Horizontal Stress Anisotropy Research Articles

Increasing accuracy of 3-D geomechanical-numerical models

SUMMARY The current crustal stress field is of key importance to understand geodynamic processes and to assess stability aspects during subsurface usage. To provide a 3-D continuous description of the stress state, linear elastic forward geomechanical-numerical models are used. These models solve the equilibrium of forces between gravitational volume forces and surfaces forces imposed mainly by plate tectonics. The latter are responsible for the horizontal stress anisotropy and impose the inverse problem to estimate horizontal displacement boundary conditions that provide a fit best to horizontal stress magnitude data within the model volume. However, horizontal stress magnitude data have high uncertainties and they are sparse, clustered and not necessarily representative for a larger rock volume. Even when Bayesian statistics are incorporated and additional stress information such as borehole failure observations or formation integrity test are used to further constrain the solution space, this approach may result in a low accuracy of the model results, that is the result is not correct. Here, we present an alternative approach that removes the dependence of the solution space based on stress magnitude data to avoid potential low accuracy. Initially, a solution space that contains all stress states that are physically reasonable is defined. Stress magnitude data and the additional stress information are then used in a Bayesian framework to evaluate which solutions are more likely than others. We first show and validate our approach with a generic truth model and then apply it to a case study of the Molasse foreland basin of the Alps in Southern Germany. The results show that the model's ability to predict a reliable stress state is increasing while the number of likely solutions may also increase, and that outlier of stress magnitude data can be identified. This alternative approach results in a substantial increase in computational speed as we perform most of the calculations analytically.

Geomechanical analysis of lost circulation control in tight formations

Unconventional oil and gas reservoirs, especially that in tight formations, contribute great parts to the global energy. During drilling in tight formations, lost circulation was one of the major problems, which can cause large amount of non operation time and millions of losses. In order to migrate the problem, lost circulation materials (LCMs) were used to prevent reopening of the fracture by isolating the fracture tip while the calculation of stress intensity factor (SIF) and fracture width is the key to LCMs design. In this paper, a dual porosity medium flow model suitable for tight formation is established to calculate the pressure distribution in fracture, and the fracture width and fracture reopening pressure (FROP) is then calculated by using the semi-analytical fracture mechanics model. Sensitivity analysis of critical parameters, for example, fracture length, wellbore radius, LCMs permeability, viscosity, wellbore pressure, and two rock-mechanics-related properties are implemented. The fracture width is larger in the formation with large horizontal principal stress anisotropy, low Young’s modulus and Poisson’s ratio. The increase in fracture length, wellbore radius and wellbore pressure also contributes to fracture opening. Meanwhile, we compared the situation before and after fracture plugging and the results emphasize that the fracture reopening is less likely to occur under the conditions of high viscosity and low permeability LCMs. The method proposed in this study can be used to calculate fracture width and FROP, which has potential significant application for lost circulation control in tight formation.

Penecontemporaneous polygonal faulting triggered by sand overloading onto unconsolidated clays: Evidence from the northern South China Sea

AbstractLayer‐bound polygonal fault systems (PFS) are a prevalent feature in fine‐grained sediments across many continental margin basins worldwide, yet their origin remains enigmatic. In this study, we report on the structural characteristics of polygonal faults recently discovered in Middle Miocene mudrocks of the Yinggehai Basin, northern South China Sea. Our data reveal that the polygonal arrays of normal faults, which comprise master faults and minor synthetic/antithetic faults with complex tiers, exhibit either straight or curvilinear traces with frequent orthogonal intersections, forming a highly interconnected fault network. We observe several sub‐circular to elliptical‐shaped depressions that lie above the faulted interval and are filled with syn‐deformation deposits, with the long axis of these depressions aligned sub‐parallel to the structure contour lines. Our findings suggest that the polygonal faults emerged during the sediment deposition and compaction preceding the deposition of overlying sediments. The faults were created through the nucleation of penecontemporaneous faults due to the overloading of sandy sediments onto unconsolidated clays, followed by the propagation of the faults along with continuous sediment deposition. The cessation of fault propagation coincided with the termination of sedimentation in the faulted interval. Additionally, the local horizontal stress anisotropy resulting from topographic‐gravitational effects may have played a crucial role in the development of polygonal faults. Our study provides novel insights into early sediment deformations in the northern South China Sea region and sheds light on the timing and genesis of PFS.

Hydraulic Fracture Propagation and Proppant Transport Mechanism in Interlayered Reservoir

Hydraulic fracture is crucial for assuring well production from unconventional reservoirs. For the optimization of hydraulic fracture geometry and the ensuing production of an interlayered reservoir, vertical hydraulic fracture propagation path has been analyzed. However, an effective fluid channel cannot be formed if the proppant is unable to reach the area where the fracture propagates. This paper presents a numerical model using the lattice-based method to investigate the hydraulic fracture propagation and proppant transport mechanism in interlayered reservoirs. The hydraulic fracture propagation model was simulated under different geological and fracturing engineering factors. The results indicate that interlayer Young’s modulus and horizontal stress anisotropy are positively correlated with longitudinal propagation and proppant carrying ability in interlayered formations. The fracturing injection rate has an optimal solution for fracture propagation and proppant carrying since a too low injection rate is unfavorable for fracture penetration of the interlayer, while a too high injection rate increases fracture width instead of further fracture penetration. In conclusion, attention is drawn to fine particle size proppants used in multi-layer reservoirs for fracturing fluid to carry proppants as far as possible to obtain maximum propped area.

Natural fractures and their relationships to structure, stress, and permeability in the Raton Basin

Fractures in Cretaceous and early Tertiary strata record several deformation events that were imposed on the formations that fill the Raton Basin in Colorado and New Mexico. A regional, generally WNW-ESE striking extension-fracture set is present across much of the basin, but fracturing also includes both dip-slip and strike-slip conjugate shear-fracture sets as well as irregular deformation-band shear fractures. In some areas of the basin, the extension fractures are dynamically-compatible with associated conjugate shear fractures, both recording a maximum compressive stress that was horizontal, trending predominantly WNW-ESE. Fracture strikes vary from NW-SE to ENE-WSW but are approximately normal to the front of the Laramide thrust-fault system that forms the western edge of the basin, implying that fracturing was the result of a horizontal compressive stress anisotropy created by indentation of the thrust system into the basin margin. Fracture anomalies occur over local structures including a N-S basement wrench-fault system that connects two large anticlines within the basin, the Tercio and Vermejo Park anticlines, where N-S strike-slip offset along the basement wrench faults caused folding and fracturing in the overlying strata. The Laramide stress system in the basin changed from thrust-related WNW-ESE horizontal compression to the present-day N-S maximum horizontal compressive stress in mid-Tertiary time as the thrust system became inactive and was replaced by regional E-W extension. No new fracture sets were formed by the re-oriented stress system, although stress-release fractures normal to the regional set formed in outcrops as overlying strata were eroded. Fracture datasets were derived from three sources during this study: outcrops, image logs, and cores. Each source provides a somewhat different perspective on the fractures that enhance permeability in Raton Basin reservoirs. Taken together, the three datasets provide the basis for a relatively complete conceptual model of the Raton fracture system. Fracture-controlled permeability anisotropy will be greatest in the WNW-ESE direction, parallel to the strike of the dominant set of Laramide-age natural fractures, but hydraulic stimulation fractures will propagate N-S, across the strike of those fractures under the influence of the present-day stress system. The apertures of the WNW-ESE fractures will be susceptible to closure under that stress system since the maximum horizontal compressive stress is approximately normal to fracture strike.

Fracability Evaluation Method and Influencing Factors of the Tight Sandstone Reservoir

Fracability evaluation is the basis of reservoir fracturing and fracturing zone optimization. The tight sandstone reservoir is characterized by low porosity and low permeability, which requires hydraulic fracturing to improve industrial productivity. In this study, a systematic model was proposed for the fracability evaluation of tight sandstone reservoirs. The rock mechanics tests and sonic tests demonstrated that tight sandstone reservoir is characterized by high brittleness, high fracture toughness, and weak development of natural fractures. Numerical simulation was used to analyze the change of reservoir parameters during hydraulic fracturing and the influence of in situ stress on fracture propagation. The results showed that when the horizontal stress anisotropy coefficient is small, natural fractures may lead hydraulic fractures to change direction, and complex fracture networks are easily formed in the reservoir. The horizontal stress anisotropy coefficient ranges from 0.23 to 0.52, and it is easy to produce fracture networks in the reservoir. A new fracability evaluation model was established based on the analytic hierarchy process (AHP). The fracability of tight sandstone reservoir is characterized by the fracability index (FI) and is divided into three levels. Based on the model, this study carried out fracability evaluation and fracturing zone optimization in the study area, and the microseismic monitoring results verified the accuracy of the model.

SERA: An ISIP analysis approach to estimate fracture height and correlate stress escalation and relaxation in multifrac horizontal wells

In hydraulic fracturing of horizontal wells in unconventional reservoirs, every fracturing stage by increasing the minimum horizontal stress modifies the vertical stress contrast as well as reduces the stress anisotropy. The induced stress gradually declines thereafter due to fracture closure caused by fluid leak-off. The time-dependent stress behavior with localized spatial effects generated by fixed but dynamic sources offers an incentive to fracturing optimization studies. Here, drawing upon the stress superposition principle, an analytical model is derived for Stress Escalation and Relaxation Analysis (SERA) to correlate the stress state at the wellbore. The SERA's model possesses a set of tuning parameters that are adaptively adjusted based on perforation design, waiting time between stage treatments, and ISIP of all completion stages. As a computationally efficient ISIP analysis technique, SERA estimates the average height of wet fracture and sets an upper limit to the critical stress state. In case of a dominant vertical stress contrast, SERA allows to determine explicitly the conditions under which the onset of wet fractures into the bounding layers of lower stress takes place, and when the stress re-orientation controls the fracturing regime, it narrows down the range of estimates of the horizontal stress anisotropy. Furthermore, it provides a tool to design optimal timing and sequencing strategies for alternating fracturing by correlating post-treatment stress relaxation parameters. We validate the model with numerical simulations and perform a sensitivity analysis on the influential parameters that can be carefully designed to give more control over stress shadowing.

Modelling of sensitivity of underground space stability to the in situ stress uncertainties: case study at the Bukov underground research facility phase II (Rozna mine, Czechia)

Numerical modelling has been widely used in the underground excavation design, where the in situ stress state plays a crucial role in the stability analysis. However, determination of an exact stress state for a specific geological region remains uncertain due to the complicated tectonic nature and measurement limitations. The stability is thus better analysed by considering the in situ stress as a finite spectrum and pinpointing the possible worst-case scenario. The most probable scenarios of in situ stress states in the Rožná mine area were analysed based on the varying trends in principal stress ratio and mean stress values obtained from four different measurement/analysis campaigns. The influence of different in situ stress judgement on the deformation and failure characteristics of the Bukov Underground Research Facility (URF) (Phase II, Czech) were investigated by the finite volume program FLAC3D. Results show that the increased horizontal stress anisotropy and the mean stress level jointly increase the overall deformation and lower the URF stability. Such influences on the roadway horizontal convergence are more considerable than the vertical ones. A mathematical model considering mean stress and horizontal stress ratio was proposed to quantitatively describe the overall stability, especially useful for excavations possessing complicated configuration.

Integrated Workflow of Geomechanics, Hydraulic Fracturing, and Reservoir Simulation for Production Estimation of a Shale Gas Reservoir

In this research, an integrated workflow from geomechanics to reservoir simulation is suggested to accurately estimate performances of a shale gas reservoir. Rather than manipulating values of hydraulic fracturing such as fracture geometry and transmissibility, the workflow tries to update model parameters to derive reliable hydraulic fracturing results. A mechanical earth model (MEM) is built from seismic attribute and drilling and diagnostic fracture injection test results. Then, the MEM is calibrated with microseismic measurements obtained in a field. Leakoff coefficient and horizontal stress anisotropy are sensitive parameters of the MEM that influence the propagation of the fracture network and gas productions. Various combinations of calibration parameters from a single-well simulation are evaluated. Then, an appropriate combination is chosen from the whole simulation results of a pad to reduce the uncertainty. Finally, production estimations of the four wells which have slightly different fracture design are compared with seven-year production history. Their results are reasonably matched with actual data having 8% of global error due to successful development of the reservoir model with geomechanical parameters.

Implications of the in situ stress distribution for coalbed methane zonation and hydraulic fracturing in multiple seams, western Guizhou, China

With 59 sets of well testing data from 32 wells, 70 coal seam gas data from 9 wells, and production data from 17 wells, the in situ stress distribution within depths of 136-1244 m and its implications for coal permeability (0.0001-1.56 mD), gas content (5-22 m 3 /t) and gas productivity in western Guizhou were investigated. Three major depth intervals with different stress regimes were identified. At depths of 800-1244 m, the horizontal stresses increased to high values with depth due to the compressional zone near the axis of the syncline. Permeability changes with depth were consistent with the effective stress variations. The 500–800 m depth interval with a normal faulting stress regime was favorable for good permeability (0.008-0.57 mD, mean 0.2 mD) and a stable pressure gradient (approximately 1 MPa/100 m), and the gas content generally increased with depth to a peak value at 800 m. For the 200-500 m and 800-1244 m depth intervals, extremely low permeability (0.0001-0.17 mD, mean 0.03 mD) resulted in discontinuous changes in gas content and pressure gradient (0.47-1.71 MPa/100 m). Overall, the stress release zone at depths of 500–800 m was favorable for coalbed methane extraction, which agreed with the measured production data. Low horizontal stress anisotropy in western Guizhou contributes to complex hydraulic fracture networks, individual seam fracturing with low proppant concentration and high fracturing fluid volume is suggested for multilayer commingled production. • In-situ stress distribution characteristics of coals in the Western Guizhou. • Permeability and resource distribution show similar belting as with stress regimes. • Stress release zone at 500–800 m depths are favorable for CBM extraction. • Low proppant concentration and high fluid volume are suggested for hydrofracturing. • Individual-seam fracturing is the most effective productivity enhancement method.

Dispersion of flexural waves in a borehole with a tensile fracture in an anisotropic stress environment

ABSTRACTThe effect of tensile fracture in a vertical borehole under anisotropic horizontal stress conditions is numerically investigated in terms of the dispersion of flexural wave generated in dipole sonic logging. Our three‐dimensional model comprises a borehole filled with water and a tensile fracture intersecting the borehole in the borehole axial direction. Two shear waves are excited individually to produce particle displacements polarized in two orthogonal radial directions using two dipole sources aligned in the two polarized directions. A vertical array of equispaced dipole sensors is placed at the centre of the borehole along the borehole axis. We assumed that the surrounding formation possesses transversally isotropic anisotropy with the isotropy plane parallel to the borehole axis due to horizontal stress anisotropy. We examined the dispersion of flexural waves travelling along a borehole in our numerical models that include either fast or slow formation with various depths of tensile fractures. Our numerical results show that the deeper the penetration depth of a tensile fracture, the higher the slowness of shear waves polarized perpendicular to the tensile fracture for both slow and fast formation models. Our results indicate that the flexural dispersion behaviour could be used to investigate the depth of penetration of a tensile fracture that can be produced by either drilling or hydraulic fracturing.

Failure Patterns and Mechanisms of Hydraulic Fracture Propagation Behavior in the Presence of Naturally Cemented Fractures

In this study, we use the extended finite element method (XFEM) with a consideration of junction enrichment functions to investigate the mechanics of hydraulic fractures related to naturally cemented fractures. In the proposed numerical model, the lubrication equation is adopted to describe the fluid flow within fractures. The fluid-solid coupling systems of the hydraulic fracturing problem are solved using the Newton-Raphson method. The energy release rate criterion is used to determine the cross/arrest behavior between a hydraulic fracture (HF) and a cemented natural fracture (NF). The failure patterns and mechanisms of crack propagation at the intersection of natural fractures are discussed. Simulation results show that after crossing an NF, the failure mode along the cemented NF path may change from the tensile regime to the shear or mixed-mode regime. When an advancing HF kinks back toward the matrix, the failure mode may gradually switch back to the tensile-dominated regime. Key factors, including the length of the upper/lower portion of the cemented NF, horizontal stress anisotropy, and the intersection angle of the crack propagation are investigated in detail. An uncemented or partially cemented NF will form a more complex fracture network than a cemented NF. This study provides insight into the formation mechanism of fracture networks in formations that contain cemented NF.

A three-dimensional numerical study of hydraulic fracturing with degradable diverting materials via CZM-based FEM

Both microseismic interpretation and post-frac production analysis have confirmed that initial hydrocarbon production and ultimate recovery in reservoirs are mainly associated with the degree of fracture complexity. Hydraulic fracturing with degradable diverting materials is an effective technology for enhancing fracture complexity. However, the physics behind the effectiveness of this technology is not fully understood at present. In this paper, we first present a comprehensive workflow to model hydraulic fracture by accounting for interactions with numerous crosscutting natural fracture or joint sets, as well as the effect of temporary plugging in opened fractures. This model is a fully coupled seepage flow in porous media, fluid flow in fractures, and rock deformation finite element model with adaptive insertion of cohesive elements as crosscutting natural fracture or joint sets. In addition, spring elements are used to model the temporary plugging behavior that can be automatically activated once the diverters are injected into the opened fractures. Numerical simulations that correspond to the orthogonal and non-orthogonal approach angles have been implemented for the model. Our analysis shows that key factors such as the tensile and shear strength of natural fracture or joint sets, horizontal stress anisotropy, and the position of temporary plugging can play a significant role in the enhancement of fracture complexity and the diversion into natural fracture or joint sets. Adding degradable diverting materials has a particularly noticeable effect on enhancing fracture complexity and helps overcome cohesive resistance at the intersections. This investigation provides new insight into the formation mechanism of fracture networks in naturally fractured formations.

DEM-continuum mechanics coupled modeling of slot-shaped breakout in high-porosity sandstone

Mechanism for slot-shaped borehole breakouts observed from drilling experiments in high-porosity sandstone is investigated using the hybrid DEM-continuum mechanics modeling approach. The rock material in DEM is modeled as an assembly of particles with flat joints at contacts. The failure mechanism for the slot-shaped breakout is mainly attributed to particle crushing, which is different from the conventional dilatant V-shaped breakout where shear failure plays the primary role. The damage evolution of the slot-shaped breakout can be divided into three stages, i.e., breakout initiation induced by the stress concentration near the excavated borehole, formation and propagation of the slot-shaped borehole breakout as a result of particle crushing and removal, and formation of a compaction band at the tip of the breakout when the breakout length ceases to grow. The dynamic change of stress during the breakout slot extension is modeled. The continuous particle breakage at the tip of the breakout maintains the same level of stress concentration as the breakout propagates. The majority of the particle loss is caused by the particle crushing and subsequent removal. The evolution of microcrack strike suggests change of the preferential failure orientation from approximately parallel to the maximum horizontal stress direction to forming a small angle to the minimum horizontal stress direction. Parametric studies show that increasing the horizontal stress anisotropy, broadening the range of particle size, and increasing the rock porosity will all increase the length of the slot-shaped breakout. The normalized breakout length tends to increase with the increase of borehole size before reaching a stable value.

A 3D approach to study the interaction between hydraulic and natural fracture

A real 3D model is developed to study the interaction between hydraulic fracture and natural fracture. The hydraulic fracture propagation behavior and fracture geometry under impact of natural fracture are studied through a series of numerical results, different horizontal stress anisotropies, intersection angles, friction coefficients as well as hydraulic parameters. The final results demonstrate the key role of horizontal stress anisotropy on the interaction between hydraulic fracture and natural fracture. Under a higher intersection angle, it is difficult for hydraulic fracture to cross the natural fracture with low horizontal stress anisotropy. In addition, the natural fracture at a lower intersection angle has a higher probability to capture the hydraulic fracture. Normally, the natural fracture is more likely to be activated in the top branch at a low intersection angle, as the lower branch is located in the induced stress shadow of hydraulic fracture. By contrast, two different branches,+ especially two symmetrical braches at the intersection of 90°, are activated. On the other hand, a higher friction coefficient slows down the propagation in natural fracture, even prevents the activation of natural fracture. Besides, increased injection rate as well as fluid viscosity will promote the fracture propagation under the same conditions to lead to a longer and wider fracture.

Seismic analysis of polygonal fault systems in the Great South Basin, New Zealand

Abstract Polygonal fault systems (PFS) consisting of networks of layer-bound normal faults are analyzed for Eocene strata in the Great South Basin, New Zealand. An advanced fault enhancement and skeletonization method is applied to delineate faults using 3D seismic data from the Great South Basin. The process sharpens structural and stratigraphic discontinuities and smears the incoherent noise on coherence to obtain a skeletonized fault probability volume which can be directly used to extract and map fault geometries. The characteristics of the polygonal faults in cross-section, planform, and 3D space were studied. The faults dip approximately 50°, with displacements on a single fault ranging from tens of meters to a hundred meters. The fault planform patterns are polygonal but also related to the regional horizontal stress anisotropy caused by slope changes of the basal unit. The patterns vary from linear for high slopes, rectangular for moderate slopes and polygonal for low slopes. Two preferred orientations may reflect the superposition of deep Cretaceous trends on the polygonal system in the Eocene strata. A local concentric pattern with outward dipping normal faults is related to passive or active draping above a circular plutonic-volcanic structure. The genetic mechanism of the polygonal fault systems is interpreted to be related to volume loss and shear failure related to Opal A/CT transition within the sediments.

Development of a full 3D numerical model to investigate the hydraulic fracture propagation under the impact of orthogonal natural fractures

Although hydraulic fracturing has been massively studied and applied as a key technique to enhance the gas production from tight formations, some problems and uncertainties exist to accurately predict and analyze the fracture behavior in complex reservoirs, especially in the naturally fractured reservoirs like shale reservoirs. This paper presents a full 3D numerical model (FLAC3D) to study hydraulic fracturing behavior under the impact of preexisting orthogonal natural fractures. In this numerical model, the hydraulic fracture propagation direction is assumed perpendicular to the minimum principal stress and activated only by tensile failure, whereas the preexisting natural fractures can be activated by tensile or shear failure or a combination of them, and only tensile failure can open the natural fracture as well. The newly developed model was used to study the impact of preexisting orthogonal natural fractures on hydraulic fracturing behavior, based on a multistage hydraulic fracturing operation in a naturally fractured reservoir from the Barnett Shale formation, northwest of Texas in USA. In this multistage operation, two more representative stages, i.e., stage 1 with a relatively large horizontal stress anisotropy of 3.3 MPa and stage 4 with a comparatively small one of 1.3 MPa, were selected to conduct the simulation. Based on the numerical results, one can observe that the interaction between hydraulic and natural fracture is driven mainly by induced stress around fracture tip. Besides, the horizontal stress anisotropy plays a key role in opening the natural fracture. Thus, no significant opened fracture is activated on natural fracture in stage 1, while in stage 4 an opened fracture invades to about 90 m into the first natural fracture. Conversely, the hydraulic fracture length in stage 1 is much longer than in stage 4, as some fluid volume is stored in the opened natural fracture in stage 4. In this work, the shear failure on natural fractures is treated as the main factor for inducing the seismic events. And the simulated seismic events, i.e., shear failure on natural fractures, are very comparable with the measured seismic events.

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.

Determine In-Situ Stress and Characterize Complex Fractures in Naturally Fractured Reservoirs from Diagnostic Fracture Injection Tests

Estimation of in-situ stresses has significant applications in earth sciences and subsurface engineering, such as fault zone studies, underground CO2 sequestration, nuclear waste repositories, oil and gas reservoir development, and geothermal energy exploitation. Over the past few decades, Diagnostic Fracture Injection Tests (DFIT), which have also been referred to as Injection-Falloff Tests, Fracture Calibration Tests, and Mini-Frac Tests, have evolved into a commonly used and reliable technique to obtain in-situ stress. Simplifying assumptions used in traditional methods often lead to inaccurate estimation of the in-situ stress, even for a planar fracture geometry. When a DFIT is conducted in naturally fractured reservoirs, the stimulated natural fractures can either alter the effective reservoir permeability within the distance of investigation or interact with the hydraulic fracture to form a complex fracture geometry, this further complicates stress estimation. In this study, we present a new pressure transient model for DFIT analysis in naturally fractured reservoirs. By analyzing synthetic, laboratory and field cases, we found that fracture complexity and permeability evolution can be detected from DFIT data. Most importantly, it is shown that using established methods to pick minimum in-situ stress often lead to over or underestimates, regardless of whether the reservoir is heavily fractured or sparsely fractured. Our proposed “variable compliance method” gives a much more accurate and reliable estimation of in-situ stress in both homogenous and naturally fractured reservoirs. By combining the unique pressure signatures associated with the closure of natural fractures, a lower bound on the horizontal stress anisotropy can be estimated.

Wellbore breakouts: Mohr-Coulomb plastic rock deformation, fluid seepage, and time-dependent mudcake buildup

Borehole breakout is a time-dependent failure process which includes breakout initiation, propagation, and stabilization. Plastic rock deformation, fluid seepage, and time-dependent mudcake buildup on the wellbore wall affect the near-wellbore stress state and, therefore, the breakout behavior. In this paper, a hydro-mechanical model was developed for breakout prediction taking into account these factors. Filtration tests were conducted to obtain time-dependent permeability and thickness of mudcake, and the experimental testing results were incorporated to embody the dynamic mudcake buildup process. A sensitivity analysis was performed to investigate the effects of horizontal stress anisotropy, drilling mud pressure, and time-dependent fluid flow and mudcake buildup on breakouts in a vertical borehole. The simulation results show increased possibility of breakouts with larger horizontal stress anisotropy. Additionally, fluid seepage between the wellbore and the surrounding formation makes the breakout a time-dependent process. For low mud pressure, the initial breakout shape immediately after drilling is very similar to the final breakout shape after reaching steady state seepage. However, for high mud pressure, the wellbore may experience significant breakout propagation after drilling, owing to considerable fluid seepage associated with the larger differential pressure between the wellbore and the formation. Time-dependent mudcake buildup on the wellbore wall can effectively reduce the likelihood of borehole breakout by acting as a low-permeability barrier that mitigates fluid seepage across the wellbore wall and reduces changes in formation pore pressure. Disregarding the mudcake or considering a perfectly impermeable mudcake can lead to overestimating or underestimating the risk of borehole breakout, respectively. The proposed model provides a useful approach to understand and assess borehole breakout for drilling design.