Fluids injection for hydraulic stimulation and fracturing, typical in the development of enhanced geothermal systems (EGS) in granites, can reactivate deep faults and induce seismicity. Such faults typically contain chlorite coatings as an alteration product that may impact styles of deformation—aseismic through seismic. We performed low velocity shear experiments on simulated granite fault gouges under conditions typifying a geothermal reservoir at ∼4-km depth with a confining pressure of 110 MPa, a temperature of 150°C, fluid pressures of 21–80 MPa, and chlorite contents of 0–100%, to investigate the influence of variation in effective stress and mineral composition on fault strength and stability. Our results show a transition from velocity-strengthening to velocity-weakening behavior in simulated granite gouge when the effective confining pressure was reduced from 89 to 30 MPa, characterized by a transition from fault compaction to dilation—as revealed by microstructural observations—with implications in enabling unstable failure. Conversely, increasing chlorite content stabilizes slip but reduces frictional strength. The microstructures of these mixed gouges exhibit shear localized on chlorite-enriched planes and promoting fault sliding. These results suggest that earthquake ruptures occurring during fluid injection can be facilitated by effective stress variations and that both controlling fluid overpressures (effective stresses) and being aware of the presence of alteration minerals are both important controls in mitigating such injection-induced seismic risks.

中文翻译：

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有效应力变化和绿泥石化对花岗岩断层摩擦和稳定性的竞争控制：对流体注入引发的地震活动的影响

用于水力增产和压裂的流体注入，在花岗岩中增强地热系统 (EGS) 的开发中是典型的，可以重新激活深层断层并诱发地震活动。此类断层通常包含绿泥石涂层作为蚀变产物，可能会影响变形方式——通过地震影响变形。我们在模拟花岗岩断层泥的条件下进行了低速剪切实验，典型条件是约 4 km 深度的地热储层，围压为 110 MPa，温度为 150°C，流体压力为 21-80 MPa，绿泥石含量为0-100%，研究有效应力和矿物成分变化对断层强度和稳定性的影响。我们的研究结果表明，当有效围压从 89 MPa 降低到 30 MPa 时，模拟花岗岩泥中的速度强化行为向速度减弱行为转变，其特征是从断层压实到膨胀的转变——微观结构观察表明——这在启用不稳定的故障。相反，增加绿泥石含量会稳定滑动，但会降低摩擦强度。这些混合泥的微观结构在富含绿泥石的平面上表现出局部剪切并促进断层滑动。