Thermo-Hydro-Mechanical Modeling of Working Fluid Injection and Thermal Energy Extraction in EGS Fractures and Rock Matrix

Abstract

Reliable performance predictions of EGS reservoirs require accurate and robust modeling for strongly coupled thermal-hydrological-mechanical (THM) processes. Conventionally, these types of problems have been solved using operator-splitting methods, usually by coupling a subsurface flow and heat transport simulators with a solid mechanics simulator via input files. An alternative approach is to solve the system of nonlinear partial differential equations that govern multiphase fluid flow, heat transport, and rock mechanics simultaneously, using a fully coupled, fully implicit solution procedure, in which all solution variables (pressure, enthalpy, and rock displacement fields) are solved simultaneously. This paper describes numerical simulations used to investigate the poro- and thermal- elastic effects of working fluid injection and thermal energy extraction on the properties of the fractures and rock matrix, using a novel simulation software FALCON, a finite element based simulator solving fully coupled multiphase fluid flow, heat transport, rock deformation, and fracturing. Investigations are also conducted on how these poro- and thermal- elastic effects are related to fracture permeability evolution and potential fracture creation/closing during production. Case studies illustrate how these effects affect the long-term sustainability of EGS as well.