Pore Pressure Perturbations Research Articles

On the role of poroelastic stressing and pore pressure diffusion in discrete fracture and fault system in triggering post-injection seismicity in enhanced geothermal systems

Injection-induced seismicity has become one of the most critical challenges for the widespread deployment of Enhanced Geothermal Systems (EGS). In particular, some EGS development projects have led to large, damaging earthquakes that unexpectedly occurred far off the stimulated reservoir region and, in particular, after stopping fluid injection. Yet, the causative mechanisms of these seismicity patterns remain highly elusive. Here, we identify a combination of mechanisms that could explain delayed seismicity in EGS sites by conducting fully-coupled hydromechanical simulations of the hydraulic stimulation of a naturally-fractured granitic reservoir. The model comprises a sparse network of long, variably-oriented fractures interacting with a nearby, critically-oriented fault. The results show that the presence of fractures introduces notable nonlinearities in the flow field and rock deformation and significantly expands the rock volume affected by fluid injection. First, the stimulated fracture network provides highly-permeable conduits for communicating elevated pore pressure over long distances. Second, the anisotropic expansion of fractures generates shear stress that is transmitted almost instantaneously across the reservoir. The pore pressure and stress perturbations can not only cause slip along fractures, inducing (micro)seismicity during injection, but also affect the stability of nearby faults, which may not necessarily be pressurized during injection. The transferred poroelastic stresses can increase or decrease the slip tendency along different fault segments. However, the fault may reactivate only after several months following injection when a progressive pore pressure diffusion modulated by the transient fault permeability evolution brings a critically-stressed fault segment to failure conditions. We also find that the spatiotemporal evolution of seismicity depends largely on the nearby fault orientation, hydromechanical properties, and hydraulic connection with the fracture network, as well as the initial state of stress. We conclude that accurate subsurface characterization and continuous monitoring during and after injection should allow for managing the risks posed by injection-induced seismicity and safely unlocking the immense potential for clean and sustainable geothermal energy.

Surface Deformation and Seismicity Linked to Fluid Injection in the Raton Basin.

It is suggested that in addition to seismicity deep fluid injection may cause surface uplift and subsidence in oil and gas-producing regions. This study uses the Raton Basin as an example to investigate the hydromechanical processes of surface uplift and subsidence during wastewater injection. The Raton Basin, in southern central Colorado and northern central New Mexico, has experienced wastewater injection related to coalbed methane and gas production starting in 1994. In this study, we estimate the extent and magnitude of total vertical deformation in the Raton Basin from 1994 to 2020 and incremental deformation between the years 2017 to 2020. Results indicate a modeled uplift as much as 15 cm occurring between 1994 and 2020. Between 2017 and 2020, up to 3 cm of uplift occurred, largely near the northwestern injection wells. Most modeled uplift between 1994 and 2020 occurred near the southern wells, where the greatest cumulative volume of wastewater was injected. However, modeled subsidence occurred around the southern and eastern wells between 2017 and 2020, after the rate of injection decreased. Modeling indicates that while the magnitude of modeled uplift corresponds to cumulative injection volume and maximum rate in the long-term, short-term incremental deformation (uplift or subsidence) is controlled by changes in the rate of injection. The number of yearly earthquake events follows periods of rapid modeled uplifting throughout the Basin, suggesting that measurable surface deformation may be caused by the same injection-induced pore pressure perturbations that initiate seismicity.

Bedrock Fractures Control Groundwater‐Driven Mountain Slope Deformations

AbstractSeasonal deformation of mountain rock slopes can be driven by groundwater infiltration and depletion. Such processes could explain our field observation in the Aletsch Valley, Switzerland, where GNSS‐derived 3D annual displacement amplitudes reach 3.4 cm. However, the physical mechanisms behind such groundwater‐driven surface displacements are not well understood. Here, we develop a fully coupled hydromechanical model to simulate the relevant processes in a valley slope embedded with numerous fractures of variable sizes. The magnitude and orientation of transient annual slope surface displacement obtained from our model are in overall agreement with the field observations. The key geological factors controlling the type and magnitude of reversible mountain slope deformations are fracture network geometry, fracture aperture, and regional stress field. We show that the heterogeneity and anisotropy of bedrock hydromechanical responses, originating from depth‐dependent variations of fracture properties, play a critical role in groundwater recharge and valley slope deformation. During recharge events, pore pressure perturbations migrate downward from the groundwater table and toward the receiving stream and the deep subsurface. This process driven by pressure diffusion and poroelastic stressing develops in the subsurface with a great reach of up to a few kilometers, called critical hydromechanical response zone, and controls surface deformation patterns. During groundwater recession, this hydromechanical response zone expands downward and ground surface displacement vectors rotate upwards. Our results suggest that slope surface deformation can inform about subsurface permeability structures and pore pressure fluctuations, which have important implications for understanding groundwater flow in fractured bedrock slopes.

3D coupled hydro-mechanical modeling of multi-decadal multi-zone saltwater disposal in layered and faulted poroelastic rocks and implications for seismicity: An example from the Midland Basin

Knowledge of subsurface coupled pore pressure and stress perturbations due to saltwater disposal (SWD) is crucial for understanding and managing induced seismicity. Current studies choose Biot poroelasticity to implement fluid-rock coupling and perform numerical modeling in relatively simple settings. In this study, we first introduce a general poromechanical framework to capture a broader range of hydro-mechanical coupling and then apply it to model real-world SWD problems with geological and operational complexities. We demonstrate a viable workflow through the case study of a 2500 km2, layered, faulted, and recently seismically awaken area that has undergone multi-decadal multi-zone injection by nearly 200 SWD wells in the Midland basin. The example provides a unique opportunity to study the respective impact of shallow and deep injections across depths and the role of inter-zone interactions, among others. Our modeling suggests over 500 and 250 psi increases in pore pressure and Coulomb stress, respectively, in the deep disposal zone in the last decade, which were 5 to 10 times greater than those accumulated over the past four decades in the two shallower disposal zones. Vertically, shallower injections impacted mostly shallower zones, whereas the deep injection produced changes across depths and dominated especially in the deep zone and the upper segment of the basement. Towards the mid-segment of the basement, changes due to shallow and deep injections were comparable, with the latter 2 to 5 times higher. Laterally, in-zone changes were dominated by pore pressure diffusion in the near field and poroelastic stressing in the far field in shallower zones but were dominated by pore pressure diffusion everywhere in the deep zone, owing to their different hydraulic diffusivities. Faults acted as pore pressure and Coulomb stress sinks for disposal zones and sources for non-disposal zones, and fault-zone permeability structure had a minor impact. Finally, seismological modeling suggests that seismicity in the area was most likely driven by the deep injection.

Pore Pressure Diffusion and Onset of Induced Seismicity

AbstractThe Raton Basin of Southern Colorado and Northern New Mexico experienced a significant increase in seismicity over the last several decades, including two M5+ earthquakes. The increase in seismicity started soon after an increase in wastewater disposal associated with coal‐bed methane production. Twenty‐nine wastewater injection wells have operated in the Basin, with several high‐rate injectors located within a kilometer of the most seismically active regions. We developed a pore pressure model to investigate the relationship between pore pressure diffusion and onset of induced seismicity, with an emphasis on the Trinidad fault zones. Our model indicates that sufficient pore pressure increase had accumulated on the northern extent of the Trinidad fault zone to induce seismicity at the time of early‐stage seismic activity. The hydrogeologic architecture of the Basin played a key role in controlling pore pressure perturbation. The basal sedimentary interval with moderately low diffusivity restricted vertical pore pressure diffusion, creating a substantial lag time between wells becoming active and the expression of perturbation in the fault zones. The spatiotemporal relationship between early‐stage seismicity and wastewater injection in conjunction with modeled pore pressures suggests that the early‐stage seismicity in the Raton Basin was likely triggered by pore pressure diffusion.

Investigating seismicity rates with Coulomb failure stress models caused by pore pressure and thermal stress from operating a well doublet in a generic geothermal reservoir in the Netherlands

Abstract The utilisation of geothermal energy in the Netherlands is primarily focused on deep sedimentary aquifers, which are often intersected by major faults. Geothermal operations (i.e. fluid production and injection) may alter the effective stress state along these faults and trigger induced seismic events. Pore pressure perturbations have been generally considered the main driver of injection-induced seismicity. However, thermal stresses caused by temperature gradients between the re-injected cold fluid and the reservoir rock may also contribute to the triggering of earthquakes in geothermal reservoirs. While existing geothermal power plants operating in sandstone reservoirs did not produce any major induced seismicity, it is a matter of debate whether a reduction in the temperature of the re-injected fluid could increase the seismic hazard potential. In this study, we applied modified Gutenberg–Richter statistics based on frictional Coulomb stress variations implemented in a coupled thermo-hydro-mechanical model to estimate the seismic hazard caused by the operation of a geothermal doublet. We conducted a systematic parametric study to assess and rank the impact of different intrinsic (geological) and extrinsic (operational) parameters on the induced seismic hazard potential. We identified a competing mechanism between induced variations in pore pressure and thermal stress within the reservoir in controlling induced seismicity. We found that stress changes induced by pore pressure variations are the main cause of seismic hazard, although thermally induced stresses also contribute significantly. The results indicate that by optimising the operational parameters it is possible to increase production efficiency while maintaining a long-term control over the fluid injection-induced seismicity.

Slip Characteristics of Induced Earthquakes: Insights From the 2015 Mw 4.0 Guthrie, Oklahoma Earthquake

AbstractTo better quantify how injection, prior seismicity, and fault properties control rupture growth and propagation of induced earthquakes, we perform a finite‐fault slip inversion on a Mw 4.0 earthquake that occurred in April 2015, the largest earthquake in an induced sequence near Guthrie, Oklahoma. The slip inversion reveals a rupture with slip patches that are anti‐correlated to the locations of prior seismicity. The prior seismicity driven by low pore pressure changes and static stress changes occurred on weaker portions of the fault, while the Mw 4.0 earthquake likely ruptured relatively stronger portions of the fault. To resolve if pore pressure changes or the initial underlying stress distribution and fault strength controlled the final slip distribution of the Guthrie Mw 4.0 earthquake, we compare strike‐slip events of similar magnitude from tectonically active regions and previously inactive regions. Earthquakes on reactivated faults exhibit different slip distributions than active regions, they have more prominent and well separated slip patches, a behavior often associated with faults of lower fault maturity. Pore pressure shows little effect on the distributions. These observations suggest that the initial underlying stress distribution and fault strength of reactivated faults in low deformation regions is the primary controlling factor of the slip distribution with pore pressure perturbations and earthquake interactions being secondary. Therefore, Guthrie Mw 4.0 earthquakes slip distribution was enhanced by pore‐pressure perturbations and earthquake interactions by creating an optimal stress state for its failure, but the slip distribution itself is controlled by its fault's initial stress and strength state.

Stress Drop Variations of Induced Earthquakes near the Dallas–Fort Worth Airport, Texas

Abstract We estimate stress drops for injection-induced earthquakes near the Dallas–Fort Worth Airport in the Fort Worth basin (FWB), Texas, to investigate source properties in response to fluid injection. The stress drops for the Airport sequence show three unique characteristics compared to those estimated for other earthquake sequences in the FWB: (1) stress drops have lower mean and median values; (2) stress drops increase with moment magnitude; and (3) stress drops increase in size over the first 1.5 km in radial distance from the injection point. The low stress drop Airport events occurred shortly after the initiation of injection near a fault within hundreds of meters of the well. Pore pressure perturbations in the Airport area are 1 order of magnitude lower than those from the other sequences, suggesting that absolute pore pressure changes may not be the main factors of stress drop variations. We suggest that the low stress drop events may be related to transition from aseismic slip to seismic rupture previously observed in laboratory and field experiments.

Effects of fracture network distribution on excavation-induced coupled responses of pore pressure perturbation and rock mass deformation

We perform a numerical study of the spatio-temporal evolution of rock damages and fracture displacements around underground excavation in fractured rocks. We model fracture networks based on a power law model and explore different values of the length exponent and fracture intensity. An elasto-brittle constitutive relation is employed to characterise the deformation/failure of intact rocks, while a non-linear stress–displacement relation is used to mimic the normal/shear displacement of pre-existing fractures. Fluid flow through fractured porous media is simulated based on Darcy’s law. Both direct coupling (via poroelasticity) and indirect coupling (via changes of rock mass properties) are considered, with excavation-induced stress alteration, pore pressure variation, damage evolution, and fracture deformation realistically captured. It is observed that, fracture systems dominated by large fractures in general accommodate significant sliding along pre-existing fractures but minor damage in intact rocks, while fracture systems governed by small fractures can have profound damage and slippage simultaneously. Excavation-induced stress variation near the tunnel can instantly cause pore pressure fluctuations in low-permeability rock matrix, promoting fracture shear and rock damage. In addition, we investigate the microseismicity generated during and after the excavation as well as analyse the mechanisms of excavation and drainage-induced disturbances. Our results have important implications for underground excavation activities involved in many geoengineering applications.

Changes in Fault Slip Potential Due to Water Injection in the Rongcheng Deep Geothermal Reservoir, Xiong’an New Area, North China

The Xiong’an New Area is abundant in geothermal resources due to its unique geological structure. To address whether large-scale deep geothermal exploitation will induce a fault slip, we first determined the initial in situ stress field using shallow (~4000 m) in situ stress measurements from the North China plain. After characterizing the in situ stress field, we analyzed the initial stability of the main active faults in the sedimentary strata of the Rongcheng deep geothermal reservoir based on the Mohr–Coulomb failure criteria. Assuming that this area will be subjected to forty years of continuous fluid injection, we calculated the excess pore pressure in the deep geothermal reservoir and, subsequently, estimated the fault slip potential of the main active faults in this region from 2021 to 2060. Our results indicated that both the in situ stress field in the shallow crust of the Xiong’an New Area and the Middle-Late Pleistocene active faults will initially maintain a stable state. With constant fluid injection for forty years at six geothermal wells in the Rongcheng deep geothermal reservoir, the maximum superposed excess pore pressure at a single well is 18 MPa; this excess pore pressure value impacts the stress state of faults within 8 km of the well location. These pore pressure perturbations heavily impact the F5-10, F5-11, and F9-2 segments of the Rongcheng uplift boundary fault, with FSP values of 92%, 23%, and 47% in 2060, respectively. Porosity exacts little impact on the fault slip potential on the boundary fault segments of F5-10 and F9-2 in the Rongcheng deep geothermal reservoir, while an enhanced permeability can weaken the FSP values for these faults. The predicted maximum moment magnitude of an induced earthquake due to continuous injection of forty years can be up to Mw 5.0 with a 5% fluid loss in the Rongcheng deep geothermal reservoir. Long-term water injection may increase the ambient thermoelastic stress to the point where faults in a critical (or subcritical) stress state become unstable. The results can provide a reference for geothermal development in terms of injection rate and locations of geothermal wells.

Controls of fault geometry and thermal stress on fault slip modes: Implications for permeability enhancement and injection-induced seismicity

Fluid pressurization within the fault zone generates increasing pore pressure and stress change which is liable to create shear and/or brittle fractures within the reservoir volumes and subsequently generating earthquakes of varying magnitudes. Here, we explored time-dependent fault weakening processes in the fault zone which are dependent on several factors, including the rate of cold-water injection, modes of injection (hydromechanical (HM) and thermo-hydro-mechanical (THM) interactions), and changing fault spatial configurations using data from Niger Delta Basin. The variation in the stability of different fault models in withstanding stresses induced by HM and THM fluid interactions is evident. Fault permeability enhancement and the behaviour of slip event under isothermal and non-isothermal conditions revealed that stress and pore pressure perturbations have a first order control on the rate of fault dilation and compression. It is observed that the progressive cooling of the reservoir induced thermal stress which induced the timing of slip by unloading the fault to earlier seismic rupture in the non-isothermal case, and accelerates the magnitude of the fault reactivation and the accompanied induced seismicity. Owing to increased tendency of shear failure during injection, fracture opening through shear dilation is more enhanced in THM simulation as the fracture permeability is significantly higher than in HM. This effect becomes increasingly more dominant with intermediate fault angle and joint orientation. Certain fault/joint configurations which were resistant to shear failure under isothermal injection had their frictional resistance broken by thermal stress. The results also indicate that there is higher pore pressure build-up in THM than in HM as the injection rate increases and reservoir temperature drops during cold injections.. This study has demonstrated the importance of fully characterizing the fracture geometries and configurations of normal faulting regime in addition to fluid injection conditions when developing fractured reservoirs to mitigate seismic risks and hazards that could result from early fault reactivation.

The Role of Fluid Pressure‐Induced Aseismic Slip in Earthquake Cycle Modulation

AbstractThe evolving state of fault stress during and after the perturbation of fluid pressure gives rise to an intriguing interplay of seismic and aseismic slip on the fault. A better understanding of the possible role of fluids in the triggering mechanism of seismicity is pivotal to effective seismic hazard mitigation, particularly in the context of induced seismicity. Through numerical modeling, we investigate the effect of pore pressure perturbations on the spatio‐temporal evolution of fault slip and the modulation of earthquake cycles. Pressure perturbations are imposed at different magnitudes and different times during a selected interseismic period. Results show a wide range of aseismic responses which can lead to both time advancement and delay of subsequent earthquakes. Specifically, even pressure perturbation <5% of the average event stress drop can trigger aseismic slip that leads to considerable time delay in the next earthquake even when perturbation occurs late in the interseismic period. We find that earthquakes that are delayed in time are associated with large aseismic moment release. Our study highlights the importance of close monitoring of aseismic fault slip in regions prone to the influence of pore fluids and provides physical insights into identifying critical aseismic responses associated with certain triggering outcomes.

Stress-Drop Estimates for Induced Seismic Events in the Fort Worth Basin, Texas

ABSTRACTEarthquakes in the Fort Worth basin (FWB) have been induced by the disposal of recovered wastewater associated with extraction of unconventional gas since 2008. Four of the larger felt earthquakes, each on different faults, prompted deployment of local distance seismic stations and recordings from these four sequences are used to estimate the kinematic source characteristics. Source spectra and the associated source parameters, including corner frequency, seismic moment, and stress drop, are estimated using a modified generalized inversion technique (GIT). As an assessment of the validity of the modified GIT approach, corner frequencies and stress drops from the GIT are compared to estimates using the traditional empirical Green’s function (EGF) method for 14 target events. For these events, corner-frequency residuals (GIT−EGF) have a mean of −0.31 Hz, with a standard deviation of 1.30 Hz. We find consistent mean stress drops using the GIT and EGF methods, 9.56 and 11.50 MPa, respectively, for the common set of target events. The GIT mean stress drop for all 79 earthquakes is 5.33 MPa, similar to estimates for global intraplate earthquakes (1–10 MPa) as well as other estimates for induced earthquakes near the study area (1.7–9.5 MPa). Stress drops exhibit no spatial or temporal correlations or depth dependency. In addition, there are no time or space correlations between estimated FWB stress drops and modeled pore-pressure perturbations. We conclude that induced earthquakes in the FWB occurring on normal faults in the crystalline basement release pre-existing tectonic stresses and that stress drops on the four sequences targeted in this study do not directly reflect perturbations in pore-fluid pressure on the fault.

4D Travel-Time Tomography as a Tool for Tracking Fluid-Driven Medium Changes in Offshore Oil–Gas Exploitation Areas

The monitoring of rock volume where offshore exploitation activities take place is crucial to assess the corresponding seismic hazard. Fluid injection/extraction operations generate a pore fluid pressure perturbation into the volume hosting the reservoir which, in turn, may trigger new failures and induce changes in the elastic properties of rocks. Our purpose is to evaluate the feasibility of reconstructing pore pressure perturbation diffusion in the host medium by imaging the 4D velocity changes using active seismic. We simulated repeated active offshore surveys and imaged the target volume. We constructed the velocity model perturbed by the fluid injection using physical modeling and evaluated under which conditions the repeated surveys could image the velocity changes. We found that the induced pressure perturbation causes seismic velocity variations ranging between 2–5% and 15–20%, depending on the different injection conditions and medium properties. So, in most cases, time-lapse tomography is very efficient in tracking the perturbation. The noise level characterizing the recording station sites is a crucial parameter. Since we evaluated the feasibility of the proposed 4D imaging strategy under different realistic environmental and operational conditions, our results can be directly applied to set up and configure the acquisition layout of surveys aimed at retrieving fluid-induced medium changes in the hosting medium. Moreover, our results can be considered as a useful starting point to design the guidelines to monitor exploitation areas.

Fluid-Triggered Aftershocks in an Anisotropic Hydraulic Conductivity Geological Complex: The Case of the 2016 Amatrice Sequence, Italy

The mechanism by which faults interact each other is still a debated matter. One of the main issues is the role of pore-pressure diffusion in the delayed triggering of successive events. The 2016 Amatrice-Visso-Norcia seismic sequence (Central Apennines, Italy) provides a suitable dataset to test different physical mechanisms leading to delayed events. The sequence started on August 24, 2016, with the Amatrice mainshock (MW = 6), and was followed after more than 60 days by events in Visso (MW = 5.4) and Norcia (MW = 5.9). We analysed the contribution of the static stress change and the role of fluids in the delayed triggering. Through 3D poroelastic modelling, we show that the Amatrice mainshock induced a pore-pressure diffusion and a normal stress reduction in the hypocentral area of the two aftershocks, favouring the rupture. Our parametric study employs a simple two-layered conductivity model with anisotropy in the seismogenic layer, characterized by larger conductivity values (K>10-5 m/s) along the NNW-SSE direction. The one-way coupled pore-pressure 3-D diffusion modelling predicts the maximum increase of the pore pressure at the location of the two Visso earthquakes 60 days after the mainshock. The occurrence of anisotropic diffusivity is supported by the pattern of active faults and the strong crustal anisotropy documented by S-wave splitting analysis. We conclude that the temporal evolution of the sequence was controlled by the anisotropic diffusion of pore-pressure perturbations through pre-existing NNW-trending fracture systems.

Stress Path Analysis for Characterization of In Situ Stress State and Effect of Reservoir Depletion on Present-Day Stress Magnitudes: Reservoir Geomechanical Modeling in the Gulf of Suez Rift Basin, Egypt

A reservoir geomechanical modeling has been attempted in the hydrocarbon-bearing Miocene formations in the offshore Badri field, Gulf of Suez, Egypt. Pore pressure established from the direct downhole measurements indicated sub-hydrostatic condition in the depleted mid-Miocene Hammam Faraun and Kareem reservoirs. Vertical stress (Sv) estimated using bulk density data yielded an average of 0.98 PSI/feet (22.17 MPa/km) gradient. Magnitudes of minimum (Shmin) and maximum (Shmax) horizontal stresses were deduced from the poro-elastic model. Relative stress magnitudes (Sv ≥ Shmax > Shmin) reflect a normal faulting tectonic stress in the Badri field. Pore pressure and stress perturbations (ΔPP and ΔSh) in the depleted reservoirs investigated from actual measurements recognized ‘stress path’ values of 0.54 and 0.59 against the Hammam Faraun and Kareem Formations, respectively. These stress path values are far away from the normal faulting limit (0.68), indicating induced normal faulting or fault reactivation to be unlikely at the present depletion rate.

Earthquakes Induced by Wastewater Injection, Part II: Statistical Evaluation of Causal Factors and Seismicity Rate Forecasting

ABSTRACTWastewater disposal has been reported as the main cause of the recent surge in seismicity rates in several parts of central United States, including Oklahoma. In this article, we employ the semi-empirical model of the companion article (Grigoratos, Rathje, et al., 2020) first to test the statistical significance of this prevailing hypothesis and then to forecast seismicity rates in Oklahoma given future injection scenarios. We also analyze the observed magnitude–frequency distributions, arguing that the reported elevated values of the Gutenberg–Richter b-value are an artifact of the finiteness of the pore-pressure perturbation zones and a more appropriate value would be close to 1.0. The results show that the vast majority (76%) of the seismically active blocks in Oklahoma can be associated with wastewater disposal at a 95% confidence level. These blocks experienced 84% of the felt seismicity in Oklahoma after 2006, including the four largest earthquakes. In terms of forecasting power, the model is able to predict the evolution of the seismicity burst starting in 2014, both in terms of timing and magnitude, even when only using seismicity data through 2011 to calibrate the model. Under the current disposal rates, the seismicity is expected to reach the pre-2009 levels after 2025, whereas the probability of a potentially damaging Mw≥5.5 event between 2018 and 2026 remains substantial at around 45%.

Role of Fluid Injection on Earthquake Size in Dynamic Rupture Simulations on Rough Faults

AbstractAn outstanding question for induced seismicity is whether the volume of injected fluid and/or the spatial extent of the resulting pore pressure and stress perturbations limit rupture size. We simulate ruptures with and without injection‐induced pore pressure perturbations, using 2‐D dynamic rupture simulations on rough faults. Ruptures are not necessarily limited by pressure perturbations when (1) background shear stress is above a critical value, or (2) pore pressure is high. Both conditions depend on fault roughness. Stress heterogeneity from fault roughness primarily determines where ruptures stop; pore pressure has a secondary effect. Ruptures may be limited by fluid volume or pressure perturbation extent when background stress and fault roughness are low, and the maximum pore pressure perturbation is less than 10% of the background effective normal stress. Future work should combine our methodology with simulation of the loading, injection, and nucleation phases to improve understanding of injection‐induced ruptures.

On the Role of Poroelasticity in the Propagation Mode of Natural Fractures in Reservoir Rocks

Almost all reservoir rocks contain natural fractures. The presence of such discontinuities affects reservoir stimulation to a certain degree. The interaction between the natural and hydraulic fractures determines the overall fracture trajectory and the stimulated reservoir volume. In contrast to conventional hydraulic fracturing, natural fractures do not propagate in a tensile-dominated regime. Propagation of mechanically closed fractures lies in the mixed-mode propagation regime which differs from that of open fractures. Therefore, the response of natural fractures to pore pressure and stress perturbations should be carefully addressed within the context of reservoir stimulation. This study aims at investigating the response of closed natural fractures to water injection in poroelastic rocks. A poroelastic displacement discontinuity (DD) model is developed and used to shed light on the propagation of closed natural fractures. Mohr–Coulomb contact elements are used in this study to identify the contact status of the elements in the transverse direction. Moreover, fracture propagation is effectively accounted for based on linear elastic fracture mechanics. Hydro-mechanical coupling is achieved by combining an implicit finite difference scheme with the poroelastic DD model. Our simulation results indicate that natural fracture shear slip and propagation are likely to occur when the closed natural fractures are subject to direct water injection. The injection pressure required to maintain the propagation in poroelastic rocks was found to be consistently higher than the pressure in its elastic counterpart. The propagation trajectory of the wing cracks was, on the other hand, found to be similar to that in elastic rocks. It was found in our results that the formation of wing cracks and shear (secondary) cracks is an integral part of the reservoir stimulation in geothermal systems where natural fractures are in direct contact with the injection wells. This point is often overlooked or poorly treated and has led to erroneous interpretations from the reservoir stimulation standpoint.

Sensitivity of Acoustic Emission Triggering to Small Pore Pressure Cycling Perturbations During Brittle Creep

AbstractFluid‐induced stress perturbations in the crust at seismogenic depths caused by sources such as tidal or seasonal loading may trigger earthquakes. We investigate the role of small periodic pore pressure (Pp) perturbation in rupture nucleation by performing laboratory triaxial creep experiments on Fontainebleau sandstone, saturated in water, under sinusoidal Pp variations. Results show that recorded acoustic emissions (AEs) correlate with Pp as the rock approaches failure. More interestingly, AEs occur significantly more when Pp is decreasing, that is, when strain rate is maximum with a progressive increase of Pp‐AEs correlation in time as the rock approaches failure. This suggests that the correlation of small stress perturbations and AEs not only depends on Pp amplitude but also on the criticality of the rock. Observations at the laboratory scale support field observations where tidal loading may have modulated seismic rates during the nucleation phase of the 2004 Sumatra‐Andaman and 2011 Tohoku‐Oki earthquakes.