Numerical Simulation of Fracture Propagation Morphology in Hydraulic Fracturing Development of Geothermal Reservoirs Based on the Cdem-Thm3d

Abstract

Reservoir fracturing stimulation is the key to constructing an enhanced geothermal system (EGS) for geothermal development in hot dry rock (HDR) reservoir. To clarify the crack propagation law of HDR fracturing, a 3D thermo-hydro-mechanical coupling simulation model of fracture propagation is produced based on the continuum–discontinuum element method (CDEM-THM3D). The correctness of the CDEM-THM3D model is validated by the theoretical solution of the nonisothermal soil consolidation model and Penny fracture model. Then, hydraulic fracturing numerical simulations are performed to analyse the influence of controlling variables on fracture propagation. The results indicate that the thermal tensile stress induced by injecting cold water can decrease reservoir fracture pressure and fracture extension pressure, causing an increasement in fracture width and a reduction in fracture length. Increasing thermal expansion coefficient and temperature difference enhances the effect of thermal stresses and even creates new branch fractures. A large elastic modulus favours an increase in fracture length, while large rock tensile strength and minimum horizontal stress lead to a decrease in fracture length. With increasing injection flow rate and fracturing fluid viscosity, the reservoir fracture pressure and the fracture width rise significantly, and the fracture easily breaks through the barrier of the high-stress compartment.

Keywords: Geothermal reservoirs, Hydraulic Fracturing, Fracture propagation, Thermo-hydro-mechanical coupling, Continuum-discontinuum element method, Thermal tensile stress