Study on Seismic Events Induced by Loss of Fault Confinement Due to Rock Temperature Decrease in EGS Reservoirs

Abstract

Heat is extracted from geothermal reservoirs and the altered thermal conditions of rock perturb in-situ stresses and may induce slip (shear displacement discontinuity) on pre-existing faults. Noticeable seismic events may arise as a consequence of the thermally induced slip. This paper focuses on impacts of cooling on the loss of fault confinement and triggering a rupture (seismic slip). We simulate a preexisting, strike-slip fault and calibrate the dynamic rupture modeling by checking results against an analytical solution. The calibrated model is then used for simulating thermally induced rupture along faults with different initial stresses. We explore effects of cooling rock on triggering a seismic slip along the fault while the temperature of surrounding rock continuously decreases from 400 ºC until slip occurs. We study fault models with which initial shear stresses on the fault are six different fractions of the fault peak shear strength. An energy-based numerical methodology, previously developed and verified by the authors, is employed to simulate the intensity of rupture through calculating radiated seismic energy from each fault model. Results show that the fault with initial shear stresses closer to the fault shear strength is activated by smaller disturbances in rock temperature and generates rupture with higher radiated seismic energy. The induced rupture generates less radiated seismic energy for faults that are not close to the brink of failure and thus require a higher thermal drawdown before activation.