–Brown and Mogi–Coulomb failure criteria and numerically using a Fish program which is implemented in FLAC. The allowable drilling mud pressures that resulted from these methods are compared with each other and then with the field data: two wellbores in two Iranian oilfields are investigated as case studies to assess the results. Finally the criterion that matches closely field data is identified. It is shown that the minimum allowable drilling mud pressure obtained from the Mogi–Coulomb criterion is close to actual data collected during drilling operation. Hence, this criterion is considered more appropriate for wellbore stability analysis than the other two criteria.

Transient thermo-poroelastic finite element analysis of borehole breakouts

A thermo-poroelastic model which is fully coupled to conductive and convective transport processes is employed to analyze wellbore stability and borehole breakout. Using the two-dimensional finite element method, stress distribution around an open borehole subjected to non-isothermal, non-hydrostatic loading is investigated. Four strength criteria: Mohr–Coulomb, Drucker–Prager, modified Lade, and Mogi–Coulomb, are utilized to the stability of boreholes. Borehole breakout propagation is assumed to occur as successive spalling of thin layers of rock caused by stress concentration and redistribution around boreholes and breakouts. Stabilization of breakouts is examined by deploying Mogi–Coulomb failure criterion for different wellbore fluid pressures and temperatures. To evaluate the influence of various prominent factors on borehole breakouts, a parametric study is conducted. Results indicate that Mohr–Coulomb criterion offers the highest breakout depth, so it underestimates rock strength and requires the highest value of the least mud weight necessary for wellbore stability and Drucker–Prager provides the lowest estimation of the breakout depth. Borehole breakout stabilization is concluded to occur as consequences of reduction in effective tangential stress and increase in confinement effect of radial stress around breakout periphery. Moreover, confinement effect of borehole fluid appears to be more influential in borehole stability than pore pressure effect. Parametric analysis reveals that drilling a borehole with small radii, applying overbalanced fluid pressure condition, and maintaining drilling fluid temperature lower than rock temperature boost the creation of a favorable borehole stability condition.