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

Abstract

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.

1. Introduction

In deep coal mines, geothermal energy is primarily regarded as a high-temperature heat hazard, with its potential remaining largely underutilized [1–4]. As a green energy source unaffected by seasonal or environmental variations, mine geothermal energy has become one of China’s key strategic energy development priorities [5–8]. Meanwhile, most deep coal mines employ the caving mining method, which generates large volumes of collapsed and fractured rock masses [9–11]. These collapsed spaces remain underutilized, leading to significant wastage of underground spatial resources [12–14]. Thus, transforming the dual challenges of heat hazards and fractured rock voids in deep coal mines from “waste” into “valuable assets” has become an urgent priority for the sustainable development of deep mining operations.

China’s research on the causes of mine heat hazards and the evolution patterns of temperature fields in deep coal mining started relatively late. It was not until the 1970s that the country first proposed methods to mitigate mine heat hazards and explored converting this thermal energy into usable forms. Ning et al. [15] established a backfill heat exchanger in a mine goaf. They investigated and confirmed the optimal tube spacing arrangement for a multi-tube series configuration. Furthermore, their study demonstrated that the temperature difference between the inlet and outlet of the heat extraction space is positively correlated with the backfill’s thermal conductivity. But negatively correlated with the flow velocity. Zhang et al. [16] investigated the synergistic optimization of mechanical and thermal properties in backfill materials. They conducted numerical simulations of heat transfer between the backfill and cold fluid during thermal discharge processes. Their study revealed the temperature distribution patterns of backfill under varying thermal storage/release durations, and elucidated the corresponding heat transfer mechanisms. Yin et al. [17] proposed a backfilled fracture-enhanced geothermal system for hot dry rock extraction.