Numerical insights into impact of rock matrix and fracture characteristics on sCO2-enhanced geothermal heat extraction

Utilizing supercritical CO2 (sCO2) as the heat transfer medium in Enhanced Geothermal Systems (EGS) offers several advantages over water. However, the impact of changes in permeability, porosity, and fracture aperture on the heat transfer characteristics of sCO2 remains poorly understood. This study numerically investigates the dynamic variation of reservoir fractures, the changes in rock matrix properties (permeability and porosity), and their combined effects on the heat exchange behavior of sCO2-EGS via a newly developed sCO2-EGS model. Our results indicate that hydraulic fractures directly connecting the injection and production wells exhibit significantly higher heat transfer rates compared to fractures formed by natural networks, but result in lower overall heat extraction quality and incur higher associated costs. The simulations also reveal that higher reservoir porosity and permeability improve the overall effectiveness of convective heat transfer between sCO2 and the reservoir, but reduce the production temperature because fluids flow too fast within the reservoir. This leads to increased production rates but results in lower output temperatures. Additionally, the impact of the fracture aperture is crucial as it significantly influences fluid characteristics flowing through the reservoir. Larger apertures may cause sCO2 to move too quickly through the reservoir, limiting sufficient heat exchange and resulting in lower production temperatures and reduced heat extraction quality. Several other factors also influence temperature distribution and its variations within the reservoir, including the thermal conductivity of the reservoir matrix, heterogeneity of natural fractures, initial reservoir temperature, and geothermal gradient. Specifically, higher thermal conductivity accelerates heat removal around fractures by sCO2. Furthermore, accounting for the heterogeneity of natural fractures results in higher temperatures at the production well and an increase in the average reservoir temperature compared to when this heterogeneity is ignored.