Thermo-Hydro-Mechanics in Shear Fracturing in Geothermal Reservoirs

Abstract

This paper presents a novel method to investigate shear stimulation at an injection well in Enhanced Geothermal Systems (EGS). Nowadays, the technique of EGS has been extensively used for extracting thermal energy from the earth. As the intrinsic permeability of the rock is usually too low to allow an economic flow, stimulation for fractures is incorporated. The connectivity of fracture networks around boreholes dominates the system behaviour. In theory, stimulations including both tensile (mode I) and shear (mode II) fracturing are desired, so that sufficient surface area for heat exchange is produced. However, shear stimulation is considered a safer choice than tensile fracturing in terms of possibility of inducing local earthquakes. This study investigates shear fractures only, from a slip-line field point of view. The rock is modelled as elasto-viscoplastic material with damage mechanics coupled. A numerical simulator REDBACK, based on the MOOSE framework, is employed to solve this coupled multi-physics involved problem. With injection pressure imposed on the interior of a borehole, slip lines grows in the form of logarithmic spirals, indicating the potential trace of shear fractures. Imperfections are imposed on the boundary as seeding for the spirals. Cases with and without thermo-mechanical coupling are compared, indicating the essential role of shear-heating feedback in enhancing shear fractures. Bifurcation analysis for various Arrhenius numbers is performed, demonstrating a clear exponential relationship between critical injection pressure and the local temperature of host rock.