Fault/fracture slip and seismicity caused by fluid injection are of major interest to subsurface science and engineering. However, some fundamental aspects of the temporal and spatial evolutions of ...induced seismicity remain unresolved. In this paper, we present the results of a laboratory injection‐induced shear test conducted on a rough granite fracture with concurrent acoustic emission (AE) monitoring. The results demonstrate a sequence of aseismic‐seismic‐aseismic fracture motion during fluid injection. It is observed that the temporal evolution of AE/microseismic activities was accompanied by the changes of slip velocity, stress drop, and friction coefficient. The seismic slip phase consists of three subphases, namely, a first quasi‐static slip, a dynamic slip, and a second quasi‐static slip. The dynamic slip occurred with an AE mainshock, simulating earthquake instability triggered by injection in deep crystalline rocks. In addition, slip heterogeneity controlled by the fracture surface roughness is directly evidenced by the spatially heterogeneous distribution of AE hypocenters and fracture surface topography.
Plain Language Summary
Inducing fracture slip, dilation, and propagation to create a highly conductive fracture network by injection has been viewed as an effective reservoir stimulation approach for subsurface energy extraction. Fault/fracture slip can cause seismic/microseismic events, which can be used to estimate the stimulated volume. There are also some concerns regarding seismic hazard related to fluid injection. In order to better understand the underlying physics of fault slip and induced seismicity, we performed laboratory experiments. Here, we report results from an injection‐induced shear test conducted on a rough granite fracture, in which the fracture deformation, fluid flow, and acoustic emission were all concurrently measured. Our results demonstrate that faults/fractures may slip in different modes (from creep, slow quasi‐static slip, to fast dynamic slip) during fluid injection and each slip mode is reflected by a different AE/microseismic response and frictional behavior (slip weakening or slip strengthening). Moreover, it is observed that the spatially heterogeneous distribution of induced AE events is directly linked with the fracture surface topography and the shear‐induced asperity damage, indicating the controlling effect of surface topography on fault slip heterogeneity.
Key Points
A laboratory injection‐induced shear test has been conducted on a rough granite fracture with concurrent acoustic emission monitoring
Aseismic‐seismic transition accompanied by temporal evolution of slip velocity, stress drop, and friction coefficient has been observed
The link between slip heterogeneity and fracture roughness is revealed by AE hypocenters distribution and fracture surface topography
Shear stimulation, so‐called hydroshearing, is believed to reactivate critical or near‐critical preexisting fractures by pressurized fluid injection, causing them to slip and dilate for economic ...production from geothermal and unconventional petroleum reservoirs. However, little or no experiments have been conducted to directly probe the coupled hydromechanical responses and permeability evolution during fracture shearing. In this paper, we present the results of novel injection‐induced shear tests on cylindrical granite samples each containing a single tensile or saw cut fracture. The shear test on rough fractures demonstrates that retainable permeability enhancement can be achieved through dilatant shear slip. The characteristic stick‐slip behavior of the fracture during shearing is also observed. Fracture aperture controlled by effective normal stress dominates the fluid flow in stick state, while in slip state, the major contributor to production increase is the irreversible normal dilation caused by fracture shear slip. The experimental data show that depending on the fracture roughness, the fracture slip process includes two quasi‐static slip intervals with a slip velocity of ~10−9 to ~10−6 m/s and a dynamic slip interval with a slip velocity of ~10−7 to ~10−5 m/s. The fracture slip correlates well with the associated stress relaxation: a faster fracture slip induces a quicker stress relaxation, and vice versa. The influence of surface roughness on the hydromechanical responses during fracture shearing and the shear‐induced asperity degradation are also studied. The various experimental observations on rough and smooth fractures clearly indicate that the key component to shear stimulation is fracture self‐propping by asperities.
Key Points
Dilatant shear slip and retainable permeability enhancement have been demonstrated in advanced injection experiments on granite fractures
The permeability evolution, slip characteristics, and stress relaxation during injection‐induced fracture shearing have been revealed
Shear‐induced asperity degradation and the influence of surface roughness on hydromechanical coupled responses of fractures have been studied
•A fully-coupled displacement discontinuity (DD) model with robust fracture propagation capability has is developed.•The model is used to simulate secondary crack propagation associated with natural ...fracture slip The secondary cracks form as wing cracks and/or shear cracks.•The secondary cracks form as wing cracks and/or shear cracks•The predominance of wing cracks and their lengths and paths are controlled by the relative value of differential and mean stresses.•The predominance of wing cracks and their lengths and paths are controlled by natural fracture length, and natural fracture frictional properties.•Natural fracture coalescence could be achieved through the secondary cracks in both the tensile and shear modes•In reservoir with low differential stress and/or high mean stress, shear crack propagation could play a major player in network formation and permeability enhancement
Permeability enhancement via shear slip is commonly accepted as the main stimulation mechanism. However, the mechanism of permeability increase appears to have been understood to be limited to shear dilation and perceived to exclude the propagation in tensile and shear mode of the natural fractures that experience slip. This has led to the claims of the discovery of a new stimulation mechanism, namely, stimulation via wing crack propagation. The root cause of the misconceptions is likely the inability to model natural fracture propagation and coalescence. However, natural fracture propagation in general and wing cracks in particular are to be viewed as an integral part of the shear slip stimulation mechanism because shear slip increases the stress-intensity at the fracture tips, potentially leading to fracture propagation. In an effort to better illustrate the underlying mechanisms in the geothermal reservoir stimulation process, a displacement discontinuity (DD) model is developed and used to simulate secondary crack propagation associated with natural fracture slip. The model uses Mohr-Coulomb joint (contact) elements and rigorously accounts for fracture propagation. The model is applied to explore the conditions conducive to shear and tensile mode fracture propagation. When natural fractures undergo shear slip due to direct and indirect water injection, out-of-plane wing (tensile) cracks form and propagate at injection pressures below the minimum in-situ stress level and turn toward the maximum in-situ stress direction as they grow longer. It was found in our results that the injection pressure is stabilized at approximately the minimum in-situ stress level. The secondary cracks form as wing cracks and/or shear cracks. The predominance of wing cracks and their lengths and propagation paths were found to be controlled by the relative value of differential and mean stresses, natural fracture length, as well as rock and natural fracture frictional properties. In deeper geothermal reservoir with relatively low differential stress conditions and/or high mean stress levels, the shear crack propagation could play a major role in fracture network formation and permeability enhancement.
Identifying distributed strain sensing (DSS) patterns (or signatures), particularly those arising from different hydraulic fracture geometries, has gained significant attention and research effort. ...Recent works have generated a catalogue of signatures for planar hydraulic fractures in an elastic rock formation. Yet, in numerous cases (e.g., fault motion and some geothermal reservoir stimulation), the main mode of deformation is a shear on a fracture or a network of natural fractures (particularly during low pressure injection/circulation). However, the specific fiber signatures that result from such shear deformation have not been studied. In this study, we use a three-dimensional poroelastic hydraulic fracture simulator to capture the strain signatures resulting from the shear deformation of fractures in various orientations with respect to the monitoring well. Five key cases are examined: one where the fracture strike is perpendicular to the fiber, another with the strike running parallel to the fiber, a third case where the fracture strike is at 45 degrees to the fiber, a fourth case with a strike slip fault perpendicular to the fiber, and a fifth case where fiber is intersecting the fracture. Theoretically and physically meaningful results were obtained in all five cases, which completely differ from the heart-shaped signature of tensile fracture propagation. It was discovered that the strain pattern changes with the shear deformation direction with respect to the fiber. The model is then used to simulate the response of a fracture network at Utah FORGE to injection to assess whether a signature might be expected in response to the planned injection and circulation rates, and, if so, what strain pattern might be expected. The simulation confirms that a strain response can indeed be observed. More importantly, the fiber response that would be detected in the monitoring well would be a combination of strain signatures from dilation and shear deformation of differently oriented natural fractures. The results in this study provide useful insights on the application of fiber to other stimulation and/or circulation scenarios where shear deformation of a fracture or fracture network plays a major role.
The interaction between a natural fracture (NF) and a hydraulic fracture (HF) has been studied extensively, both experimentally and numerically, to better understand the potential for crossing and ...arrest of a hydraulic fracture intersecting a natural fracture. However, the actual mechanical interaction between a hydraulic and a natural fracture or a bedding plane has not been studied, particularly under triaxial stress and injection conditions. Analysis of field microseismic data recorded during hydraulic fracturing shows that the bedding plane could slip due to the approaching hydraulic fracture. In this paper, we present the results of some lab-scale experimental work, demonstrating HF/NF interaction with an emphasis on the slippage of a discontinuity surface. Injection pressure, stress applied, and the sample deformation are monitored during the tests. Acoustic Emission (AE) technology is employed to record the AE signals generated during fracture initiation, propagation, and during the sliding of the joint. In addition, strain gauges are used to measure the slippage on the natural fracture. The tests are carried out on 101.6 mm diameter cylindrical samples of PMMA, shale, and granite. The calculated displacement based on the recorded strain clearly shows a jump at the breakdown point, which is accompanied by increased AE activity and stress drop. Analysis of the data clearly shows the occurrence of slippage on the joint in response to an approaching hydraulic fracture. Expectedly, the degree of shear slip varies with natural fracture dip and friction angle.
An important technical issue in the enhanced geothermal system (EGS) is the process of fracture shear and dilation, fracture network propagation and induced seismicity. EGS development requires an ...ability to reliably predict the fracture network’s permeability evolution. Laboratory and field studies such as EGS Collab and Utah FORGE, and modeling simulations provide valuable lessons for successful commercial EGS design. In this work we present a modeling analysis of EGS Collab Testbed Experiment 1 (May 24, Stim-II ≅ 164 Notch) and interpret the stimulation results in relation to the creation of a fracture network. In doing so, we use an improved 3D discrete fracture network model coupled with a 3D thermo-poroelastic finite element model (FEM) which can consider fracture network evolution and induced seismicity. A dual-scale semi-deterministic fracture network is generated by combining data from image logs, foliations/micro-fractures, and core. The natural fracture properties (e.g., length and asperity) follow a stochastic distribution. The fracture network propagation under injection is considered by an ultrafast analytical approach. This coupled method allows for multiple seismic events to occur on and around a natural fracture. The uncertainties of seismic event clouds are better constrained using the energy conservation law. Numerical simulations show that the simulated fracture pressure profiles reasonably follow the trend observed in the field test. The simulations support the concept that a natural fracture was propagated from the injection well connecting with the production well via intersection and coalescence with other natural fractures consistent with plausible flow paths observed on the field. The fracture propagation profiles from numerical modeling generally match the field observation. The distribution of simulated micro-seismicity have good agreement with the field-observed data.
Field experience has demonstrated that infill well fractures tend to propagate towards the primary well, resulting in well-to-well interference, or so-called “frac-hits”. Frac-hits are a major ...concern in horizontal well refracturing because they adversely affect the productivity of both wells. This paper provides a 3D geomechanical study of the problem for the first time in order to better understand frac-hits in horizontal well refracturing and its mitigating solutions. To our knowledge, this is the only refracturing study focused on fracture mechanics and within the context of coupled proroelasticity using a single model. The modeling is based on the fully coupled 3D model, GeoFrac-3D, which is capable of simulating multistage fracturing of multiple horizontal wells. The model couples pore pressure to stresses, and makes it possible to create dynamic models of fracture propagation. The modeling results show that production from production well fractures leads to a nonuniform reduction of the reservoir pore pressure around the production well and in between fractures, leading to an anisotropic decrease of the total stress, potentially resulting in stress reorientation and/or reversal. The decrease in the total stress components in the vicinity of the production fractures creates an attraction zone for infill well hydraulic fractures. The infill well fractures tend to grow asymmetrically towards the lower stress zone. The risk of frac-hits and the impact on the “parent” and “infill” well production vary according to the reservoir stress regime, in situ stress anisotropy, and production time. By optimizing well and fracture spacing, fracturing fluid viscosity, and the timing of refracturing job, frac-hit problems can be minimized. The simulation results demonstrate that the risks of frac-hits can be potentially mitigated by repressurization of the production well fractures before fracturing the infill well.
The dilatant shear slip of preexisting fractures/faults by injection at pressures below the minimum in situ stress has been recognized as an important permeability creation mechanism during reservoir ...stimulation. However, interconnected fracture network generation through the propagation and coalescence of preexisting fractures during shear stimulation has been rarely studied through laboratory experiments. To examine permeability enhancement through the propagation and coalescence of preexisting fractures, we conducted novel triaxial‐injection experiments under representative crustal stress conditions on cylindrical granite samples containing single or double preexisting fracture(s)/flaw(s). In the sample with a single fracture, SW‐1, new cracks propagated from the preexisting fracture due to injection. In the sample with two fractures, SW‐2, a highly conductive fracture network was created by the coalescence of newly formed cracks with preexisting fractures. As a result, the equivalent permeability of the sample was enhanced by 17 to 35 times. Both tensile wing cracks and shear and/or mixed‐mode secondary cracks were detected from the scanning electron microscope images of the tested samples. Our results suggest that in addition to dilatant shear slip, propagation and coalescence of preexisting fractures significantly contribute to reservoir permeability creation during low injection pressure stimulation in fractured rocks. Although the experiments focused on reservoir stimulation, the results have relevance to crustal permeability evolution and induced seismicity.
Key Points
The propagation and coalescence of preexisting fractures in granite under triaxial‐injection at pressures below the minimum principal stress has been experimentally demonstrated
An interconnected fracture network has been created by injection resulting in a significant permeability enhancement
Both tensile wing cracks and shear and/or mixed‐mode secondary cracks have been observed in the tested samples
Numerical simulations of multistage hydraulic fracturing usually neglect poroelastic effects. However, in case of low permeability reservoirs, where hydraulic fracturing is usually carried-out using ...relatively low viscosity fluids and high injection rates, coupled poroelastic mechanisms need be included for better understanding of the fracturing process, which can involve rock failure and/or reactivation of natural fractures. In this paper, we present a fully coupled three-dimensional poroelastic analysis of multiple fracture propagation from horizontal wells. The numerical model uses the indirect boundary element method of displacement discontinuity for poroelastic response of the rock, the finite element method for fracture fluid flow, and the linear elastic fracture mechanics approach for fracture propagation. The model accounts for the mechanical interactions among multiple fractures, mixed-mode propagation, fluid diffusion into the reservoir matrix, and the effects of fluid diffusion on the rock mechanical response. The model is verified with analytical solutions, and numerical examples of simultaneous and sequential fracturing of single and multiple horizontal wells in the Niobrara Chalk formation are presented. The results show the created fracture network geometries are strongly influenced by the mechanical interactions among the fractures. It is also demonstrated that the poroelastic effect increases the net fracture pressure and causes a reduction in fracture volume. The poroelastic model illustrates the transient character of stress shadow, and is particularly useful for re-fracturing analysis since it readily calculates the stress variations due to reservoir depletion.
•A fully coupled three-dimensional poroelastic hydraulic fracture model is developed.•The model uses the 3D displacement discontinuity method, and linear elastic fracture mechanics.•The model simulates simultaneous or sequential multiple fractures and refracturing of horizontal wells.•Simulation results illustrate that mechanical interaction is a critical design parameter for horizontal well fracturing.•The fluid diffusion into reservoir rocks impacts the stress shadowing among multiple fractures.
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