Discrete Fracture Modeling of Hydraulic Stimulation in Enhanced Geothermal Systems

Abstract

Hydraulic stimulation in Enhanced Geothermal Systems is performed by injecting water at high pressure into low permeability, typically crystalline, rock. In most cases, the elevated pore pressure induces slip on preexisting fractures, resulting in an increase in permeability. This process involves interacting hydraulic, thermal, mechanical, and thermoelastic processes. It is complicated by the need to account for the complex geometry of the preexisting fracture network. This paper describes an investigation of three factors that affect the way that the stimulation propagates through the formation: the stresses induced by fracture slip, variability in the frictional properties of the fractures, and thermal stresses. We found that the first two factors tend to create a reservoir that is more poorly connected with significant bottlenecking of flow. We found that the third factor, thermal stress, induces tensile normal force on a fracture within a zone of cooling and compressive normal force on a fracture around a zone of cooling. Complex discrete fracture networks were generated stochastically and discretized, and flow simulator and displacement discontinuity (DD) codes were written to compute the pressures, temperatures, and stresses. An efficient method for calculating thermal stresses along a fracture is presented.