Wellbore Stress Research Articles

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

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

Transient thermoporoelastic model under local thermal non-equilibrium

Wellbore stability is a key challenge for oil and gas industry since it adds a great deal of additional cost to the industry. Traditional wellbore stability models such as elastic and poroelastic models may not produce reliable stress field around a wellbore since they consider isothermal wellbore condition. During the drilling phase, the drilling fluid temperature is different from the formation temperature due to geothermal gradient and circulation of the fluid inside the wellbore. Therefore, the assumption of isothermal condition will not predict the correct wellbore stability condition, especially for high pressure and high temperature wells. Hence, consideration of a thermoporoelastic model in stress estimation is more appropriate. Although there are several thermoporoelastic models in the literature, they employ the assumption of local thermal equilibrium (LTE) in their modeling. However, thermal properties of phases in a porous medium are rarely similar. This dissimilarity evokes the validation of LTE assumption. The local thermal non-equilibrium assumption ignores additional pore and thermal stresses in the porous medium caused by temperature variations of the fluid and solid phases. Under pure heat conduction, LTE is valid if thermal boundary conditions in a medium is constant or thermal conductivities of phases are similar according to Vadasz (2005) 1. Hence, using a local thermal non-equilibrium (LTNE) approach is required to establish each phase temperature, and update thermoporoelastic model accordingly. In this paper, the effects of induced thermal stresses on wellbore stress resolution and LTNE on thermoporoelastic model around a wellbore during drilling phase are investigated in low and high permeable formations. The LTNE thermoporoelastic model developed herein is a transient model considering conductive and convective heat transfers.

Contemporary tectonic stress pattern of the Persian Gulf Basin, Iran

The contemporary crustal stress pattern is important to several aspects of earth sciences such as reservoir geomechanics, neotectonic deformation and geodynamic modelling. The Persian Gulf Basin is a petroliferous region in southern Iran that hosts hundreds of petroleum wells and, hence, the knowledge of the present-day stress is critical for any exploration and production in this region. In addition to geo-resources, this region has a great economic importance and the in-situ stresses provide key insights on neotectonic and seismic hazard of the basin. However, there is limited in-situ stress information in the Persian Gulf region. In this study, we analyse >8 km of borehole image logs in 20 wellbores in the Persian Gulf in order to provide a clear picture of the stress pattern in this region. Our wellbore stress analyses provide 35 quality-ranked maximum horizontal stress orientations (SHmax) data records. In addition, we compile 27 earthquake focal mechanisms in the study region from different seismic catalogues and published literature, and present the first comprehensive quality-ranked in-situ stress map of the Persian Gulf Basin with 62 data records, which can be used as a valuable source of information for future exploration and seismic hazard assessment. The resulting database suggests that the SHmax azimuth in central and western Persian Gulf is NE-SW (parallel to the Arabia-Eurasia tectonic plate convergence), which demonstrate the key role of relative plate motion in the present-day stress pattern of these areas. However, the SHmax azimuth in the eastern part of the study area is NW-SE, which is different from the central and western Persian Gulf. The NW-SE SHmax azimuth is consistent with other geological and geophysical observations that suggest an indentor model for Musandam Peninsula (Oman) in the Zagros-Makran syntaxis.

Breakdown pressure prediction with weight function method and experimental verification

Accurate estimation of fracture gradient is critical for both drilling and hydraulic fracturing. Fracture gradient by traditional methods is greatly overestimated due to the ignorance of preexisting fractures, nonlinear near wellbore stress concentration acting on fracture surfaces, or non-uniform fluid pressure distribution inside fractures. To overcome the limitations of current methodologies, a weight function method is firstly introduced to calculate breakdown pressure where there are a pair of symmetrical radial fractures collinear with borehole axis emanating from the borehole. The results are compared against both measured breakdown pressure in both field and laboratory and results of classical fracture models. Parametric studies are conducted to investigate the influence of pre-existing crack length, orientation, in-situ stress contrast, fracture toughness, and internal pressure distribution on breakdown pressure. It indicates that breakdown pressure (1) does not always decrease with increasing dimensionless crack length at different stress contrast, (2) for the fracture with non-uniform pressure is higher than the one with constant internal pressure, and (3) increases with increasing absolute value of inclination angle1It means an angle between the local maximum horizontal principal stress and the axis of the fracture.1 and fracture toughness. Comparison of breakdown pressure by traditional and weight function methods demonstrates that breakdown pressure by traditional methods is overestimated. The proven weight function method simplifies the prediction of breakdown pressure, and shows a great potential in improving the accuracy of breakdown pressure prediction because it captures more physical factors during rock fracturing process.

Short-Term and Long-Term Mechanical Models of Wellbores Considering Cement Consolidation and Formation Creep

Summary With oil and gas wells extending deeper and deeper, downhole conditions become increasingly complicated, and thus increasingly sophisticated wellbore models are needed. Current wellbore models usually neglect the coupling effect in the cement–consolidation process and do not sufficiently consider the whole operation process of the wellbore. To overcome these shortcomings, short–term and long–term mechanical wellbore models while considering the relevant stages in wellbore life are built. In the short–term model, wellbore–operation stages include casing running, cement displacement, and cement consolidation. The governing equation of cement consolidation while considering the coupling effect between cement hardening and volume change is presented. In the long–term model, the governing equation of formation creep while considering prestresses and initial strains is given. The elastic/viscoelastic–correspondence principle and stress–superposition method are used to simplify the derivation. Next, the effects of relevant factors on short–term and long–term wellbore stresses are analyzed. The results show that wellbore stresses caused by cement consolidation will be underestimated when the coupling effects are neglected. The most vulnerable positions for wellbore failure are on different cylinder elements under different wellbore stages. Wellbore properties, short–term stresses, and formation creep greatly affect wellbore mechanical behaviors. Therefore, the new model provides an important basis for wellbore–failure prediction and optimal design.

Dynamic Wellbore Stability Analysis Under Tripping Operations

The loadings which act on the wellbore are more frequently dynamic than static, such as the surge/swab pressure caused by tripping operations. The changing rate of loading could induce a change in wellbore stress and result in wellbore instability. The conventional Kirsch solution to calculate wellbore stress is only applicable to the steady state without considering the coupled deformation–diffusion effect. To account for these deficiencies, this paper introduces a coupled poroelastodynamics model to obtain wellbore stress distribution under dynamic loading. The model is solved by the implicit finite-difference method with the surge/swab pressure caused by tripping operations taken as the specific dynamic loading source. Then, failure criteria are applied to analyze dynamic wellbore stability and the hemisphere plots of minimum mud density (MMD) to avoid wellbore collapse are generated. The effect of in-situ stress regimes, failure criteria, and permeable properties on the instability of the borehole has been investigated, and the applicability and accuracy of the conventional failure criteria for dynamic loading conditions have been studied by the extensive triaxial compression tests performed on Bedford limestone under different strain rates. Furthermore, two specific cases have been analyzed and results show that compared with the poroelastodynamics model, the conventional method either overestimates or underestimates the MMD, the difference of which can be as significant as 0.11 g/cm3. Not limited to the tripping operations, the developed poroelastodynamics model can be applied to any specific dynamic loading conditions with the fluid inertia neglected.

Effect of rock anisotropy on wellbore stresses and hydraulic fracture propagation

Using a newly developed 2D numerical model based on the displacement discontinuity method (DDM) and the fictitious stress method (FSM), hydraulic fracture propagation near and away from a horizontal wellbore in anisotropic mudstones is studied. In addition to elastic anisotropy, the effect of fracture toughness anisotropy on hydraulic fracture propagation is included and fluid flow in fractures is fully coupled with rock deformation. Simulations show wellbore failure pressure, fracture trajectory, and near wellbore fracture apertures are significantly affected by elastic and fracture toughness anisotropy. A simple plane strain analytical solution might indicate that near wellbore fracture opening is enhanced in vertically transversely isotropic (VTI) rock. However, near wellbore fracture turning in a rock with high fracture toughness anisotropy results in severe constriction of fracture apertures. This effect becomes more severe with increase in fracture toughness anisotropy and perforation misalignment angle. Simulations of multiple hydraulic fractures from horizontal wellbores suggest that fracture lengths and apertures are affected by their orientation with respect to the directions of elastic symmetry. The spatial extent of the induced normal stresses perpendicular to the fracture surface increases when the fracture is aligned with the direction of least Young's modulus. The larger spatial extent of induced stresses in anisotropic rock leads to early termination of the fractures emanating from the inner perforation clusters, resulting in fracture networks with dominant outer fractures. In the presence of fracture toughness anisotropy, the fractures deviate towards the plane of least fracture toughness under low differential stress conditions.

Dynamic stress distribution around the wellbore influenced by surge/swab pressure

The calculation of dynamic stress distribution around the wellbore under surge/swab pressure is an unsolved issue. The conventional method to calculate wellbore stress is only applicable to the steady state without considering the coupled effect of deformation-diffusion. In this paper, an elastodynamics model and a poroelastodynamics model have been established to overcome these deficiencies. The numerical solution is provided by the implicit finite difference method, and it is verified by the analytical solution achieved from Laplace transform and Crump inversion. Through sensitivity analysis, it shows that the period of the sine function (inner boundary condition), Biot coefficient, hydraulic diffusivity, and rock types could influence the stress distribution. Then the dynamic surge/swab pressure predicted based on Lubinski's method is implemented into these two models. Two field cases with different depths have been analyzed, and it demonstrates that the surge/swab pressure is more significant for a shallow formation because the static formation pressure and in-situ stresses are relatively small. If the tripping velocity is high, the stress distribution will be significantly changed. Finally, the stress calculated by these two models is compared with the stress obtained using the conventional solution. The comparison shows that the poroelastodynamics solution is different from the elastodynamics solution because pore pressure is significant in the poroelastodynamics solution. The poroelastodynamics solution is also significantly distinguishable from the conventional solution, which demonstrates the importance of the poroelastodynamics model. Overall, it fills the gap in calculating dynamic wellbore stress under tripping conditions, which can be applied to wellbore stability analysis and contribute to a reduction in both tripping time and drilling costs.

Organophilic-Clay-Free Invert-Emulsion Fluid Helps Drill Record-Length Well in the UAE

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 186917, “Breaking Records Using an Innovative Organophilic-Clay-Free Invert-Emulsion Fluid To Drill the Longest Well to Date in the United Arab Emirates,” by Mena Nasrallah and Matteo Vinci, SPE, Halliburton, prepared for the 2017 SPE/IATMI Asia Pacific Oil and Gas Conference and Exhibition, Bali, Indonesia, 17–19 October. The paper has not been peer reviewed. This paper discusses the successful design, laboratory testing, and performance of an innovative, low-solids, organophilic-clay-free invert-emulsion fluid (OCF-IEF) used to drill the reservoir section of an extended-reach-drilling (ERD) well. This specially designed drill-in fluid helped maintain the key ERD factors within the specifications necessary and set new limits for drilling performance, thus maximizing the horizontal-section displacement and reservoir drainage and production output. Introduction ERD is an operative practice for drilling high-angle wellbores with long horizontal displacement. Typically, a well design is classified as ERD when the horizontal displacement is at least two times the vertical-displacement true vertical depth (TVD). The longest well drilled in the UAE had a measured depth of 35,800 ft, with 18,800 ft of horizontal displacement and a 7,894-ft TVD (Fig. 1). This technique is considered one of the more technically challenging in the industry. However, because of in-creased reservoir contact, it is generally preferred to conventional well designs. Proper fluid selection is fundamental during the ERD-well-planning phase. Software support is necessary to predict the well conditions and simulate the fluid behavior to maintain the equivalent circulating density (ECD) lower than the fracture gradient, maximize hole cleaning, provide wellbore stability throughout the well execution, and prevent losses. The different drilling parameters used to drill the horizontal section should also be considered during the planning phase to help enable correct simulations and optimize the execution accordingly. Fluid Properties During drill-in-fluid design, formation damage and production rates should be considered while maintaining the optimal mud properties for drilling and rate-of-penetration (ROP) enhancement. A comprehensive testing and validation process was compiled using a systematic approach to customize this specific well design. Clay-free invert-emulsion fluids (CF-IEFs) are designed to provide a much stronger gel structure than traditional IEFs. However, with the application of force, these gels break easily, thus reducing the wellbore stresses and pressure spikes. Progressive yet fragile gel strengths are important elements of a CF-IEF system that allow the cuttings to be suspended while maintaining low ECD and minimal circulation pressures compared with a clay-laden system. The organophilic clays and organophilic lignites are replaced with various organic polymers to achieve different fluid behaviors. Acid solubility to minimize formation damage was another important system-design criterion. This helped minimize formation damage and maintain proper bridging by use of ground marble with the correct particle-size distribution (PSD), determined by specialized software.

Mudcake effects on wellbore stress and fracture initiation pressure and implications for wellbore strengthening

Although a large volume of mudcake filtration test data is available in the literature, effects of mudcake on wellbore strengthening cannot be quantified without incorporating the data into a stress-analysis model. Traditional models for determining fracture initiation pressure (FIP) either consider a wellbore with an impermeable mudcake or with no mudcake at all. An analytical model considering permeable mudcake is proposed in this paper. The model can predict pore pressure and stress profiles around the wellbore, and consequently the FIP, for different mudcake thickness, permeability, and strength. Numerical examples are provided to illustrate the effects of these mudcake parameters. The results show that a low-permeability mudcake enhances FIP, mainly through restricting fluid seepage and pore pressure increase in the near-wellbore region, rather than by mudcake strength. Fluid loss pressure (FLP) should be distinguished from FIP when a mudcake is present on the wellbore wall. Fracture may occur behind the mudcake at FIP without mudcake rupture. The small effect of mudcake strength on FIP does not mean its effect on FLP is small too. Mudcake strength may play an important role in maintaining integrity of the wellbore once a fracture has initiated behind the mudcake.

Stress estimation in reservoirs using an integrated inverse method

Estimating the stress in reservoirs and their surroundings prior to the production is a key issue for reservoir management planning. In this study, we propose an integrated inverse method to estimate such initial stress state. The 3D stress state is constructed with the displacement-based finite element method assuming linear isotropic elasticity and small perturbations in the current geometry of the geological structures. The Neumann boundary conditions are defined as piecewise linear functions of depth. The discontinuous functions are determined with the CMA-ES (Covariance Matrix Adaptation Evolution Strategy) optimization algorithm to fit wellbore stress data deduced from leak-off tests and breakouts. The disregard of the geological history and the simplified rheological assumptions mean that only the stress field, statically admissible and matching the wellbore data should be exploited. The spatial domain of validity of this statement is assessed by comparing the stress estimations for a synthetic folded structure of finite amplitude with a history constructed assuming a viscous response.

Analytical Solution to Study Depletion/Injection Rate on Induced Wellbore Stresses in an Anisotropic Stress Field

During production or injection, the state of stresses within the reservoir as well as around the wellbore changes. It is therefore important to evaluate the impact of the induced stresses on stability of wellbores. This study proposes an analytical solution to estimate the influence of production or injection rate on stresses around a wellbore in an anisotropic stress field. For considering the effect of production/injection rate, pseudo steady state flow was assumed within the reservoir that occurs more frequently than transient or steady state flow in a reservoir with an expanding drainage radius. Until now stress equations have not been developed in which the effect of production/injection rate being included. The results showed that the impact of the depletion and injection rate on induced stresses around the wellbore is significant and should be considered in the geomechanical analysis especially in low permeability reservoirs. Application of the proposed solution on a typical sandstone reservoir showed that in relatively high production rate, the radial and tangential stresses near wellbore decrease whereas the vertical and pore pressure increase. An opposite trend was observed for the effect of injection rate on near wellbore stresses. This results are consistent with field observation and means that in rapid production, the vertical stress will increase and horizontal stresses will decrease leading to surface subsidence.

A numerical study on the feasibility of evaluating CO2injection wellbore integrity through casing deformation monitoring

AbstractDistributed sensing technologies have been increasingly used for downhole monitoring in the oil field in recent years. A casing deformation monitoring system based on distributed strain sensors has been developed and proposed for wellbore integrity monitoring. However, due to the lack of field data on casing deformation induced by various exploration and development activities, the relationship between casing deformation and wellbore integrity remains unclear. To study the feasibility of evaluating short‐term CO2injection wellbore integrity through casing deformation monitoring, a staged finite element analysis is conducted for an existing well in the Weyburn field to study the relationship between casing deformation and wellbore integrity during CO2injection. The model is generated with HypermeshTMand the analysis is conducted with AbaqusTM. The staged finite element model applies loads and materials in multiple steps to replicate the actual drilling process. The wellbore stress, casing strain, and interface gap distance change at the surface casing shoe and production casing shoe are calculated. A parametric study is also conducted to study the effect of various parameters on CO2injection wellbore integrity in the Weyburn field. The result shows that the casing deformation can be correlated to the wellbore integrity by cement expansion/shrinkage through its effect on the interface gap distance change and the stress difference between wellbore components induced by CO2injection. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.

Research on wellbore stress in under-balanced drilling horizontal wells considering anisotropic seepage and thermal effects

Under-balanced drilling (UBD) is widely used in petroleum drilling engineering, and regarding wellbore safety, it is necessary to analyze stress distribution around the wellbore during drilling procedures. Aiming at seepage and thermal effects during UBD, a model of pore pressure distribution under anisotropic seepage of horizontal wells was derived using conformal transformation. Combined with elastic theory, a new model of seepage stress around wellbores has been constructed. Moreover, based on a heat transfer model solved by finite difference methods, this paper studied wellbore and formation temperature fields during horizontal drilling when heat sources (circulating pressure drops and mechanical friction) are considered. This study shows that both pore pressure and seepage stress are present in an elliptical manner under anisotropic seepage. The annulus temperature of the horizontal section is greater than the geothermal temperature, thus compressive thermal stress forms in the surrounding rock; in contrast, tensile thermal stress develops when heat sources are excluded. The two factors, therefore, should be considered sufficiently important. The heat sources with thermal effects and anisotropy of seepage, which are usually omitted, have been treated again, leading to more accurate results of horizontal UBD and providing a theoretical basis for subsequent wellbore stability analysis.

Analytical methodology for wellbore integrity assessment considering casing-cement-formation interaction

A wellbore analysis presents several challenges regarding the drilling, construction and production phases. Owing to that, the cement and casing stress states during the wellbore lifespan are also difficult to predict. Nevertheless, analytical solutions can reach good results. Additionally, once analytical approaches are exhibited as mathematical expressions, they facilitate the assessment of the interaction between variables and the impact on results. On the other hand, wellbore analytical solutions do not usually consider all the phases involved. In this work, an analytical solution is developed to assess the stresses during the drilling, construction and production phases. Three models are adopted to be compared with a numerical approach. Thus, single and double casing configurations are analyzed. In both approaches, the initial stress states in the formation and in the cement, as well as the drilling operation, the casing run, the pressure changes, and the pore pressure variations are considered. The solutions consider plane strain conditions, continuous homogeneous isotropic media and linear elastic materials. In addition to the casing integrity, radial and tangential stresses for casing, cement and formation are presented. In conclusion, this analytical solution reaches excellent results when compared to the numerical model, therefore, becoming an alternative for the stress evaluation for everyday scenarios. Capable of simulating different scenarios regarding the wellbore stresses and integrity, this analytical solution is a good tool for a preliminary design.

Wellbore Stability Problems in Deepwater Gas Wells

The analysis of wellbore stability in deepwater gas wells is vital for effective drilling operations, especially in deepwater remote areas and for modern drilling technologies. Wellbore stability problems usually occur when drilling through hydrocarbon formations such as shale, unconsolidated sandstone, fractured carbonate formations and HPHT formations with narrow safety mud window. These problems can significantly affect drilling time, costs and the whole drilling operations. In deepwater gas wells, there is also the possible of gas hydrate problems because of the low temperature and high pressure conditions of the environment as well as the coexistence of gas and water inside the wellbore. These hydrates can block the mud line, surface choke line and even the BOP stack if no hydrate preventive measures are considered. In addition, the dissociation of these hydrates in the wellbore may gasify the drilling fluid and reduce drilling mud density, hydrostatic pressure, change mud rheology and cause wellbore instabilities. Traditional wellbore stability analysis considered the formation to be isotropic and assumed that the rock mechanical properties are independent of in-situ stress direction. This assumption is invalid for formations with layers or natural fractures because the presence of these geological features will influence rock anisotropic properties, wellbore stress concentration and failure behavior. This is a complicated phenomenon because the stress distribution around a wellbore is affected by factors such as rock properties, far-field principal stresses, wellbore trajectory, formation pore pressure, reservoir and drilling fluids properties and time. This research work reviews the major causes of wellbore stability problems in deepwater gas wells and outlines different preventive measures for effective drilling operation, because real-time monitoring of drilling process can provide necessary information for solving any wellbore stability problems in a short time.

In situ stress distribution and mechanical stratigraphy in the Bowen and Surat basins, Queensland, Australia

Abstract We present regional in situ stress analyses based on publicly available log and pressure data from coal seam gas developments in the Permian Bowen basin, Australia. Together with earlier data from the eastern part of the Jurassic Surat basin, our results show a broad, but systematic, rotation of S Hmax azimuths in this part of eastern Australia as well as systematic changes in stress state with depth. Overall, the geomechanical state of the region appears to reflect the interplay between basin-controlling structures and a complex far-field stress regime. At the reservoir level, within and between Permian coal seams, this stress complexity is reflected in highly variable stress states both vertically and laterally. Stress data, including direct pressure measurements and observations of borehole failure in image logs, have been used to calibrate sonic-derived one-dimensional wellbore stress models that consistently exhibit a change in tectonic stress regime with depth. Shallow depths (<600 m) are characterized by a reverse-thrust stress regime and deeper levels are characterized by a strike-slip regime. Changes in the stress state with depth influence the mechanical stratigraphy of rocks with widely contrasting mechanical attributes (coals and clastic sediments). Our results highlight the interdependency between regional tectonic, local structural and detailed rheological influences on the well scale geomechanical conditions that have to be taken into consideration in drilling and completion designs. Supplementary material: Database of additional wells with image log data are available at https://doi.org/10.6084/m9.figshare.c.3785849

In SituStress and Active Faulting in Oklahoma

Abstract Appreciable injection‐induced seismicity has been occurring in north‐central Oklahoma since 2009. To better understand these earthquakes, we have compiled new information on the state of stress in the state to compare it with both mapped faults and faults inferred from earthquake epicenters and focal plane mechanisms. Seventy‐five new in situ stress orientations are available from wellbores throughout the state. In the north‐central part of the state where the induced seismicity is occurring, stress orientation and relative magnitude from focal mechanism inversions show excellent agreement with the wellbore stress orientations. All of the data show remarkably uniform stress directions. The azimuth of S Hmax , the maximum horizontal stress, is about N85°(±5°)E. Strike‐slip faulting is occurring in central Oklahoma, with strike‐slip/normal and normal faulting observed in northern Oklahoma and southern Kansas. As very few of the thousands of M ≥2.5 earthquakes that have recently occurred are located on, or near, already mapped faults, we utilize the stress information to interpret the likely fault planes associated with over 300 well‐constrained focal plane mechanisms. In the vicinity of the January 2016 sequence of magnitude 4 and 5 earthquakes in the Fairview, Oklahoma, region, we illustrate how knowledge of the stress field can be used to identify the faults responsible for the seismicity and better evaluate the potential hazard associated with possible future earthquakes in the area. The September 2016 M w 5.8 Pawnee event occurred on an unmapped N70°W trending strike‐slip fault expected to be active in the local stress field.

Thermoporoelastic Modeling of Time-Dependent Wellbore Strengthening and Casing Smear

One of the most critical aspects in the drilling operation is to reduce the nonproductive time and to avoid the borehole instability issues such as kicks, blow outs, lost circulation, stuck pipe, and breakouts. To investigate these problems, one has to understand the formation properties, fluid hydraulics, and the basic mechanics behind drilling a well. In the previous research on this field, the factors were widely discussed and results obtained were related to the formation properties. However, while considering the stresses in the wellbore, the mechanical factors such as the RPM and contact of casing at different positions in wellbore have usually been neglected. In furtherance to this study, the importance of thermal condition, fluid loss, and filter cake formation study cannot be out ruled. This work includes a new insight toward understanding the stress redistribution due to pipe contact by the wellbore and smear mechanism. Additionally, it presents the numerical analysis of influence of casing contact and downhole thermal conditions using the finite-element analysis. The classical equations used to obtain the wellbore stresses include very few parameters such as the far-field stresses, pore pressure, and wellbore geometry. They do not consider the influence of casing contact while drilling, mud-cake permeability, and elastic and inelastic properties of the formation. To take into account the effects of these parameters, finite-element analysis is carried out considering the above-mentioned parameters in various scenarios. The main objective of these simulations is to investigate the hypothesis of the increase in hoop stress considering casing contact with regard to formation stresses orientation. The study of different cases shows the variation of a few hundred psi of hoop stress. However, the thermal effect on the near-wellbore stress regions can be important for drilling in deep water and other complex drilling environments. To see the thermal effect, this study develops a thermoporoelastic model. It is found that there is decrease in radial stress and hoop stress in near-wellbore region with time. This reduction will have a considerable impact on fracture initiation pressure in the near-wellbore region. Also, the smearing effect will be influenced by stress changes due to change in temperature.

Wellbore Failure During Water-Alternating-Gas Injection by Use of Flow-Stress Coupling Method

Summary Accurate evaluation of failure pressure is crucial in the design of injection wells. Besides, in-situ stresses play an important role in obtaining the results. Pressure and rock stresses are related together as the role of effective-stress theorem. In fact, pressure changes with stress alteration caused by change in porosity and permeability. Therefore, it should be obtained with the coupling method. Moreover, to calculate pressure, temperature, and stress in the fully coupling method, a huge matrix should be solved, and it takes long processing time to implement it. Therefore, this study developed a wellbore geomechanical model for stability during injection by use of the iterative coupling method. The processing speed was enhanced in this study because the parameters were calculated separately. The parameters of pressure, temperature, saturation, and stress were obtained for the multiphase-flow condition with numerical modeling. Furthermore, in this study, the finite-difference method (FDM) had been used to solve pressure, temperature, and saturation, whereas the finite-volume method (FVM) was applied to solve the wellbore stress. On top of that, the iterative coupling method was used to improve the accuracy of the stress results. As a result, a correction of approximately 20 psi (0.14 MPa) was noted for pressure in relation to stress, which is 1 psi (6.89 kPa). Moreover, the Drucker-Prager failure criterion was used to model the fracturing on the basis of the stress results. Other than that, sensitivity analysis on horizontal maximum (σH) and minimum (σh) stresses showed that by increasing σH, the maximum injection pressures to avoid fracturing had been reduced, whereas in the case for σh, an increment was observed.