Sensitivity analysis of development parameters on the production temperature of karst geothermal reservoirs: A case study of the Qingfeng area in Henan, China

The temperature of produced geothermal water serves as a pivotal indicator influencing the efficiency of geothermal energy utilization, with a higher temperature corresponding to more effective geothermal reservoir exploitation. For karst geothermal reservoirs, recognized as a significant type of geothermal reservoir, the temperature of produced geothermal water is influenced by reservoir parameters. Focusing on the Ordovician karst geothermal reservoirs in the Qingfeng area, Henan Province, this study established a geological model of the reservoirs, followed by numerical simulations based on thermal flow and well-reservoir coupling using the data of seven geothermal wells. The purpose is to explore the effects of the geological conditions and reinjection parameters of geothermal reservoirs on the temperature of produced geothermal water. The results reveal that the presence of faults accelerates the diffusion of moderate-low-temperature zones toward production wells, leading to an early thermal breakthrough. In the late stage of geothermal exploitation, the temperature of produced geothermal water was higher in the presence of faults, and its curve exhibited a higher slope compared to the case where faults are absent. This result indicates that faults contribute to an elevated temperature of produced geothermal water during thermal equilibrium in the Qingfeng area, ultimately conducive to an increase in extracted heat. Presently, it is generally considered that a smaller spacing between production and reinjection wells (also referred to as the well spacing) is associated with a lower temperature of produced geothermal water. However, in this study, the temperature of the produced geothermal water exhibited a rapid downward trend with increasing exploitation time under reinjection rates exceeding 50 kg/s. Most especially, the temperature under a well spacing of 835 m was even higher than that under a well spacing of 1000 m. Finally, this study determined the optimal reinjection parameter combination for the specific scenario in the Qingfeng area, involving a reinjection temperature of 20°C, a reinjection rate of 50 kg/s, and a well spacing of 835 m. The results of this study will serve as a valuable guide for preparing schemes for the exploitation of karst geothermal reservoirs in the Qingfeng area or similar geological settings.