Legacy mines: geothermal energy exploitation and storage

<p indent="0mm">Mining plays a crucial role in human society, encapsulated by the saying, “Everything we use is either grown or mined.” However, once resources are depleted, most mines are inevitably closed and abandoned, posing significant risks to both human health and the environment. Simple closure and improper management of these legacy mines result in significant resource wastage and create numerous safety, environmental, and social challenges. Unlike well-developed mining countries such as the USA, UK, Canada, and Germany, which have been repurposing legacy mines for decades, China recently began the regenerative utilization of closed and abandoned mines. This delay has resulted in a weaker foundation of basic theory and key technology in this field. Among the various models for repurposing legacy mines, photovoltaic and wind power generation, pumped hydroelectric energy storage, compressed air energy storage, and geothermal energy exploitation and storage have garnered significant attention. Abandoned mines, especially those that become flooded, can easily form low-enthalpy mine water ‘hydrothermal reservoirs’ characterized by high permeability and cost-effective drilling. However, a lack of understanding of these shallow to medium-depth geothermal reservoirs—including their development potential and feasibility—hinders the widespread implementation of this approach, particularly in obsolete collieries. This study analyzes the diachronic and spatial characteristics of mine-oriented geo-thermal energy exploitation and storage. It introduces a new concept: geothermal energy exploitation and storage throughout the entire life cycle of a mine within Earth Critical Zone. From a temporal perspective, projects should be implemented sequentially during the exploration, construction, production, and legacy phases of a mine. Spatially, the mine-oriented geothermal energy exploitation and storage process involves the migration of various substances (solid: soil/rock; liquid: water; gas: wind and compressed air) and energy conversion (wind, solar, heat, geopotential, and mechanical energy) within Earth Critical Zone (atmosphere, hydrosphere, biosphere, pedosphere, and lithosphere). A key scientific challenge lies in understanding the migration of Earth’s fluids and energy conversion in near-earth-contact environments during mine-oriented geothermal energy exploitation and storage. To address this challenge, it is essential to analyze the similarities and differences between mine and traditional geothermal reservoirs in terms of heat source, genesis pattern, and compensation and drainage conditions. Specifically, large-diameter, high-flow, single-hole, and steady-state pumping tests are recommended to obtain basic hydrogeological parameters. These can be used to create a 3D geological model of the targeted mine strata, which can simulate and evaluate the quasi-water seal effect during the mine-water heat storage process in legacy shafts. Theoretical analyses, numerical simulations, and model experiments are urgently needed to understand the formation and dissipation mechanisms of thermohaline stratification in large-scale mine water bodies, as well as its impact on heat and mass transfer. Solving these issues will enhance the scientific understanding of geothermal resources in legacy mines, strengthen the technical support for mine geothermal energy exploitation and storage, and accelerate the development of new high-quality productivity in the transformation of the local legacy mine industry in China.