Numerical Study on Heat Transfer of Multibranch U-shaped Wells for Closed-Loop Geothermal Systems

Deep geothermal energy is a renewable energy source with broad distribution, vast resource potential, and promising development prospects. The current main extraction methods include enhanced geothermal systems（EGS）, annular heat exchange well systems（AGS）, fault zone fluid circulation extraction, and coaxial casing extraction methods. However, challenges such as unstable heat extraction power, high seismic risks, and low heat extraction efficiency persist. Addressing the bottlenecks in current deep geothermal extraction technology, we adheres to the principle of energy exchange without material exchange during the extraction process and aims for large-scale, sustainable, and stable development of deep dry hot rock geothermal resources. We propose the clustered multi-branch U-shaped well heat extraction method（UMW-DGS） and its key technologies. On this basis, an axisymmetric thermal conduction model for the wellbore is established. We calculated the spatiotemporal evolution of the temperature field and heat extraction power around the well under constant wellbore diameter conditions and analyzed the effects of three sensitive factors—temperature difference, thermal conductivity, and wellbore diameter—on heat extraction power. In addition, to address the boundary value problem of the UMW-DGS, a three-dimensional thermo-hydro-mechanical coupling numerical algorithm based on the finite volume method（FVM） was developed. This algorithm was used to study the heat exchange efficiency of a single horizontal well section of the UMW-DGS and the spatiotemporal evolution of the temperature field under different injection flow conditions. By analyzing the effective heat exchange amount, duration, and power at different flow rates, we found that increasing the injection flow rate decreases the effective heat exchange energy and duration while causing the effective heat exchange power to first increase and then decrease. The research results indicate that deep geothermal energy development requires designing injection flow rates under the condition of balancing heat exchange temperature and power to achieve optimal heat exchange efficiency.