Fault Reactivation Due to Thermal Drawdown in Enhanced Geothermal Reservoirs

Abstract

Late-stage seismic-slip in geothermal reservoirs has been shown as a potential mechanism for inducing large seismic events [Gan and Elsworth, 2014]. The propagation of fluid pressures and thermal stresses are investigated in a prototypical geothermal reservoir, containing a centrally-located critically-stressed fault from a doublet injector and withdrawal well. Two bounding modes of fluid injection are defined at a given temperature; these bounding modes are related to either low- or high-relative flow rates. The pressure pulse travels slowly at low relative flow rates, the pressure-driven changes in effective stress are muted but thermal drawdown propagates through the reservoir as a distinct front. This results in the lowest likelihood of pressure-triggered events but the largest likelihood of late-stage thermally-triggered events. Conversely, at high relative flow rates the propagating pressure pulse is larger and migrates more quickly through the reservoir but the thermal drawdown is uniform across the reservoir and without the presence of a distinct thermal front, and less capable of triggering late-stage seismicity. We evaluate the uniformity of thermal drawdown as a function of a dimensionless flow rate Qd that scales with fracture spacing s(m), injection rate q(kg/s), and the distance between the injector and the target point L (Qd~qs^2/L ). This parameter enables the reservoir characteristics to be connected with the thermal drawdown response around the fault and from that the corresponding magnitude and timing of seismicity to be determined. This dimensionless scaling facilitates design for an optimum value to yield both significant heat recovery and longevity of geothermal reservoirs while minimizing associated induced seismicity.