Heat extraction analysis of a novel multilateral-well coaxial closed-loop geothermal system

We propose a novel multilateral-well coaxial closed-loop geothermal system (CCGS) to help realize the commercial exploitation of hot dry rock. Compared with the enhanced geothermal system, it avoids the high-cost fracturing and environmental problems by closed-loop circulation. Also, the multilateral-well CCGS can significantly increase heat production than traditional vertical-well or horizontal-well CCGSs via lateral wellbores. Then, we establish a new 3D transient model and validate it with experimental data. The temperature field is studied. The Influences of injection flow rate, number of lateral wellbores, wellbore size, thermal conductivity of insulation pipe, and reservoir temperature are analyzed. Results show that the lateral wellbores are vital to improving heat extraction, and a viable lateral-wellbore spacing is determined. The total energy output rises linearly with time, while the extracted heat in the initial stage is limited though the thermal power is high. There is an appropriate injection flow rate by comparing the total energy output and dissipation. A smaller wellbore size is preferred due to the minor effect. A reasonable thermal conductivity exists for the insulation pipe. Particularly, the high-temperature geothermal resources at various depths can be efficiently developed through the multilateral-well CCGS given the effect of reservoir temperature.