Hydraulic stimulation of Enhanced Geothermal Systems (EGS) aims at boosting permeability to facilitate fluid circulation, while keeping a low induced seismicity. However, some stimulations have led to poor permeability enhancement or too high induced earthquakes, which suggests that further understanding is needed on poromechanical processes during stimulation. Here, we model a highly-monitored test performed at the Bedretto Underground Laboratory to investigate the impact of fluid injection on permeability enhancement and induced microseismicity. We examine three models: (1) a homogeneous fracture whose transmissivity is manually calibrated to reproduce the observed pressure evolution at the injection borehole (this model fails to capture the spatial distribution of pressure and the corresponding poromechanical processes); (2) an elastic fracture approach, where transmissivity changes locally as a function of fracture aperture following the cubic law (this model overestimates pressure after the onset of fracture slip); and (3) a viscoplastic fracture approach with strain weakening and dilatancy that yields an additional permeability enhancement after shear reactivation. The viscoplastic model captures the spatio-temporal coupled response of the fractured rock to hydraulic stimulation before and after shearing both in terms of pressure and microseismicity. Subsequently to the onset of shear failure, microseismic events occur in every injection cycle as the reactivation front advances when plastic strain and, thus, permeability surpass the previously achieved maximum value. This viscoplastic model permits estimating the extent of the stimulated fracture, the permeability enhancement and its impact on the local state of stress and pore pressure at surrounding fractures, representing a useful tool for the design of effective hydraulic stimulation.

中文翻译：

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Bedretto 地球科学和地球能源地下实验室裂隙结晶岩水力刺激数值模拟

增强型地热系统（EGS）的水力增产旨在提高渗透性以促进流体循环，同时保持较低的诱发地震活动性。然而，一些改造导致渗透率增强效果不佳或诱发地震过高，这表明需要进一步了解改造过程中的孔隙力学过程。在这里，我们对贝德雷托地下实验室进行的一项高度监控的测试进行了建模，以研究流体注入对渗透率增强和诱发微震的影响。我们研究了三种模型：（1）均质裂缝，其透射率经过手动校准，以重现注入井眼处观察到的压力演化（该模型无法捕获压力的空间分布和相应的孔隙力学过程）；(2) 弹性裂缝方法，其中透射率根据立方定律随裂缝孔径的函数而局部变化（该模型高估了裂缝滑移开始后的压力）；(3) 具有应变弱化和剪胀性的粘塑性断裂方法，在剪切再激活后产生额外的渗透率增强。粘塑性模型捕捉了剪切前后裂隙岩石对水力刺激在压力和微震活动方面的时空耦合响应。在剪切破坏开始后，当塑性应变和渗透率超过先前达到的最大值时，随着再激活前沿的推进，每个注入周期都会发生微震事件。该粘塑性模型可以估计压裂裂缝的范围、渗透率增强及其对周围裂缝的局部应力状态和孔隙压力的影响，是设计有效水力增产的有用工具。