Fault zone exploitation in geothermal reservoirs: Production optimization, permeability evolution and induced seismicity

This study employs a coupled thermal-hydrological-mechanical model to investigate the permeability evolution and fault reactivation of a critically stressed fault in geothermal reservoir. The fracture permeability sensitivity analysis for permeabilities of the fault damage zone that hydro-mechanical processes have dominating control on permeability evolution and the associated timing of slip on the fault plane for high fracture permeability. The mode of thermal sweep in the reservoir is dominated by advection, as the high fracture permeability permitted higher velocity of fluid flow. For the condition of low fracture permeability, heat transfer process has pronounced control on permeability evolution and timing of slip due to heat conduction process. With reduction in fluid flow and hydro-mechanical effects, heat transfer in the reservoir is dominated by heat conduction, as the temperature difference between the fault zone and the country rock becomes negligible. For the intermediate fracture permeabilities, the induced thermal unloading due to conduction could prompt the onset of failure. Changing the locations of the injection well along the fault zone shows that shear failure on the fault/fracture plane occurs earlier for lower stress state and vice versa. The evolutions of production rate and power generation are also influenced by the stress state at the injection and production wells. The elevated pore pressures in the fault zone due to fluid injection causes distributed seismicity on the fault/fracture planes which all have moment magnitudes that are below 2.5.