Numerical Simulation of Heat Extraction of CO2-EGS with THM Coupling Method Based on Discrete Fracture Models

Abstract

The enhanced geothermal system (EGS) becomes the key technology of extracting heat from Hot Dry Rock (HDR) as it constructs an artificial thermal reservoir by hydraulic fracturing. Furthermore, geothermal development using carbon dioxide as the work fluid can achieve the CO2 geological sequestration and concurrently bring additional benefits. In this paper, using the local thermal non-equilibrium theory, a numerical approach is presented to simulate the thermal-hydraulic-mechanical (THM) coupling processes and heat production performance of CO2-EGS. The reservoir models consist of impermeable matrix and synthetic fracture networks with a power-law length distribution. A sequence of fracture network topologies and three different injection pressures are considered to investigate the influences of the geometrical distribution and hydraulic regimes. By analyzing the evolutions of equivalent permeability and heat production, the fracture network with a high connectivity still performs a wide thermal sweep under the high pressure gradient but that with a low connectivity gains a rapid decline in heat extraction efficiency with the pressure gradient increasing. The results show the geometrical and hydraulic effects play critical roles in the heat extraction efficiency and service life. In addition, the observation of two interactively superimposed processes has important implications for estimating the effectiveness of hydraulic fracturing in CO2-EGS.