Mechanism of seepage heat transfer in thermal reservoirs of high temperature metamorphic rocks in the Yanggao-Tianzhen basin

A typical high-temperature metamorphic rock geothermal reservoir was newly discovered in the Tianzhen geothermal field in Datong City, China. However, due to the complex geological structure and thermal properties, the seepage heat transfer mechanism of metamorphic rock reservoirs is still unclear, seriously impeding the efficient development and sustainable utilization of geothermal resources. This study established a percolation-heat transfer model through tracer testing and numerical simulation to reveal the percolation mode and heat transfer mechanism of high-temperature metamorphic rock reservoirs during the reinjection process. We also predicted the distribution characteristics of various physical fields in the geothermal reservoir after the geothermal system has been in operation for 100 a, analyzed the influence of different reinjection schemes on seepage heat transfer in the geothermal reservoir, and proposed an optimization strategy for the reinjection scheme. The results show that: (1) The connectivity between production and injection wells is poor, and there are water-conducting fractures connecting the shallow and bottom layers; (2) The seepage channels through fractures guide the migration of the reinjected fluid and form a cold front surface with a protruding shape towards the mining well in the temperature field, resulting in temperature changes in the production well; (3) Seepage heat transfer in thermal reservoirs is greatly affected by the reinjection flow rate and the distance between production and reinjection wells, but less by the reinjection temperature. As the reinjection temperature drops, the flow rate increases, the well spacing decreases, and the temperature variation range of the production well becomes greater; (4) Under the current reinjection test conditions, the temperature of the mining well decreased by approximately 4°C after 100 a of geothermal reinjection operation, and a thermal breakthrough occurred at 78 a. Under the condition of maintaining a reinjection flow rate of 60 m³/h and a reinjection temperature of 80°C unchanged, the well spacing should be no less than 470 m to ensure that the well temperature does not cause a thermal breakthrough during the reinjection operation for 100 a. This research provides a theoretical basis and optimization methods for the efficient development of high-temperature metamorphic rock thermal reservoirs.