A Parametric Study of L-shape Coaxial Closed-Loop Geothermal Systems with Reservoir Convection

Abstract

Closed-loop geothermal systems (CLGS) have become of interest because they do not require reservoir permeability, which may increase the number of sites where they can be utilized. In addition, the working fluid is constantly recirculated and therefore conserved. This gives the flexibility of selecting different working fluids, such as water and supercritical CO2, which can directly drive a turbine for generating electricity. However, for conduction-only reservoirs, CLGS require deep wellbores and long laterals to overcome the limited heat transfer area. This may not be feasible due to the geothermal location or the cost of drilling. To enhance the heat extraction, wet-rock geothermal systems have been proposed. This idea suggests that the convection (either natural or forced) in the porous rock will improve the heat extraction of the system by flow of in-situ water in the reservoir. Previous work has shown that natural convection can improve the performance of the geothermal systems with U-tube configuration, but only in cases of high reservoir permeability. In this work, we developed a numerical model for a wet-rock CLGS with a coaxial configuration where liquid convection is allowed in the rock. We then compared the heat extracted from the dry rock coaxial CLGS and the wet rock U-tube CLGS to determine if there are any advantages to having convection in the coaxial configuration. The effects on performance of working fluid mass flow rate, permeability, and geothermal temperature gradient in the reservoir have been investigated. The advantage of natural convection highly depends on the permeability and the geothermal temperature gradient. The U-tube system performs better than the L-shape coaxial system. SAND2024-13209A