Heat Extraction Through an Advanced Closed-Loop Geothermal System

Abstract

Heat production through conventional closed-loop geothermal systems (CLGSs) is constrained by the limited contact area available for heat exchange between rock formations and the wellbore containing circulating fluid. To address this challenge, an advanced closed-loop geothermal system (ACGS) has been proposed to enhance heat production in this research. The ACGS incorporates a hydraulic fracture, partitioned by a horizontal insulator for vertical zonal isolation of fluid flow in the fracture, into the closed-loop system's fluid circulation. Since working fluid flows through the partitioned fracture, convective heat transfer from rock to fluid in the fracture having a large surface area is introduced to the closed-loop system, which will significantly enhance the temperature of fluid produced from the system. To accurately assess the heat production performance of the ACGS, a comprehensive numerical study is performed. Initially, a three-dimensional hydrothermal model of the ACGS is developed and numerically validated. This numerical model is utilized to simulate heat production through the ACGS incorporating a double-wing fracture for different key parameters, including fracture dimensions and tubing thermal conductivity. Then, heat production performances of two main ACGS configurations respectively incorporating a branched fracture and a multiple-wing fracture are analyzed. Lastly, simulation results of the ACGS under different conditions were compared to determine the design parameters for ACGS yielding the highest heat production performance. Compared with the scenario without a fracture, the near-wellbore temperature of the ACGS has decreased significantly, indicating that the geothermal reservoir is cooled much more efficiently. Due to incorporation of a double-wing fracture, the cumulative extracted heat of a closed-loop system over 20 years is enhanced by up to 162.94%. Increasing the fracture half-length and fracture height can both enhance heat production efficiency of the ACGS considerably. Vacuum-insulated tubing with extremely low thermal conductivity performs better than polymeric insulation tubing in avoiding heat loss through tubing. Compared with a multiple-wing fracture, a branched fracture results in better heat production through the ACGS, with a larger number of fracture branches leading to more efficient heat production. A branched fracture can improve the cumulative extracted heat of a closed-loop system over 20 years by up to 321.77%. Therefore, the proposed ACGS emerges as a promising solution to overcome the limitations faced by closed-loop systems in heat production.