The effect of thermo-elastic stress re-distribution on geothermal production from a vertical fracture zone

Abstract

Injection of cold water into fracture zones in Enhanced Geothermal Systems (EGS) induces contractive thermo-elastic strains in the host rock, locally altering the stress state of the reservoir. Building on previous studies with horizontal fractures we examined how such thermo-elastic effects act during injection and production from a doublet intersecting a vertically extensive fracture zone embedded in impermeable host rock with a vertical geothermal gradient and with vertical aperture variations in response to stress increase with depth. As in horizontal fracture zones, contractive strains propagate outward from point of injection of cold water and are surrounded, due to stress re-distribution, by a ring zone with increased effective normal stress and, therefore, reduced aperture. The reduced aperture ring affects production temperature evolution to different degrees: for fracture zones with initially homogeneous transmissivity the effect on temperature decline is most pronounced for injection in the deeper well while it is small to negligible for injection in the shallower well. Fractures zones with initially heterogeneous transmissivity distribution experience highly channelized flow and the enhanced normal stresses develop preferentially at the sides of the channels, having little effect on flow along the channel and temperature decline. The effects show systematic variation with the rock's Young's modulus as does the tendency to induce shear failure by thermo-elastic effects. The simulated temperature evolutions deviate significantly from previous models that assessed the economics of EGS with sub-horizontal wells intersecting vertically extensive fractures in which the geothermal gradient and thermo-mechanical were not included.