Safe Mud Window Research Articles

1D Geomechanical Model and Wellbore Stability Analysis for Kafr El-Sheikh Formation at Sapphire Field, West Delta Deep Marine, Egypt

The Messinian Abu Madi Formation considered the most prospective reservoir target in the Nile Delta, is situated under the Pliocene Kafr El-Sheikh Formation, where most non-productive time takes place. Nevertheless, reaching the desired reservoir presents a greater level of complexity than first perceived. To mitigate the occurrence of significant non-productive time caused by wellbore instability problems, including stuck pipes, tight holes, pack-offs, and jarring or fishing operations, there has been an increased focus on geomechanical analysis. Therefore, this work aims to offer a comprehensive 1D mechanical earth model for a specific set of wells situated in the sapphire field, which is situated inside the West Delta Deep Marine region. The mechanical parameters of the rock were determined by the use of empirical equations, since the absence of core sample data necessitated reliance on wireline data. The analysis of the borehole image has provided clarification that the orientation of the maximum horizontal stress is aligned with the NNE-SSW direction. The calibration of this model was conducted using data obtained from wellbore stability events that occurred during the drilling operations of these wells. This model would be valuable in forecasting and minimizing prospective problems by predicating the proper safe mud window. Sapphire-Da, the primary key, is utilized in this particular piece of writing.

Building 1D Mechanical Earth Model for Subba Oilfield in Iraq

At the Subba oil field, wellbore stability is the main concern while drilling. The wellbore's instability causes several issues, including: (inefficient hole cleaning, tight hole, stuck pipe, mud losses, caving, bad cementing, and well kick or blowout). This increases Non-Productive Time (NPT) and well-drilling costs; hence the operator's main goal is to create a drilling program that reduces these problems and therefore reduce drilling cost. The study aims to build a 1D mechanical earth model to predict the wellbore failure and design optimum mud weight to improve the drilling efficiency for future wells. The model includes pore pressure, stress state, and rock mechanical parameters (such as UCS, angle of friction, Young-Modulus, and Poisson-Ratio). To achieve this aim, the study utilised offset well data including log data (Gama-Ray Logs (GR), Caliper Logs (CALI), Density Logs (RHOZ), and Compressional Sonic (DTCO) and Shear Sonic (DTSM)), Core tests, Mini-frac field tests, Drilling Reports, Mud Reports, and Mud Log Reports (master log) to estimate and calibrate the profiles of formation pore pressure, rock mechanical properties, and in-situ stresses. The 1D mechanical earth model was built using the Excel program for three wells data set, where all the necessary parameters to create the model was calculated, calibrated for the calculated variables with core data and pressure test points, and finally the safe mud window was detected. The results showed that the Eaton Slowness method to predict pore pressure perfectly matches the pressure test points. The most common fault regimes in the Subba oilfield are normal and strike-slip faults. The Modified Lade criteria showed a compatible match with drilling events and calliper log in predicting the failure zones, so it is the best criterion in determining minimum and maximum mud weight. Based on the results of this study and in comparison with the mud window used in drilling operations in the field, it is necessary to change the mud window used in drilling and adopt the safe MWW of this study in drilling new wells in this field and the area adjacent to the field.

Analyzing in situ stresses and wellbore stability in one of the south Iranian hydrocarbon gas reservoirs

This study aims to analyze in situ stresses and wellbore stability in one of the Iranian gas reservoirs by using well log data, including density, sonic (compressional and shear slowness), porosity, formation micro-image (FMI) logs, modular formation dynamics tester (MDT), and rock mechanical tests. The high burial depth, high pore pressure, and strike-slip stress regime of the field require an optimal design of geomechanical parameters based on an integrated data set consisting of static and dynamic data, which is available for this study. Firstly, poroelastic modulus and vertical stress were calculated. Afterward, the Eaton’s equation was used to estimate pore pressure from well logging data. The geomechanical parameters were also calibrated through the interpretation of image data, the use of the modular formation dynamics tester (MDT), and laboratory rock mechanic tests. Employing poroelastic equations, the lowest and highest horizontal stresses were calculated. It was shown that the maximum horizontal stress and minimum horizontal stress correspond to sigma H and sigma h, indicating the strike-slope fault regime. The findings of this research indicated that the equivalent mud weight (EMW) resulted in 10–13 ppg suitable for the Kangan Formation and 11–14 ppg suitable for the Dalan Formation. Additionally, the well azimuth in the NE-SW direction provided the best stability for drilling the encountered formations. Therefore, the results of this study serve as cost-effective tools in planning adjacent wells in carbonate formations of gas field to predict the wellbore stability and safe mud window.

A hybrid data-driven solution to facilitate safe mud window prediction

Safe mud window (SMW) defines the allowable limits of the mud weights that can be used while drilling O&G wells. Controlling the mud weight within the SMW limits would help avoid many serious problems such as wellbore instability issues, loss of circulation, etc. SMW can be defined by the minimum mud weight below which shear failure (breakout) may occur (MWBO) and the maximum mud weight above which tensile failure (breakdown) may occur (MWBD). These limits can be determined from the geomechanical analysis of downhole formations. However, such analysis is not always accessible for most drilled wells. Therefore, in this study, a new approach is introduced to develop a new data-driven model to estimate the safe mud weight range in no time and without additional cost. New models were developed using an artificial neural network (ANN) to estimate both MWBO and MWBD directly from the logging data that are usually available for most wells. The ANN-based models were trained using actual data from a Middle Eastern field before being tested by an unseen dataset. The models achieved high accuracy exceeding 92% upon comparing the predicted and observed output values. Additionally, new equations were established based on the optimized ANN models’ weights and biases whereby both MWBO and MWBD can be calculated without the need for any complicated codes. Finally, another dataset from the same field was then used to validate the new equations and the results demonstrated the high robustness of the new equations to estimate MWBO and MWBD with a low mean absolute percentage error of 0.60% at maximum. So, unlike the costly conventional approaches, the newly developed equations would facilitate determining the SMW limits in a timely and economically effective way, with high accuracy whenever the logging data are available.

Geomechanical analysis of an oil field: Numerical study of wellbore stability and reservoir subsidence

Geomechanics as the knowledge of rock deformation and stability is an indispensable part of all field development plans. Conducting geomechanical analyses leads to a safer and more efficient operation otherwise different kinds of instability and distortion might occur. In this study, the geomechanical behavior of Ilam and Sarvak formations of an oil field in southwest of Iran was investigated. The research objectives can be summarized as wellbore stability evaluation and predicting the value of reservoir subsidence due to pressure drop as a result of reservoir fluid production. To fulfill these, a set of petrophysical logs run in the exploration well of this green field were collected. Next, using empirical correlations and statistical methods, required data for evaluating wellbore stability during drilling, specifying safe mud window to discover reservoir breakdown pressure, predicting the possibility of wellbore collapse in field lifetime, and assessing reservoir subsidence were determined. The results revealed that the average subsidence value as the consequence of production within 21 years is 0.275 ft Which is not significant. In terms of wellbore stability, it was concluded that all horizontal and vertical wells remain stable during this time period. Briefly to conclude, field development is not associated with alarming incidents from geomechanical aspect.

A robust approach to pore pressure prediction applying petrophysical log data aided by machine learning techniques

Determination of pore pressure (PP), a key reservoir parameter that is beneficial for evaluating geomechanical parameters of the reservoir, is so important in oil and gas fields development. Accurate estimation of PP is also essential for safe drilling of oil and gas wells since PP data are used as the input for safe mud window determination. In the present study, empirical equations along with machine learning methods, namely random forest algorithm, support vector regression (SVR) algorithm, artificial neural network (ANN) algorithm, and decision tree (DT) algorithm, are employed for PP prediction applying well log data. To this end, 2827 data records collected from three wells (Well A, Well B, and Well C) drilled in one of the Middle East oil fields are used. The dataset of Wells A and B is used for models’ training, validating, and testing, while Well C dataset is applied for evaluating the models’ generalizability in PP prediction in the field under study. To construct the predictive algorithms, 12 input variables are initially considered in the study. A feature selection analysis is conducted to find the most influential input variables set for developing PP predictive models. The results obtained suggest that the 9-input-variable set is the most efficient combination of inputs used in the ML models construction. Among all the four ML algorithms proposed, the DT algorithm presents the most accurate predictions for PP, delivering R2 and RMSE values of 0.9985 and 14.460 psi, respectively. Furthermore, the model generalization analysis results reveal that the 9-input-variable DT model developed can be used for PP prediction throughout the field of study since it presented an excellent accuracy performance in predicting PP when applied to Well C dataset.

An integrated geomechanics approach to evaluate and manage wellbore stability in a deep graben formation in Tarim Basin

An ultra-deep exploration well has been drilled in the Tarim Basin, northwest of China, with an Ordovician target zone is at a depth of over 7500 m. The Ordovician formation is within a graben structure. Extensive bedding planes and natural fractures develop in the target formation according to seismic analysis and image logging data from several reference wells, indicating a high risk of borehole instability or other complications while drilling through the fragmentized Ordovician zone. To mitigate potential borehole instability problems and associated nonproductive time during drilling the well, an integrated geomechanical study is conducted that combines geological, seismic, logging, and drilling data. The pore pressure, collapse pressure, and fracture pressure of the well are estimated and a safe mud window is provided to guide drilling operations in the Ordovician graben formation. A fracture development index is also calculated to locate the fracture distribution zones. The factors affecting wellbore stability are analyzed by establishing a discrete element model. With the guidance of this study, the well was successfully drilled and the stability of the wellbore was well controlled during drilling in the fractured formation.

Evaluation of Mud Weight Using Safe Mud Window Concept Based on Well Log Data: A Case Study of Well OP-002 in the North Sumatra Basin Area, Indonesia

In the drilling operation of well OP-002 which is located in the North Sumatra Basin at a depth interval of 2887 - 3186 m occurred partial loss, and caving at a depth interval of 500 - 1650 m, where the drilling problem is caused by the use of inappropriate mud weight. Safe mud window analysis is carried out by processing well log data to build PPFG (Pore Pressure Fracture Gradient) and 1D Geomechanics model using several calculation methods. Furthermore, the results of the calculation of pore pressure and fracture gradient are validated with well test data from the well OP-002, so the safe mud window can be determined, and can be used as a basis in the analysis of the drilling problems that occur. The optimum mud weight can minimize wellbore instability, with a limit value that must be greater than the collapse pressure, but not exceeding the minimum insitu stress limit. From the results of the mud safe window analysis, it can be concluded that at a depth interval of 500 - 1650 m caving occurs, because the density value used is smaller than the shear failure gradient, and at a depth interval of 1619 - 2829 m, the density value used is greater than Shmin. To overcome this problem, a mud wight with a safe mud window concept is recommended, namely the selection of the optimum mud weight to be used must be greater than the pore pressure and shear failure gradient and does not exceed the minimum horizontal stress and fracture gradient values.

Geomechanical model and wellbore stability analysis utilizing acoustic impedance and reflection coefficient in a carbonate reservoir

One of the most important oil and gas drilling studies is wellbore stability analysis. The purpose of this research is to investigate wellbore stability from a different perspective. To begin, vertical stress and pore pressure were calculated. The lowest and maximum horizontal stress were calculated using poroelastic equations. The strike-slip to normal fault regime was shown by calculated in situ stress values. The 1-D geomechanical model was utilized to investigate the failure mechanisms and safe mud window estimation using the Mohr–Coulomb failure criterion. Using density and sonic (compressional and shear slowness) logs, the acoustic impedance (AI) and reflection coefficient (RC) logs were determined subsequently. The combination of layers with different AI indicates positive and negative values for the RC, zones prone to shear failure or breakout, and the mud weight in these zones should be increased, according to the interpretation of the AI and RC readings and the results of the geomechanical model. Furthermore, the zones with almost constant values of AI log and values close to zero for RC log are stable as homogeneously lithologically, but have a lower tensile failure threshold than the intervals that are sensitive to shear failure, and if the mud weight increases, these zones are susceptible to tensile failure or breakdown. Increased porosity values, which directly correspond with the shear failure threshold and inversely with the tensile failure threshold, cause AI values to decrease in homogenous zones, but have no effect on the behavior of the RC log. This approach can determine the potential zones to kick, loss, shear failure, and tensile failure in a short time.

Mud Weight Evaluation Based on Safe Mud Window in Drilling Well “X-1” to Overcome Caving and Partial Loss Problems in The Oil Field

Drilling of Well X-1 in the North Sumatra Basin at a depth of 2887-3186 m TVD occurred partial loss, and caving at a depth of 500-1650m TVD. To overcome this problem, it is necessary to use the safe mud window concept. Drilling mud density planning must be greater than the pore pressure and shear failure gradient but not more than the minimum horizontal stress and fracture pressure. The purpose of this paper is to make an accurate subsurface pressure analysis and to overcome problems caused by the mud weight planning errors used based on the safe mud window, and can be used as a reference for further drilling of wells that have field conditions and stratigraphy such as the Well X-1. In conducting safe mud window analysis, there are several parameters that need to be estimated in order to make a safe mud window, namely formation pressure, formation fracture pressure, minimum horizontal stress, maximum horizontal stress, vertical stress / overburden pressure, and shear failure gradient. From the results of the safe mud window on Well X-1, the actual mud weight data used during drilling is entered. After being analyzed, at a depth of 500-1050 m TVD caving occurs because the density value used is smaller than the shear failure gradient, while at a depth of 1050-1600m there is kaolinite mineral which causes caving. At a depth of 2829-3281 m TVD the density value is greater than minimum horizontal stress (SHmin). Here, caving occurs if the density value used is smaller than shear failure gradient and partial loss occurs if the density used is greater than SHmin. Based on the safe mud window, the optimal mud weight for drilling at a depth of 36-354.2 meter on a 20” route is 9.2-9.4 ppg. At a depth of 354.2-948 meter on a 16” route is 14.49-15.33ppg. At a depth of 948-1619 meter on a 13 3/8” route is 15.45-17.65ppg. At a depth of 1619-2829 meter on a 9 5/8” route is 17.36-17.76ppg. At a depth of 2829-3281 meter on a 7” route is 16.57-16,7ppg. And at a depth of 3281-3796.1 meter is 13.49-13.74ppg in order to avoid partial loss and caving problems.

The impact of geomechanical units (GMUs) classification on reducing the uncertainty of wellbore stability analysis and safe mud window design

The current study proposed an improved approach for application of Geomechanical Units (GMUs) classification in reducing the uncertainty of wellbore stability models. For this purpose, the optimal drilling direction and safe mud window were modeled in the Iranian Offshore oilfield, Persian Gulf basin. Two main normal faults created a graben with three geomechanical sectors in the study area including northern, middle and southern sectors. The middle sector, which is under high compression, is the main target of this study. A set of well logs and Fullbore Formation Micro Imager (FMI) data associated with the rock mechanical tests including Uniaxial Compressive Strength (UCS), Triaxial Compressive Strength (TCS) and ultrasonic results were used to establish and tune the geomechanical models. The dynamic rock elastic moduli including UCS, Poisson's ratio, Young's modulus, shear modulus and bulk modulus were estimated from well logs data. Through appropriate correlations between well logs and laboratory results, the dynamic modules were converted to static data. Five geomechanical units were obtained based on the classification of rock elastic moduli by using multi-resolution graph-based clustering.The interpretation of FMI logs and their incorporation in the geomechanical models showed that most safe drilling direction is in the 120° azimuth which is parallel to the direction of minimum horizontal stress, σh (NW-SE direction). Higher mud weight is expected at N030E azimuth to overcome collapse pressure, which is the worst drilling direction, σH (NE-SW direction). Wellbore inclination more than 60° angle is not recommended for formations comprising of GMU E and partially GMU D (mainly shales and clayey limestones). The results of this study showed that GMUs classification is an effective way of updating the wellbore stability models and reducing the uncertainty of safe mud window design.

Quantitative risk assessment for wellbore stability analysis using different failure criteria

Uncertainties in geomechanical input parameters which mainly related to inappropriate data acquisition and estimation due to lack of sufficient calibration information, have led wellbore instability not yet to be fully understood or addressed. This paper demonstrates a workflow of employing Quantitative Risk Assessment technique, considering these uncertainties in terms of rock properties, pore pressure and in-situ stresses to makes it possible to survey not just the likelihood of accomplishing a desired level of wellbore stability at a specific mud pressure, but also the influence of the uncertainty in each input parameter on the wellbore stability. This probabilistic methodology in conjunction with Monte Carlo numerical modeling techniques was applied to a case study of a well. The response surfaces analysis provides a measure of the effects of uncertainties in each input parameter on the predicted mud pressure from three widely used failure criteria, thereby provides a key measurement for data acquisition in the future wells to reduce the uncertainty. The results pointed out that the mud pressure is tremendously sensitive to UCS and SHmax which emphasize the significance of reliable determinations of these two parameters for safe drilling. On the other hand, the predicted safe mud window from Mogi-Coulomb is the widest while the Hoek–Brown is the narrowest and comparing the anticipated collapse failures from the failure criteria and breakouts observations from caliper data, indicates that Hoek-Brown overestimate the minimum mud weight to avoid breakouts while Mogi–Coulomb criterion give better forecast according to real observations.

Wellbore Stability Analysis in Oil and Gas Drilling by Mechanical, Chemical and Thermal Coupling (Case Study in the South of Iran)

Inadequate wellbore stability has always been the most important issue during drilling and can cause many drilling complications. The most important tools for studying this phenomenon are the use of geomechanical modeling and failure criteria. Drilling fluid temperature and chemical interactions between drilling fluid and wellbore are also significant factors in wellbore stability. Decreasing the fluid temperature changes the concentration of induced stresses around the well, influencing the wellbore's safe mud window. This study considered a vertical well drilled in a field in southwest of Iran to determine the mud window and find out the optimal path under different conditions. The upper and lower bound of the mud window were calculated separately and simultaneously with three criteria, namely Mohr–Coulomb Failure, Hoek–Brown, and modified Lade. In the modified Lade failure criterion that gives more realistic results, the lower bound under the initial conditions is 7.81–10.51 ppg; by applying the thermal effect, it is 7.68–10.48 ppg, and with a fluid chemistry effect, it is 8.47–10.69 ppg. Given these two factors simultaneously, the lower bound of the mud window changed to 8.34–10.66 ppg. The optimal path in all failure criteria is horizontal drilling of well and maximum in horizontal stress direction. Furthermore, the upper bound of the mud window under mechanical conditions was 21.36–31.13 ppg, and with the effect of temperature was reduced to 21.09–30.84 ppg. Moreover, after applying the chemical effect, it increased to 21.29–31.05 ppg and converted to 21.02–30.76 ppg simultaneously.

Three-dimensional numerical solution and stress cage analysis of high conductive fractures pressure sealing

Aiming at the main problems in the design and analysis of well wall strengthening for fractured formations, a three-dimensional (3D) numerical simulation scheme is proposed. The three-dimensional finite element software is used to analyze the mechanical behavior of fractures and the pressure sealing process, and evaluate the stress cage effect. The main features of the model are as follows: (1) The equivalent fractures in the analytical model represent the function sum of the mechanical behavior of all fractures on the well wall, which is a functionally equivalent crack.(2) When evaluating the stress cage effect, the shape of the crack wedge filled with the plugging agent particles is formed by simulating the fractures opening process under the injection pressure, not a given regular shape.(3) In the model of calculating bull heading of block agent, the liquid pressure on the well wall is the injection pressure, which is a variation increased with time. The fluid pressure on the well wall in the stress cage calculation model is generated by the initial pore pressure. (4) The numerical evaluation of the stress cage effect is achieved by calculating the increase amplitude FX of the minimum hoop stress on the well wall. Using this model, several sets of injection pressure design values can be used for pressure plugging numerical simulation and stress cage effect evaluation calculation, and then the optimal and accurate quantitative value of “injection pressure, block agent particle size, safe mud window upper bound” are found through comparison. Finally, through an engineering example of horizontal well drilling pressure sealing in a shale gas reservoir developed by a fracture, we use the above theoretical tools to introduce the process and results of the numerical analysis of the extended mud window drilled in the shale gas fissure reservoir.

Identification of stress regimes by using 1D geomechanical modeling of clastic and carbonate reservoirs

In this paper, data from one well is used for pore pressure prediction and geomechanical modeling in clastic and carbonate reservoirs. Pore pressure is successfully predicted in the clastic and carbonates reservoirs using trend line method. Geomechanical models showed that the direction of maximum horizontal stress is north–south, perpendicular to the direction of borehole breakout propagation. Change in magnitude of principle stresses confirmed the presence of different stress regimes in the study area. From the results of this study, it is inferred that pore pressure prediction and geomechanical modeling will help to reduce the borehole failure problems and determine safe mud window.

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.

Geomechanical Analysis to Avoid Serious Drilling Hazards in Zubair Oilfield, Southern Iraq

Zubair oilfield is an efficient contributor to the total Iraqi produced hydrocarbon. Drilling vertical wells as well as deviated and horizontal wells have been experiencing intractable challenges. Investigation of well data showed that the wellbore instability issues were the major challenges to drill in Zubair oilfield. These experienced borehole instability problems are attributed to the increase in the nonproductive time (NPT). This study can assist in managing an investment-drilling plan with less nonproductive time and more efficient well designing.
 To achieve the study objectives, a one dimension geomechanical model (1D MEM) was constructed based on open hole log measurements, including Gamma-ray (GR), Caliper (CALI), Density (RHOZ), sonic compression (DTCO) and shear (DTSM) wave velocities , and Micro imager log (FMI). The determined 1D MEM components, i.e., pore pressure, rock mechanical properties, in-situ principal stress magnitudes and orientations, were calibrated using the data acquired from repeated formation test (RFT), hydraulic fracturing test (Mini-frac), and laboratory rock core mechanical test (triaxial test). Then, a validation model coupled with three failure criteria, i.e., Mohr-Coulomb, Mogi-Coulomb, and Modified lade, was conducted using the Caliper and Micro-imager logs. Finally, sensitivity and forecasting stability analyses were implemented to predict the most stable wellbore trajectory concerning the safe mud window for the planned wells.
 The implemented wellbore instability analysis utilizing Mogi-Coulomb criterion demonstrated that the azimuth of 140o paralleling to the minimum horizontal stress is preferable to orient deviated and horizontal wells. The vertical and slightly deviated boreholes (1ess than 30o) are the most stable wellbores, and they are recommended to be drilled with 11.6 -12 ppg mud weight. The highly deviated and horizontal wells are recommended to be drilled with a mud weight of 12-12.6 ppg.

Determination of a safe drilling mud window and optimal drilling mud pressure by shear and tensile failure criteria

Wellbore drilling is the main activity to access oil reservoirs for oil production. This study aimed at presenting a safe drilling mud window using two criteria for estimating the minimum and maximum drilling mud pressure in order to prevent tensile and shear failure in oil wells. To this end, data on oil wells was first collected. To calculate the lower limit of the mud window, the oil well was modeled at different drilling mud pressures with the help of the finite difference software FLAC2D. By plotting the normalized yield zone area (NYZA) versus the mud pressure, NYZA = 1 was considered as the shear failure threshold, the corresponding pressure to which was considered as the lower limit for the drilling mud weight. To calculate the upper limit of the mud window, the wellbore was modeled with the help of the finite element software, Abaqus. To this end, the drilling mud pressure was gradually increased, and according to the increase in the crack length in the borehole, the pressure causing tensile failure was considered as the upper limit of mud weight. Ultimately, two linear relationships were proposed using the multivariate linear regression analysis in SPSS. In the first relationship, the minimum drilling mud pressure to prevent a shear failure is estimated using pore pressure, minimum and maximum horizontal stresses, cohesion, and internal friction angle. The second relationship estimates the maximum drilling mud pressure to prevent a tensile failure using pore pressure, minimum and maximum horizontal stresses, and maximum and minimum tensile strengths. Combining these two relationships, a new correlation was obtained to estimate the optimal drilling mud pressure. As its main innovative aspect, this study aimed at providing a safe window for drilling mud using two criteria for estimating the minimum and maximum drilling mud pressures in order to prevent shear and tensile failures in oil wells in south Iran oilfields.

Evaluation of mechanical, chemical, and thermal effects on wellbore stability using different rock failure criteria

During drilling operation, the occurrence of borehole instability is one of the major problems in which many parameters can affect the wellbore stability. In many cases, wellbore instability is because of a combination of mechanical and chemical instabilities. This problem may lead to complicated states, and in some cases, can cause costly operational problems (e.g. lost circulation, sidetracks, and pipe sticking). Therefore, choosing an optimum rock failure criterion for predicting safe mud window based on all mechanical, chemical and thermal effects would be of great interest to industry. The goal of this study is to comparatively evaluate six practical rock failure criteria such as modified Lade, Mogi‒Coulomb, Mohr‒Coulomb, three-dimensional (3D) Hoek‒Brown, Drucker‒Prager, and modified Wiebols‒Cook criteria considering all mechanical, chemical, and thermal effects. Through the obtained results, it is possible to determine which model is more accurate to be used in unconventional formations such as shale to effectively plan future drilling operations in the field to improve drilling efficiency and future field development. To determine the impact of poroelastic, thermal, and chemical effects on shale stability, three variables including critical mud weights, collapse stress, and pore pressure were examined. In addition, this study investigated the effects of temperature variation, changes in mud salinity, and cohesion alteration on the determination of safe mud window to stabilise the borehole in shale formations using the six rock failure criteria. The results show that all these parameters had a significant effect on the minimum mud pressure predicted by various rock failure criteria and should be considered in addition to other parameters. Lastly, the models suggested the safe mud pressure window for drilling the reservoir safely with and without considering of all mechanical, chemical and thermal effects and represented the results using stereographic projection. Also, the models represent that drilling to the orientation of minimum horizontal stress can minimise the wellbore instabilities. The results of this study can be employed for further improvement in drilling operations, openhole completions, and hydraulic fracturing treatments.

Distribution of pore pressure and fracture pressure gradients in the paleozoic sediments of Takouazet field, Illizi basin, Algeria

A comprehensive pore pressure and fracture gradient (PPFG) characterization is a basic requirement for subsurface geomechanical modeling. This study deploys indirect approaches to estimate subsurface pressure profiles in the Mesozoic and Paleozoic successions of eastern Illizi basin, Algeria. Amoco exponent has been modeled for pseudo density generation and overburden gradient has been deciphered from a composite density profile. PP calculated from sonic and resistivity logs have been calibrated and validated with direct downhole measurements. Results indicate normal pore pressure regime in the Mesozoic and Devonian sediments. Disequilibrium compaction induced abnormal pore pressure in Silurian marine shales with a maximum gradient of 12.7 PPG. Top of geopressure has been marked at Silurian top (2050m). Pore pressure drops sharply from Silurian to Ordovician sediments, across the glacial unconformity (2332 m). Fracture gradients have been interpreted by Eaton's Poisson's ratio based model and Mathews & Kelly's effective stress ratio based approach. Based on the vertical distribution of subsurface pressure, effective vertical stress, a safe mud window has been recommended for optimum drilling fluid design.