Water storage in underground mined-out space as a geothermal reservoir: Heat extraction performance and temperature evolution

As mining depth increases, the temperature of the surrounding rock rises, drawing global attention to the potential for geothermal energy extraction from high-temperature water stored in collapsed rock masses—a prospect that offers both promise and challenges. In response, this study proposes a functional backfilling method using mining solid waste to construct a high-porosity heat extraction space. The research integrates experiments, theoretical analysis, and simulations to examine the mechanical and permeability properties of solid waste backfill materials. It further aims to elucidate how flow velocity and initial temperature influence the evolution of the temperature field and the thermal performance. Results indicate that the backfill material achieves optimal mechanical strength with a glass fiber content of 10‰ and a length of 6 mm. Furthermore, the permeability of the solid waste backfill demonstrates a quadratic relationship with both axial and confining pressure. During the recovery stage, the temperature in the heat extraction space remains lower than that of the surrounding rock, with geothermal energy being extracted via convective heat transfer between the water medium and the rock. The amount of heat extracted shows a positive correlation with the flow velocity of the water medium and a negative correlation with its initial temperature.