The role of natural fracture activation in hydraulic fracturing for deep unconventional geo-energy reservoir stimulation

The presence of sealed or semi-sealed,multiscale natural fracture systems appears to be crucial for the successful stimulation of deep reservoirs.To explore the reaction of such systems to reservoir stimulation,a new numerical simulation approach for hydraulic stimulation has been developed,trying to establish a realistic model of the physics involved.Our new model successfully reproduces dynamic fracture activation,network generation,and overall reservoir permeability enhancement.Its outputs indicate that natural fractures facilitate stimulation far beyond the near-wellbore area,and can significantly improve the hydraulic conductivity of unconventional geo-energy reservoirs.According to our model,the fracture activation patterns are jointly determined by the occurrence of natural fractures and the in situ stress.High-density natural fractures,high-fluid pressure,and low effective stress environments promote the formation of complex fracture networks during stimulation.Multistage or multicluster fracturing treatments with an appropriate spacing also increase the stimulated reservoir area(SRA).The simulation scheme demonstrated in this work offers the possibility to elucidate the complex multiphysical couplings seen in the field through detailed site-specific modeling.

Hydraulic Fracturing-induced Seismicity at the Hot Dry Rock Site of the Gonghe Basin in China

Hot dry rock is becoming an important clean energy source. Enhanced geothermal systems(EGS) hold great promise for the potential to make a contribution to the energy inventory. However, one controversial issue associated with EGS is the impact of induced seismicity. In August 2019, a hydraulic stimulation experiment took place at the hot dry rock site of the Gonghe Basin in Qinghai, China. Earthquakes of different magnitudes of 2 or less occurred during the hydraulic stimulation. Correlations between hydraulic stimulation and seismic risk are still under discussion. Here, we analyze the hydraulic stimulation test and microseismic activity. We quantify the evolution of several parameters to explore the correlations between hydraulic stimulation and induced seismicity, including hydraulic parameters, microseismic events, bvalue and statistical forecasting of event magnitudes. The results show that large-magnitude microseismic events have an upward trend with an increase of the total fluid volume. The variation of the b-value with time indicates that the stimulation experiment induces small amounts of seismicity. Forecasted magnitudes of events can guide operational decisions with respect to induced seismicity during hydraulic fracturing operations, thus providing the basis for risk assessment of hot dry rock exploitation.

The role of temperature‐enhanced fault closure in promoting postinjection pressure diffusion and seismicity in enhanced geothermal systems

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 wasfirst validated against analytical solutions in terms offluid pressure diffusion and against heatedflow‐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.

Permeability evolution during pressure-controlled shear slip in saw-cut and natural granite fractures

Fluid injection into rock masses is involved during various subsurface engineering applications.However,elevated fluid pressure,induced by injection,can trigger shear slip(s)of pre-existing natural fractures,resulting in changes of the rock mass permeability and thus injectivity.However,the mechanism of slip-induced permeability variation,particularly when subjected to multiple slips,is still not fully understood.In this study,we performed laboratory experiments to investigate the fracture permeability evolution induced by shear slip in both saw-cut and natural fractures with rough surfaces.Our experiments show that compared to saw-cut fractures,natural fractures show much small effective stress when the slips induced by triggering fluid pressures,likely due to the much rougher surface of the natural fractures.For natural fractures,we observed that a critical shear displacement value in the relationship between permeability and accumulative shear displacement:the permeability of natural fractures initially increases,followed by a permeability decrease after the accumulative shear displacement reaches a critical shear displacement value.For the saw-cut fractures,there is no consistent change in the measured permeability versus the accumulative shear displacement,but the first slip event often induces the largest shear displacement and associated permeability changes.The produced gouge material suggests that rock surface damage occurs during multiple slips,although,unfortunately,our experiments did not allow quantitatively continuous monitoring of fracture surface property changes.Thus,we attribute the slip-induced permeability evolution to the interplay between permeability reductions,due to damages of fracture asperities,and permeability enhancements,caused by shear dilation,depending on the scale of the shear displacement.