Numerical Simulation of Hydraulic Fracturing of Hot Dry Rock Based on Coupled THM Model

Abstract

ABSTRACT One of the challenges of developing an enhanced geothermal system (EGS) is to create connected fracture networks by hydraulic fracturing between the injection well and the production well. However, limited work has been conducted on studying the fracture initiation and propagation of hot dry rock. To have a clear understanding of how the temperature differences between the rock and the fluid influence hydraulic fracturing, numerical simulations were conducted under different temperature conditions (100°C-300°C) based on the FDEM approach. The results indicate that the increase in the temperature difference between the rock and the injected fluid could induce the larger fracture tortuosity and fracture apertures effectively. The breakdown pressure decreased by 52% as the initial rock temperature rise from 100°C to 300°C. The present study is expected to provide an in-depth understanding of the reservoir stimulation in EGS. INTRODUCTION Geothermal energy is widely distributed across the world and is rapidly gaining interest globally as it shows great potential to decrease carbon consumption. Hot Dry Rock (HDR) is a kind of geothermal resource where no water or only a small amount of fluid exists inside the rock and the average temperature of HDR is typically above 180°C(Li et al., 2022). Enhanced Geothermal System (EGS) is currently considered the most efficient way to develop HDR reservoirs, which applies the hydraulic fracturing technique to create the fluid flow path, enabling the working fluid to pass through the fracture network transferring heat from underground to the surface(Xie et al., 2022; Mcclure and Horne., 2014). The main difference in hydraulic fracturing of the hot dry rock and other types of reservoirs is that strong thermal stress would be induced by cold water contacting with the hot rock mass, which plays a crucial role in the initiation and propagation of the hydraulic fracture(Li and Tang., 2022; Li et al., 2022). It has been investigated that the mechanical properties of the granite derogated substantially after the specimens are subject to cyclic heating and cooling(Wu et al., 2019). The large temperature difference between hot rock and cold fluid leading to a larger thermal stress contributes to damaging the granite. Therefore, it is necessary to study how thermal stress influences the hydraulic fracturing of hot dry rock.