Poroelastic Effects Research Articles

Impact of convective cooling on pore pressure and stresses around a borehole subjected to a constant flux: Implications for hydraulic tests in an enhanced geothermal system reservoir

Hydraulic tests are commonly performed prior to reservoir stimulation to determine the natural permeability of the enhanced geothermal system (EGS), which typically involves injection of cold water at low rates into the hot target EGS reservoir and measurement of the surface pressure, flow rate, and thermal drawdown simultaneously. Interpretation and analysis of the data collected during a hydraulic test are largely based on fitting type curves generated from analytical solutions. We have formulated a geomechanical model for hydraulic tests in a thermoporoelastic EGS reservoir in which the open-hole section of the borehole wall is subjected to a constant-rate flux and convective cooling. We derive the analytical solutions for injection induced axisymmetric deformation, temperature drawdown, pore pressure, and stresses in the Laplace domain, and we obtain time-domain solutions using the Stehfest inversion algorithm. We evaluate numerical examples to illustrate the competition between the thermoelastic effect and the poroelastic effect in controlling temperature, pore pressure, and stresses around the borehole wall. The numerical results indicate that temperature drawdown at the borehole wall would be significantly overestimated if a constant temperature boundary condition instead of convective cooling is used. At early times, injection induced hoop stress/pore pressure is partially offset by the corresponding ones induced by convective cooling. At late times, convective cooling plays a marginal role in influencing pore pressure and stress around the borehole. The analytical solutions are helpful to address the thermohydromechanical coupling mechanisms controlling pressure perturbations during hydraulic tests conducted in an EGS reservoir.

Hydrologic control on natural land subsidence in the shallow coastal aquifer of the Ravenna coast, Italy

Abstract. Multiple processes contributing to natural land subsidence in a shallow coastal aquifer near Ravenna (Italy) were identified by analysing the relationships among different data set time series (water table level, rainfall, land reclamation drainage, sea level, etc.) and establishing the correlations with vertical ground motion observed at a high-resolution settlement gauge. Our study highlights the presence of three deformation components related to different processes controlling land subsidence: elastic, delayed-elastic, and irreversible (plastic) components. The elastic and delayed-elastic components are closely related to water table fluctuations that change the effective stress in two portions of the coastal aquifer at a daily (in the sandy unconfined portion) and seasonal time scales (in the layered clay-rich semi-confined prodelta portion), respectively. The irreversible component represents the trend in the land subsidence time series and is due to primary consolidation (pore pressure dissipation) of the fine-grained prodelta levels above where the settlement gauge is located. The amplitudes of the elastic component can be up to 0.2–0.3 mm whereas the amplitude of the delayed-elastic component reaches 0.89 mm. The primary consolidation rate of deformation is 0.9 mm yr−1 and constrains the likely age of prodelta sediments deposition to 1300–2800 years before present. The delayed-elastic subsidence rate has similar magnitude to that due to primary consolidation and is connected to poroelastic effects in the prodelta sequence following seasonal variations in water table. Our findings are important for planning land subsidence management and monitoring strategies especially where the surface aquifer structure is heterogeneous due to different depositional settings. The natural land subsidence rate in the Holocene sediments of the shallow coastal aquifer of Ravenna (North eastern Italy) that we measured in this study accounts for 10 %–20 % of the total current land subsidence rate observed in this portion of Ravenna coastal area (10–20 mm yr−1).

A Semianalytical Poroelastic Solution To Evaluate the Stability of a Borehole Drilled Through a Porous Medium Saturated with Two Immiscible Fluids

Summary Conventional drilling design assumes that the porous rock is fully saturated with a single fluid and therefore tends to inaccurately predict the mud weight needed for borehole stability because the pore space of the porous rock may actually have two or more fluids. This paper provides a new semianalytical poroelastic solution for the case of an inclined borehole subjected to nonhydrostatic stresses in a porous medium saturated with two immiscible fluids: water and gas. The new solution is obtained under plane-strain condition. The wellbore loading is decomposed into axisymmetric and deviatoric cases. The time-dependent field variables are obtained by performing the inversion of the Laplace transforms. On the basis of the expansion of the Laplace-transform solution, we derive the unsaturated poroelastic asymptotic solutions for early times and for a small radial distance from an inclined wellbore. Sensitivity analyses are performed on different ratios of bulk modulus of two fluid phases to pore pressures of the unsaturated case. In addition, the comparative analyses of pore-pressure differences are made between the unsaturated and saturated cases. The impact of the unsaturated poroelastic effect on pore pressure, stresses, and borehole stability is investigated. Our results show that the excess pore pressure caused by the poroelastic effect is generally higher for the saturated case (water) than the unsaturated case because of the large difference between the compressibility of fluid phases (water and gas). The time dependency of the poroelastic effect causes the safe-mud-pressure window of both the unsaturated and saturated cases to narrow and approach the long-time poroelastic one with increasing time. The safe-mud-pressure window narrows with increasing initial gas saturation. Contrary to the unsaturated case, the saturated case that assumes the formation to be saturated by one fluid (e.g., water) tends to optimistically predict a wider safe-mud-pressure window required for borehole stability. This new semianalytical poroelastic solution enables the drilling engineer to more accurately estimate the time-dependent stresses and the pore pressure around a borehole, thus allowing a mud weight design that will ensure borehole stability.

Modeling the Contribution of Poroelastic Deformation to Postseismic Geodetic Signals

AbstractTo constrain the poroelastic component of postseismic deformation, we model the subsurface hydrologic response to the Mw 7.6 subduction zone earthquake that occurred on the plate interface beneath the Nicoya peninsula in Costa Rica on 5 September 2012. The model shows that poroelastic relaxation occurs on multiple time scales and the associated deformation can be up to 2 cm for the trench‐perpendicular component. By modeling the time‐dependent deformation associated with poroelastic relaxation, we can begin to remove its contribution from the observed geodetic signal. Inversions for after slip that ignore poroelastic deformation have errors of 10–20% overall and up to 50% locally. Poroelastic effects can both mute and amplify the inferred afterslip.

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.

The role of water lubrication in critical state fault slip

The conventional driving mechanisms including elevating the pore pressure or far-reaching poroelastic effects, may not necessarily explain the injection-triggered seismicity that can be up to 35–90 km far away from injection wells. In this experimental study, we examined the mechanism of water lubrication for the triggered seismicity where the fluid pressure was negligible compared to the in-situ stresses. The results show that fault slip can occur upon saturation when the ratio of the applied shear stress to the fault peak shear strength exceeds a critical threshold, which is defined to quantitatively assess the critical stress state for faults. The microscale slip test using atomic force microscopy reveals the mechanism responsible for the significant reduction of friction between granite surfaces from dry to wet state. A nanometer-thick water layer was attached on the granite surfaces, causing the shear plane shifting from the direct granite/granite interface to the water layer, and meanwhile the adhesive force between granite surfaces was significantly reduced by water intercalation, both of which played the dominate role in triggering fault slip. In practice, slip may be associated with in-situ fluid flow into far-field faults, which causes the increase of water and lubricates the fault zone.

The range of influence of the poroelastic effects in terms of dimensionless complexes for the radial hydraulic fracturing model

In our previous work the fully coupled model of the planar hydraulic fracture propagation in a poroelastic medium was developed. In present paper we simplify this model to the case of axisymmetric fracture propagation in order to find the conditions whereby the poroelastic effects are essential. We consider two dimensionless complexes responsible for the transition from 1D to 3D filtration and for the poroelastic influence on the fracture geometry. Analyzing a large set of case studies, we find the critical values for these parameters below which the aforementioned physical effects are not important. Particularly, we demonstrate that 1D pressure diffusion is preserved until the diffusion length scale is 2.4 times smaller than the fracture size. In turn, the diffusion scale plays a central role in the poroelastic stress influence on the fracture propagation. Also, we show that in case of low closure stress the pressure diffusion is dominated not by the filtration process but by the rock deformation with a slight impact on the fracture geometry.We believe that this study will become useful for the appropriate model selection in engineering practice.

Hydromechanical Modeling of Fault Reactivation in the St. Gallen Deep Geothermal Project (Switzerland): Poroelasticity or Hydraulic Connection?

AbstractIn 2013, fluid injection during the St. Gallen deep geothermal project, Switzerland, induced hundreds of seismic events, including a 3.5 earthquake on a fault hundreds of meters away from the well. Recent studies have suggested the direct pressure effect through permeable hydraulic connections and poroelastic effects as possible mechanisms for inducing seismicity on distant faults. In St. Gallen, operational, seismic, and earthquake data are available to investigate the underlying physical mechanisms using a numerical model. The results show that Coulomb stress changes at the fault can be 3 orders of magnitude greater when a hydraulic connection is present. Combining this with several field observations, we conclude that the direct pressure effect was more likely the predominant mechanism behind the seismicity induced in St. Gallen. The detection of hydraulic connections may be important for future projects as pressure can be driven far from the well and reactivate remote faults.

Poroelastic models for fault reactivation in response to concurrent injection and production in stacked reservoirs

Concurrent production and injection in stacked reservoirs as commonly conducted in unconventional resource exploitation potentially influences reactivation of nearby faults. Using three-dimensional, fully-coupled poroelastic finite-element simulations, we assessed the potential for reactivation of a barrier normal fault in a normal-faulting stress regime for twelve generic injection–production scenarios that differ in the depth of injection and production, and in the position and distance relative to the dipping fault plane. The simulations display significant variation in the Coulomb failure stress (CFS) with depth along the fault plane for these scenarios, reflecting differences in pore pressure distribution and associated poroelastic changes in normal and shear stress across the fault. Based on the CFS trends with depth we find that 1.) concurrent production and injection reduces or increases the fault reactivation potential in the injection reservoir depending on the lateral position and the distance of the wellbores relative to the fault plane; 2.) the fault is most prone to reactivation with stacked wellbores and injection into the upper reservoir within the hanging wall or the lower reservoir within the footwall, and 3.) the fault is least prone to injection-induced reactivation for stacked wellbores and injection into the lower reservoir within the hanging wall at wellbore-to-fault distances ten times the reservoir thickness. With decreasing wellbore-to-fault distance, induced poroelastic shear stresses and thus CFS increase, making injection only into the lower reservoir, without concurrent production, the most stable configuration at close distance. These simulations demonstrate the importance of the coupled poroelastic effects and of the three-dimensional arrangement of injection and production wellbores on fault reactivation. Our results are intended to provide general guidance for further detailed site-specific geomechanical evaluations needed for induced seismic hazard assessment.

Statistical based hydromechanical models to estimate poroelastic effects of CO2 injection into a closed reservoir

AbstractIn this study, a statistical‐based methodology is used to evaluate the poroelastic effects of injection during CO2 geologic sequestration in a closed reservoir. We constructed a series of hydromechanical models to evaluate the poroelastic effect of injection by quantifying total stress changes inside the reservoir, reservoir displacement, and surface uplift. The models are representative of closed carbonate reef reservoirs of the Michigan basin. A combination of experimental design for seven independent parameters (depth, caprock and reservoir Young's modulus, caprock and reservoir Poisson's ratio, pressure, and Biot's coefficient) and response surface modeling was used to develop statistical‐based reduced‐order models. We performed 147 numerical simulations to develop simplified models. The poroelastic model responses were captured using a standard quadratic model with full interaction terms, as well as a reduced model with only statistically significant coefficients. The interpretation of reduced‐order models, using R2 loss method, shows that while surface uplift depends mainly on the depth of the reservoir, stress increase depends mainly on Biot's coefficient. The developed statistical‐based models provide a quick tool to evaluate the poroelastic effect of injection into the closed carbonate reservoirs of the Michigan basin. Reduced‐order models were then combined with a Monte Carlo simulation to perform poroelastic uncertainty analyses and achieve a better understanding of the poroelastic performance of CO2 storage in the closed carbonate system of the Michigan basin. © 2020 Society of Chemical Industry and John Wiley & Sons, Ltd.

Poroelastic effects on steady state crack growth in polymer gels under plane stress

Fracture experiments on polymer gels are often conducted with thin specimens, which are close to plane stress in two-dimensional models. However, many of the previous theoretical and numerical studies on fracture of polymer gels have assumed plane strain conditions. The subtle differences between the plane stress and plane strain conditions are elucidated in this paper based on a linear poroelastic formulation for polymer gels, including the asymptotic crack-tip fields and finite element simulations of steady-state crack growth in long strip specimens. Moreover, a poroelastic cohesive zone model is adopted to study the rate-dependent fracture process of polymer gels. It is found that, without the cohesive zone model, the normalized crack-tip energy release rate at the fast crack limit is greater than the slow crack limit, suggesting reduced poroelastic toughening for fast crack growth under plane stress conditions, while the two limits are identical under plane strain conditions. With a solvent-permeable cohesive zone for the case of immersed specimens, solvent diffusion within the cohesive zone enhances the poroelastic toughening significantly as the crack speed increases, leading to a rate-dependent traction-separation relation. On the other hand, with no solvent diffusion in the cohesive zone for the not-immersed case, the poroelastic toughening effect diminishes as the crack speed increases. Based on the present study, the intrinsic steady-state fracture toughness of a poroelastic gel can be determined using long-strip pure-shear specimens, which in general is smaller than the applied energy release rate.

Diffusivity-dependent fracturing processes and microseismic activity in granite

Enhanced Geothermal Systems have relied on hydraulic fracturing to increase the permeability of rock reservoirs. The permeability enhancement depends on the connectivity between new and existing fractures. This, in turn, depends to a large extent on the interaction between the rock and the fracturing fluid, which not only pressurizes existing and new fractures but also diffuses into the rock matrix. In this research, the effect of the diffusivity of hydraulic oil on the fracturing processes and microseismicity of unconfined prismatic granite specimens was experimentally evaluated using visual and acoustic emission monitoring. The tests consisted of injecting hydraulic oil into two pre-fabricated flaws at two rates (2 ml/min and 20 ml/min), kept constant in each test. The fluid pressure inside the flaws was increased until hydraulic fractures propagated and the fluid front growing from the pre-fabricated flaws was visually monitored throughout the tests. It was observed that the fracturing pressures and patterns were injection-rate-dependent, which shows that diffusivity and poro-elastic effects play an important role in the hydraulic fracturing processes of granite. A smaller fluid front was observed for the 20 ml/min injection rate, associated to a lower volume injected and to a higher fracturing pressure when compared to the 2 ml/min injection rate. This was interpreted to be caused by the different pore pressures that developed inside of the rock matrix, which are function of the fluid front size. Microseismic activity was observed throughout the tests, becoming more intense and localized near the flaws as one approached the end of the test (i.e. visible crack propagation). While microseismic events were observed outside the fluid front region, their density was significantly larger within this area, showing that fluid diffusivity may contribute to an intensification of the microseismic activity.

Poroelastic Effects on the Time- and Rate-Dependent Fracture of Polymer Gels

AbstractFracture of polymer gels is often time- and rate-dependent. Subject to a constant load, a gel specimen may fracture immediately or after a delay (time-dependent, delayed fracture). When a crack grows in a gel, the fracture energy may depend on the crack speed (rate-dependent). The underlying mechanisms for the time- and rate-dependent fracture of gels could include local molecular processes, polymer viscoelasticity, and solvent diffusion coupled with deformation (poroelasticity). This paper focuses on the effects of poroelasticity. A path-independent, modified J-integral approach is adopted to define the crack-tip energy release rate as the energetic driving force for crack growth in gels, taking into account the energy dissipation by solvent diffusion. For a stationary crack, the energy release rate is time-dependent, with which delayed fracture can be predicted based on a Griffith-like fracture criterion. For steady-state crack growth in a long-strip specimen, the energy release rate is a function of the crack speed, with rate-dependent poroelastic toughening. With a poroelastic cohesive zone model, solvent diffusion within the cohesive zone leads to significantly enhanced poroelastic toughening as the crack speed increases, rendering a rate-dependent traction-separation relation. While most of the results are based on a linear poroelastic formulation, future studies may extend to nonlinear theories with large deformation. In addition to the poroelastic effects, other mechanisms such as viscoelasticity and local fracture processes should be studied to further understand the time and rate-dependent fracture of polymer gels.

GPS observed horizontal ground extension at the Hutubi (China) underground gas storage facility and its application to geomechanical modeling for induced seismicity

Induced earthquakes and ground displacements were seldom reported in relation to gas injection or extraction. The Hutubi underground gas storage (HUGS) facility is the largest one in China and is also a unique case with both earthquakes and ground displacements detected during multiple cycles of injection and extraction since 9 June 2013. Unlike previous studies with a primarily seismological focus, here, we conducted quantitative analysis on the geomechanics of seismicity induced by the HUGS through developing a hydrogeologic framework, which systematically integrated geodetic, geophysical and geological data. First, we measured horizontal ground extension and shortening on the order of cm in response to gas injection and extraction of the HUGS at depth using a local GPS network, which was not reported in other regions with induced seismicity. Second, we synthesized a variety of data, including seismic reflection profiles, a newly acquired local velocity model, rock physics measurements, well drilling and logging data, to build up a 2D geomechanical model for the HUGS. Third, based on fully-coupled poroelasticity, we proposed two methods to optimize the permeability of the upper aquifers as well as the reservoir porosity and permeability with constraints from well level, GPS and well pressure data. Numerical simulations using the calibrated 2D model revealed that the horizontal extension due to the reservoir dilation is larger than ground uplift. The observed seismicity on faults without hydraulic connections to the gas repository was probably induced by the poroelastic effect of reservoir dilation. Our study provided a prototype scheme for detecting and characterizing the geomechanical behavior of cyclic fluid injection and extraction in a deep reservoir, which would be applicable to other UGS facilities.

Long-Term Evolution of Coal Permeability Under Effective Stresses Gap Between Matrix and Fracture During CO2 Injection

Understanding the long-term evolution of coal permeability under the influence of gas adsorption-induced multiple processes is crucial for the efficient sequestration of CO2, coalbed methane extraction and enhanced coal bed methane recovery. In previous studies, coal permeability is normally measured as a function of gas pressure under the conditions of constant effective stresses, uniaxial strains and constant confining pressures. In all these experiments, an equilibrium state between coal matrix and fracture is normally assumed. This assumption has essentially excluded the effect of matrix–fracture interactions on the evolution of coal permeability. In this study, we hypothesize that the current equilibrium assumption is responsible for the discrepancy between theoretical expectations and experimental measurements. Under this hypothesis, the evolution of coal permeability is determined by the effective stress gap between coal matrix and fracture. This hypothesis is tested through an experiment of CO2 injection into a coal core under the constant effective stress. In this experiment, the effective stress in the fracture system is unchanged while the effective stress in the matrix evolves as a function of time. In the experiment, the coal permeability was measured continuously throughout the whole period of the experiment (~ 80 days). The experimental results show that the core expands rapidly at the beginning due to the gas injection-induced poroelastic effect. After the injection, the core length remains almost unchanged. But, the measured permeability declines from 60 to 0.48 μD for the first month. It rebounds slowly for the subsequent 2 months. These results indicate that the effective stress gap has a significant impact on the evolution of coal permeability. The switch of permeability from the initial reduction (the first 30 days) to rebound (the subsequent 50 days) suggests a transition of matrix deformation from nearby the fracture wall to further away area. These findings demonstrate that the evolution of coal permeability is primarily controlled by the spatial transformation of effective stresses between matrix and fracture.

Measurements, Metrics, and Modeling of Normal-Incidence Acoustic Interaction With Ocean Sediment

Normal-incidence echosounder signals are attractive for sediment characterization since they can be done easily and remotely over a large area. However, in addition to being sensitive to sediment type, these measurements are also sensitive to other factors such as layers, scattering, and gradients, which make sediment characterization much more difficult. In this study, a well-characterized set of normal-incidence bottom loss measurements are compared with a finite element model using measured environmental parameters such as sound speed, attenuation, and interface roughness. The acoustic measurements were taken in the midfrequency range (5–10 kHz) during the Target and Reverberation Experiment near the Florida panhandle in May 2013. To make the comparison, two sets of metrics were considered. The first was based on the statistical moments of the acoustic signal, a method commonly used for sediment characterization. The second set of metrics was based on features of the return identified by physical mechanisms. It was determined that the physically derived metrics were more robust for model-data comparison. The three physically derived metrics used were bottom loss, full-width-at-half-maximum (FWHM), and the ratio of the half-width-at-half-maximum (HWHM) before the peak to the HWHM after the peak (i.e., HWHM ratio). The finite-element model allowed for effects such as layering, gradients, and interface roughness to be disambiguated from the bulk sediment parameters, thus identifying the influence of these factors on the normal-incidence echo return. Considering all measured environmental and sediment properties, the model could not account for all of the bottom loss without a substantially lower value for the density than measured. Thus, the sediment was modeled using an effective density, a proxy for poroelastic effects. Although the low density model was consistent for most of the data for all three metrics, several areas of outliers in both FWHM and HWHM ratio indicated that other mechanisms such as volume scattering may be important in some areas.

Dynamic compliances of rigid foundation on layered poroelastic soils

Dynamic interaction between a rigid foundation and layered poroelastic soils is studied in this paper. The foundation is subjected to time-harmonic vertical horizontal and moment loading, and bonded to a layered poroelastic half-space. The interaction problem is formulated by employing a discretization technique based on the rigid body displacement boundary conditions at the foundation-soil contact surface and the displacement influence functions of a layered poroelastic half-space. The discretization method used in this paper yields a flexibility equation system for the contact traction at the foundation-soil contact area. Comparison with existing solution for a rigid square foundation on an elastic half-space is presented to verify the accuracy of the present scheme. Selected numerical results for compliances of rigid square foundations are presented to portray the influence of poroelastic effects on the dynamic interaction problem.

Modeling of PDC single cutter – Poroelastic effects in rock cutting process

When cutting a saturated rock under pressure, the PDC cutter is not only fragmenting the rock matrix, but also driving the pore fluid ahead of it. Because of the solid-fluid coupling in rock, different pore pressures induced by cutter will affect rock failure and lead to different MSE. The fact that cutting process is influenced by the pore pressure response in the rock is referred to as poroelastic effects in this paper.This paper continues the research in our previous work (Chen et al., 2018) and gives more insights into the poroelastic effects during rock cutting process. The influences of rock diffusivity coefficient and cutter speed are studied. The results show that the two parameters will affect pore pressure response in rock and further affect rock failure and MSE during cutting process. Based on the results, the cutting process can be identified as three conditions: undrained, drained and a transition zone between undrained and drained condition. In undrained and drained condition, MSE will be independent of cutter speed; while in transition condition, MSE decreases with increasing cutter speed. The transition boundaries for the three conditions are given. Cavitation in intact rock during cutting process is also studied. The results show that cavitation is easy to occur when cutting a hard rock with low original pore pressure.Cutting tests were conducted on Torrey Buff sandstone and Carthage marble to verify the poroelastic effects in cutting process. A good agreement between the model results and experiments is found. In general, the results in this paper can give a good understanding on the combined influence of formation permeability, depth of cut and RPM on cutting rock during drilling.

Pore-pressure diffusion, enhanced by poroelastic stresses, controls induced seismicity in Oklahoma

Induced seismicity linked to geothermal resource exploitation, hydraulic fracturing, and wastewater disposal is evolving into a global issue because of the increasing energy demand. Moderate to large induced earthquakes, causing widespread hazards, are often related to fluid injection into deep permeable formations that are hydraulically connected to the underlying crystalline basement. Using injection data combined with a physics-based linear poroelastic model and rate-and-state friction law, we compute the changes in crustal stress and seismicity rate in Oklahoma. This model can be used to assess earthquake potential on specific fault segments. The regional magnitude-time distribution of the observed magnitude (M) 3+ earthquakes during 2008-2017 is reproducible and is the same for the 2 optimal, conjugate fault orientations suggested for Oklahoma. At the regional scale, the timing of predicted seismicity rate, as opposed to its pattern and amplitude, is insensitive to hydrogeological and nucleation parameters in Oklahoma. Poroelastic stress changes alone have a small effect on the seismic hazard. However, their addition to pore-pressure changes can increase the seismicity rate by 6-fold and 2-fold for central and western Oklahoma, respectively. The injection-rate reduction in 2016 mitigates the exceedance probability of M5.0 by 22% in western Oklahoma, while that of central Oklahoma remains unchanged. A hypothetical injection shut-in in April 2017 causes the earthquake probability to approach its background level by ∼2025. We conclude that stress perturbation on prestressed faults due to pore-pressure diffusion, enhanced by poroelastic effects, is the primary driver of the induced earthquakes in Oklahoma.

Basement Fault Reactivation by Fluid Injection Into Sedimentary Reservoirs: Poroelastic Effects

AbstractTo investigate mechanisms of seismic fault reactivation in crystalline basement in response to fluid injection in overlying sedimentary reservoirs, we conducted three‐dimensional finite element simulations to assess the effects of direct pore pressure communication and indirect poroelastic stress transfer on the change in Coulomb failure stress of favorably oriented faults of varying permeability structure in normal, strike‐slip, and reverse faulting stress regimes. We demonstrate that the direct pore pressure effect transmitted along a hydraulically conductive fault exceeds the indirect poroelastic effect, but alone is insufficient for fault reactivation in the basement. The poroelastic effect on the Coulomb failure stress results from induced normal tractions and, to a lesser extent, from induced shear tractions that relate to the flexing of the fault as the reservoir expands poroelastically with fluid injection. Assuming a higher Biot coefficient for reservoir over basement rock as previously reported, the combined direct pore pressure and indirect poroelastic effects result in reactivation of hydraulically conductive faults in the basement in normal and strike‐slip faulting stress regimes and in the reservoir in reverse faulting regimes. Sealing normal faults that are not preferentially conductive also preferentially reactivate in the reservoir. These findings apply to injection in either hanging or footwall of normal and reverse faults. Reducing the contrast in Biot coefficient between reservoir and basement favors fault reactivation in the reservoir for injection in the footwall in normal faulting stress regimes. These simulations demonstrate that geomechanical models without coupled poroelasticity underestimate the potential of fault reactivation in crystalline basement.