The geochemical alteration of host rocks might affect the productivity and the potential for induced seismicity of geothermal systems. In addition to natural alteration, following production and heat extraction, re-injected fluids at lower temperatures and different pressures may be in chemical disequilibrium with the rock, impacting mineral solubility and dissolution/precipitation processes. In this study, we investigate the effect of geochemical alteration on the frictional behavior of granites, and their seismogenic potential, by conducting direct shear experiments using samples with varying degrees of alteration. The samples originate from the Carnmenellis granite in Cornwall, SW England, and represent the formation used in the United Downs Deep Geothermal Power Project for heat extraction. Experiments were conducted on granite powders (referred to as gouges) at room temperature and 180°C, at simulated in situ confining and pore pressures of 130 and 50 MPa, respectively (∼5 km depth). With increasing degree of alteration, the frictional strength of the gouges decreases while frictional stability increases. At high temperature, frictional stability is reduced for all samples while maintaining the trend with alteration stage. Microstructural investigation of the sheared gouges shows alteration delocalizes shear by reducing grain size and increasing clay fraction, which promotes the formation of pervasive shear fabrics. Our work suggests that, within the range of tested pressures, more alteration of granite initially causes more stable shearing in a fault. This behavior with alteration is sustained at high temperatures, but the overall frictional stability is reduced which increases the potential for induced seismicity at higher temperatures.

中文翻译：

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化学蚀变对康沃尔郡深层花岗岩地热系统中摩擦性能的影响：近原位条件下的直接剪切实验


主岩的地球化学蚀变可能会影响地热系统的生产力和诱发地震的可能性。除了自然蚀变外，在生产和热提取之后，在较低温度和不同压力下重新注入的流体可能与岩石发生化学不平衡，从而影响矿物溶解度和溶解/沉淀过程。在这项研究中，我们通过使用具有不同蚀变程度的样品进行直接剪切实验，研究了地球化学蚀变对花岗岩摩擦行为及其成地震潜力的影响。这些样品来自英格兰西南康沃尔郡的 Carnmenellis 花岗岩，代表了 United Downs Deep Geothermal Power 项目中用于热提取的地层。在室温和 180°C 下，在模拟原位围压和孔隙压力分别为 130 和 50 MPa（∼5 公里深度）的花岗岩粉末（称为凿孔）上进行了实验。随着蚀变程度的增加，气刨的摩擦强度降低，而摩擦稳定性增加。在高温下，所有样品的摩擦稳定性都降低，同时保持变化阶段的趋势。剪切凿孔的微观结构研究表明，蚀变通过减小晶粒尺寸和增加粘土分数使剪切离域，从而促进普遍剪切织物的形成。我们的研究表明，在测试压力范围内，花岗岩的更多变化最初会导致断层中的剪切更稳定。这种变化的行为在高温下是持续的，但整体摩擦稳定性会降低，这增加了在较高温度下诱发地震的可能性。