Role of Chlorite on the Friction and Stability of Granite Faults and Implications for Seismicity in Deep Geothermal Reservoirs

Chlorite is a common metamorphic mineral that is widely distributed within crustal faults transecting deep geothermal reservoirs (e.g., Pohang and Gonghe geothermal reservoirs) with implications for fracture/fault reactivation and triggered seismicity. We employed DEM (discrete element method) modelling to define controls of chlorite content and gouge thickness on the frictional strength and stability of rough faults in granite. The rolling resistance linear contact model and the flat-joint model were, respectively, adopted to simulate the contacts within the granular fault gouge and connection with the fault wall–rock. DEM modeling was completed using a reproduced rough fracture surface from Pohang sandwiching gouge. The digital DEM experiments indicate that the fault frictional strength and stability are controlled by chlorite contents. The fault frictional strength gradually decreases with increasing chlorite contents in granular fault gouge. All chlorite-filled faults show only velocity-strengthening responses and these imply inherently stable and aseismic fault sliding. However, the values of frictional stability (a–b) decrease with both increases in chlorite content and shear displacement—due to a significant slip-weakening effect. The fault frictional and stability behaviors are illuminated by the dilatancy induced by the fault surface asperity wear and gouge compaction. The size effect also affects the fault friction with a higher peak friction and shear displacement with dilatancy at peak friction increasing at a larger fault trace length. Our modelling results indicate that the abundance of chlorite significantly lowers fault frictional strength and potentially promotes fault reactivated during fluid injection.