Influencing patterns of multiple factors on the heat transfer performance of moderately deep geothermal wells

Vigorously promoting the technology for heating using moderately deep geothermal energy serves as an important measure for China’s pursuit of carbon neutrality and peak carbon dioxide emissions. A coaxial double-pipe heat transfer system for moderately deep geothermal wells can avoid damage to groundwater resources and the environment. Furthermore, the influencing factors of the wells exhibit complicated and nonlinear interactions. Based on the theoretical analysis and mathematical description of a moderately deep well with a coaxial double-pipe heat exchanger, this study established a layered heat transfer model for the geothermal well and verified its reliability. This study investigated a moderately deep geothermal well in the Guanzhong Basin in Shaanxi Province. Through numerical simulations, it systematically analyzed the heat transfer performance of the geothermal well, as well as its heat extraction capacity during continuous operation, under the influence of various factors. The results show that compared to the measured values, the outlet water temperatures calculated using the homogeneous and layered models exhibited maximum relative errors of 14.08% and 11.50%, respectively, and average errors of 7.29% and 6.93%, respectively. Therefore, the layered model enjoyed a higher calculation accuracy than the homogeneous model. Among all influencing factors, geothermal well depth, geothermal gradient, and the thermal conductivity of strata exerted the most significant influence on the heat extraction power of the geothermal well. There existed roughly linear relationships of the heat extraction power with the geothermal gradient, inlet water temperature, and the thermal conductivity of the inner tube. Moreover, the thermal conductivity of well cementing materials produced thermal resistance effects on the heat transfer process. The heat amount extracted from the moderately deep geothermal well decreased with the operating years, with the decreasing amplitude being high in the first five heating seasons and then decreasing. After 50 heating seasons, the annual average heat extraction power decreased by 15.59%. Assuming that a geothermal field is influenced if the geotemperature decreases by more than 1℃, the average influence radius of the geothermal field was about 65 m. In addition, due to the differences in strata, the contour lines illustrating the drop and recovery of formation temperature around the geothermal well exhibited stepped changes at the interface of the strata. Strata with higher thermal conductivity of rocks corresponded to a greater distance of temperature disturbance near the interfaces of strata. The results of this study can be applied to the evaluation of the heat extraction and transfer capacities of moderately deep geothermal wells and also serve as a reference for the design of coaxial double-pipe heat transfer systems for moderately deep geothermal wells.