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Fluid-induced seismicity in tectonically inactive regions has been attributed to fluid overpressurization of rock fractures during natural resource extraction and storage. We conducted a series of triaxial shear-flow experiments on sawcut fractures in granite and showed that the fracture responses can be dissimilar under various fluid pressurization conditions. For pressure-controlled fluid pressurization, a uniform fluid pressure distribution can be promoted by lowering pressurization rate and enhancing fracture permeability. However, during volume-controlled fluid pressurization, a high pressurization rate causes a drastic increase in fluid pressure before fracture failure. In this case, our analytical model reveals that the fracture area and normal stiffness also influence fluid pressure variations. The maximum seismic moment predicted by this model is well validated by the experimental data for the cases with low pressurization rates. The discrepancy between the analytical and experimental data increases with higher fluid overpressure ratio owing to the assumption of uniform fluid pressure distribution in the model. The sensitivity analysis demonstrates the importance of fracture size estimation in the maximum seismic moment prediction. Our model can potentially be applied to control the fluid overpressurization of rock fractures and to mitigate the risks of fluid-induced seismicity.

Induced seismicity associated with fluid injection has raised serious concerns for the safety and efficiency of geo-energy systems. Cyclic injection has recently been proposed as an alternative injection scheme to reduce the large magnitude injection-induced seismicity. However, the influence of cyclic injection on the activation of natural fractures in granite and the resulting seismic risk is not yet clear. This study investigates the injection-induced activation of a critically stressed natural fracture in a granite core sample, particularly focusing on the comparison between monotonic and cyclic water injection under pressure-controlled and volume-controlled conditions. Experimental results show that the acceleration and deceleration of fracture slip are modulated by the shear stress imbalance between the fixed shear stress and the evolving frictional strength of the fracture. Fracture slip affects the fluid pressure distribution on the fracture, which in turn regulates the frictional strength of the fracture. At a small total shear displacement (i.e., ~ 0.9 mm in this study), cyclic injection with a restricted peak injection pressure results in aseismic fracture slip at much smaller peak slip rates compared to that during the monotonic injection. On the one hand, the more uniform reduction in effective normal stress caused by cyclic injection encourages slow and stable fracture slip, characterized by the smaller peak slip rates. On the other hand, the flowback of injected fluid or suspension of injection could prevent the occurrence of fast-accelerated fracture slip during cyclic injection. However, the fracture can become unstable when it has experienced a considerable amount of total shear displacement (larger than ~ 0.9 mm in this study), and likely gained a significantly enhanced permeability. Continued injection after the unstable shut-in stage, signified by an unusual increase in slip rate and an accelerated drop in injection pressure, could result in rapid and unstable fracture slip.

We present a series of controlled fluid injection experiments in the laboratory on a pre‐stressed natural rough fracture with a high initial permeability (∼10−13 m2) in granite using different fluid pressurization rates. Our results show that fluid injection on a fracture with a slight velocity‐strengthening frictional behavior exhibits dilatant slow slip in association with a permeability increase up to ∼41 times attained at the maximum slip velocity of 0.085 mm/s for the highest‐rate injection case. Under these conditions, the slip velocity‐dependent change in hydraulic aperture is a dominant process to explain the transient evolution of fracture permeability, which is modulated by fluid pressurization rate and fracture surface asperities. This leads to the conclusion that permeability evolution can be engineered for subsurface geoenergy applications by controlling the fluid pressurization rate on slowly slipping fractures. Plain Language Summary Understanding the evolution of fracture permeability during hydraulic stimulation of subsurface reservoirs is the key to characterizing fluid transport and formulating strategies to limit induced seismicity. Accordingly, there is a significant interest in deciphering how the fluid pressurization rate, a constitutive operational parameter during injection, influences the transient permeability change during fracture slip. We conducted a series of experiments in the laboratory using different fluid pressurization rates on a natural rough fracture in granite under a pre‐stressed state. The fracture had a high initial permeability. Our findings show that when fluid is injected into a fracture with a slight velocity‐strengthening frictional behavior, it causes slow slipping with significant permeability enhancement. The change in hydraulic aperture caused by slip velocity is the main reason for the temporary change in permeability, and this effect is modulated by fluid pressurization rate and fracture surface irregularities. Our results suggest that we can modulate the permeability of subsurface geoenergy reservoirs by controlling the fluid pressurization rate on slowly slipping fractures. Key Points We conducted fluid injection experiments on a pre‐stressed natural rough fracture in granite at different pressurization rates The velocity‐strengthening fracture exhibits slow slip accompanied by a significant increase in permeability during fluid injection Transient fracture permeability is controlled by injection‐induced slip velocity, modulated by pressurization rate and surface asperities

The effect of normal stress variations on fault frictional strength has been extensively characterized in laboratory experiments and modelling studies based on a rate-and-state-dependent fault friction formalism. However, the role of pore pressure changes during injection-induced fault reactivation and associated frictional phenomena is still not well understood. We apply rate-and-state friction (RSF) theory in finite element models to investigate the effect of fluid pressurization rate on fault (re)activation and on the resulting frictional slip characteristics at the laboratory scale. We consider a stepwise injection scenario where each fluid injection cycle consists of a fluid pressurization phase followed by a constant fluid pressure phase. We first calibrate our model formulation to recently published laboratory results of injection-driven shear slip experiments. In a second stage, we perform a parametric study by varying fluid pressurization rates to cover a higher dimensional parameter space. We demonstrate that, for high permeability laboratory samples, the energy release rate associated with fault reactivation can be effectively controlled by a stepwise fluid injection scheme, i.e. by the applied fluid pressurization rate and the duration of the constant pressure phase between each successive fluid pressurization phase. We observe a gradual transition from fault creep to slow stick–slip as the fluid pressurization rate increases. Furthermore, computed peak velocities for an extended range of fluid pressurization rate scenarios (0.5 MPa/min to 10 MPa/min) indicate a non-linear (power-law) relationship between the imposed fluid pressurization rate and the peak slip velocities, and consequently with the energy release rate, for scenarios with a fluid pressurization rate higher than a critical value of 4 MPa/min. We also observe that higher pressurization rates cause a delay in the stress release by the fault. We therefore argue that by adopting a stepwise fluid injection scheme with lower fluid pressurization rates may provide the operator with a better control over potential induced seismicity. The implications for field-scale applications that we can derive from our study are limited by the high matrix and fault permeability of the selected sample and the direct hydraulic connection between the injection well and the fault, which may not necessarily represent the conditions typical for fracture dominated deep geothermal reservoirs. Nevertheless, our results can serve as a basis for further laboratory experiments and field-scale modelling studies focused on better understanding the impact of stepwise injection protocols on fluid injection-induced seismicity.

Thermo-mechanical drilling combines the advantages of flame thermal treatment and rotary head drilling to achieve a high penetration rate in granitic rocks and a low wear rate of drill bits. This drilling strategy has been recognized as a potential technique to develop deep geothermal energy, but further improvement is still needed to optimize the drilling efficiency, such as the reduction in drill bit wear in thermally treated rocks. We use the Cerchar abrasivity test to investigate the abrasivity characteristics of Bukit Timah granite after the thermal treatment at different high temperatures. The Cerchar abrasivity index (CAI) decreases from 3.5 at 25 °C to 3.0 at 400 °C, remains constant between 400 and 600 °C and subsequently increases to 4.1 at 800 °C. Based on the meso-scale and micro-scale observations using polarizing microscope, 3D scanner and scanning electron microscope, the reduction in CAI value at lower temperatures is due to the abrasive wear of stylus tip, while the mechanical interaction between the stylus cone and abrasive minerals causes the increase in CAI value at higher temperatures. The Cerchar abrasivity test results indicate that overheating rocks may result in the exposure of drill bits to more abrasive minerals and enhance drill bit wear.

Fracture permeability controls the fluid flow in unconventional geo-energy reservoirs, and achieving an effective and sustainable increase in permeability is a primary objective of hydraulic stimulation treatments. However, the stress dependence of fracture permeability remains insufficiently constrained. In this study, we systematically compiled and re-processed published hydro-mechanical data of tensile, shear, saw-cut, and natural fractures in representative reservoir rocks, including shale, tight sandstone and crystalline rocks, under diverse stress and deformation conditions. We calculated the fracture permeability, synthesized a unified dataset and systematically quantified the permeability variations with Terzaghi effective normal stress using a commonly adopted exponential law characterized by intrinsic permeability ( k 0 ) and normal stress sensitivity ( α ). The synthesized dataset indicates that shale fractures are associated with low k 0 and moderate α , suggesting the potential importance of proppant use for maintaining fracture permeability in shale oil/gas reservoirs; sandstone fractures possess high k 0 and low α , favoring sustainable stimulation effects; and crystalline rock fractures have intermediate k 0 and high α , requiring proppants for maintaining post-stimulation permeability. Slip events are generally accompanied by increases in both k 0 and α , enhancing permeability but making it more sensitive to pressure changes, whereas cyclic fluid pressure changes generally compact fractures and reduce both parameters. Laboratory-scale data suggests that shear stress tends to decrease fracture permeability due to asperity crushing and clogging, while field-scale evidence implies permeability enhancement of critically stressed fractures through brecciation. Overall, this study provides a systematic synthesis of existing data for interpreting stress-dependent fracture permeability, which may inform stimulation strategies designed to enhance and sustain fluid transport in unconventional geo-energy reservoirs.

Post shut‐in seismic events in enhanced geothermal systems (EGSs) occur predominantly at the outer rim of the co‐injection seismic cloud. The concept of postinjection fracture and fault closure near the injection well has been proposed and validated as a mechanism for enhancing post shut‐in pressure diffusion that promotes seismic hazard. This phenomenon is primarily attributed to the poro‐elastic closure of fractures resulting from the reduction of wellbore pressure after injection termination. However, the thermal effects in EGSs, mainly including heat transfer and thermal stress, may not be trivial and their role in postinjection fault closure and pressure evolution needs to be explored. In this study, we performed numerical simulations to analyze the relative importance of poro‐elasticity, heat transfer, and thermo‐elasticity in promoting postinjection fault closure and pressure diffusion. The numerical model was first validated against analytical solutions in terms of fluid pressure diffusion and against heated flow‐through experiments in terms of thermal processes. We then quantified and distinguished the contribution of each individual mechanism by comparing four different shut‐in scenarios simulated under different coupled conditions. Our results highlight the importance of poro‐elastic fault closure in promoting postinjection pressure buildup and seismicity, and suggest that heat transfer can further augment the fault closure‐induced pressure increase and thus potentially intensify the postinjection seismic hazard, with minimal contribution from thermo‐elasticity. We used numerical simulations to investigate the impact of thermal effects on fluid diffusion after injection and to provide insights into the thermal effects on postinjection seismicity in enhanced geothermal systems. Our results highlight the importance of poro‐elastic fault closure in promoting postinjection pressure buildup and seismicity. Heat transfer can further promote the fault closure‐induced pressure increase and potentially intensify the postinjection seismic hazard, with negligible contribution from thermo‐elasticity. Highlights We used numerical simulations to investigate the impact of thermal effects on fluid diffusion after injection and to provide insights into the thermal effects on postinjection seismicity in enhanced geothermal systems. Our results highlight the importance of poro‐elastic fault closure in promoting postinjection pressure buildup and seismicity. Heat transfer can further promote the fault closure‐induced pressure increase and potentially intensify the postinjection seismic hazard, with negligible contribution from thermo‐elasticity.

The shield-roof interaction as mining proceeds in longwall panels remains unclear, hindering the further increase of longwall productivity. To uncover the mechanisms of shield-roof interaction, using our self-developed Status of Shield and Roof IntelliSense (SSRI) system, we investigated the effects of idle time, retreating rate, setting pressure, yielding, and shearer’s cutting, as well as neighboring shields’ advance on the spatial-temporal evolution of leg pressure and leg closure of shields. Our results show that the shield-roof interaction is not only dependent on the shield capacity, but also collectively determined by the time-related factors, the geological condition, the setting pressure, yielding characteristics, and mining method. Understanding the shield-roof interaction in longwall panels enables us to apply the SSRI system for ground control in longwall coal mines. Early warning of severe roof weighting can be achieved by establishing a warning model based on the decision tree algorithm. Apart from this, we can also assess the working condition of yield valve and diagnose fluid leakage of shield cylinder using the SSRI system. Finally, we propose the research prospects on shield-roof interaction in longwall panels to achieve a more reasonable determination of shield capacity, prediction of roof fall and coal wall spalling, and self-adaptive control of the shield.

The distinction between the shear behavior of tensile- and shear-induced fractures is critical to understanding the deformation and failure of geologic discontinuities at different scales. To investigate these differences, a series of direct shear tests were performed on sandstone specimens with a continuous fracture created by either splitting or shearing. The acoustic emission (AE) technique was used to examine variations in grain-size cracking behavior between specimens with tensile- and shear-induced fractures. An increase in normal stress for both fracture types correlates with increased microcrack density and energy release. However, there are notable differences: during the shear process, tensile-induced fractures produce AE sequences similar to the seismic patterns observed along natural tectonic faults, with foreshocks, mainshocks, and aftershocks. In contrast, the AE sequence for shear-induced fractures during the shear process lacks prominent mainshocks and deviates progressively from the power-law function with time as normal stress increases. In addition, the AE b-value for tension-induced fractures initially shows a gradual decrease as the mainshock approaches and then slowly increases during the aftershock period. In contrast, the b-value remains nearly constant for shear-induced fractures due to the low roughness and heterogeneity of the fracture surface. These differences highlight the strong correlation between AE responses and fault heterogeneity, paving the way for fault characterization and risk assessment in subsurface energy extraction.HighlightsThe cracking behavior of both tensile- and shear-induced fractures in direct shear tests is investigated using the AE technique.In direct shear tests, the AE sequences of tensile fractures follow a power law, while a significant deviation from the power law is observed in the AE sequence of shear fractures.The power-law evolution of the AE sequence before and after the mainshock, together with anomalous b-values, can be used as indicators to distinguish young faults from mature faults.

In Enhanced Geothermal Systems (EGS), rock fractures in the reservoir are often subjected to cyclic changes in effective stress at elevated temperatures, causing permeability variations. In this study, the effect of cyclic normal stress on permeability evolution of rough fractures in Gonghe granite at elevated temperature was investigated through flow-through experiments. The results show that the fracture permeability decreases with increasing normal load and partially recovers as the normal load releases. Under a constant normal stress, the fractures gradually close, exhibiting viscoelastic-plastic behavior that can be characterized using the Nishihara model. Based on these characteristics, a fracture-creep deformation model has been developed considering the stress history using a combination of the Hopkins fracture closure model and the Nishihara model. This study investigated the effects of cyclic stress on the internal geometric features of fractures at elevated temperature, calculated the fracture deformation and the corresponding permeability during the loading and unloading process, and validated the accuracy of the proposed model. The study reveals the primary mechanisms responsible for fracture permeability evolution under cyclic effective stress at elevated temperatures, providing valuable insights for the sustainable development of EGS.HighlightsThe viscoelastic–plastic behavior of rough fractures in Gonghe granite subjected to cyclic normal stress has been demonstrated in the flow-through experiments.A fracture-creep deformation model for a single rough fracture is developed considering the stress history using a combination of the Hopkins fracture closure model and the Nishihara model.The fracture permeability is calculated according to the geometric characteristics of the fracture, which has been validated by laboratory permeability measurements.