Thermo-hydro-mechanical-chemical characteristics of mining-integrated geothermal systems

With continuous increase of mining depth, the deep high-temperature strata with geothermal gradients provides significant opportunities for developing high-grade geothermal resources. To explore the feasibility of closed-loop geothermal extraction using abandoned mines, a multi-field coupling model for time-dependent cemented backfill was established based on Biot's porothermoelastic theory, where a non-isothermal pipe-flow module based on the Navier-Stokes equations was implemented. A new simulation system for co-mining of coal and geothermal energy with backfill was developed. The multi-physical field response of backfill and outlet water temperature development under different mining methods and geothermal conditions, and the evolution patterns of thermal efficiency and structural stability during system operation were analyzed. The results show that backfill hydration heat is the core early-stage thermal source, contributing 47.1% of energy within 90 d under calculated conditions. Geothermal extraction enhances system stability. Under conditions of backfill/fluid temperature at 15 ℃ and surrounding rock temperature at 35 ℃, peak pore water pressure and total stress decreased by 40.6% and 22.1%, respectively, when compared to non-geothermal-extraction scenarios. The cooling effect of heat-exchange fluid causes confined contraction of pore water and reduces backfill water pressure, thereby improving system stability.