Low Temperature, Man-Made Geothermal Reservoirs in Abandoned Workins of Underground Mines

Abstract

The flooded, abandoned deep underground mines form low-temperature geothermal reservoirs, characterized by the site-specific geological, hydrogeological, and technical conditions of mining fields. The large volumes of underground open spaces with high permeabilities that are left after mining activity in areas with relatively high geothermal gradients may be considered as man-made reservoirs of warm mine waters. In the process of adaptation of mine workings for reservoir purposes
the subsidence of the mining area should be considered, which will reduce the volume of water-filled spaces in the mine, and also the high amount of total dissolved solids (TDS) and gas in mine waters. The rate of cooling of the rock mass during mining activity and the time for thermal recovery to natural, undisturbed thermal conditions are also important.

The flooded coal mine of Nowa Ruda in south-western Poland is presented as an example. In the Piast mining field the abandoned workings in the
Carboniferous Lower Zacler beds contain about 5,000,000 cubic meters of water at a temperature of 16-26?C. The exploitation of the mine water for geothermal heat pumps is planned. Two two-dimensional (2-D) numerical models based on the TOUGH2 program have been developed to introduce heat exchange modeling into this mining field. The model of temperature recovery of abandoned workings shows the time required to return to natural thermal conditions in a rock massif that has cooled down during coal mining. The expected time for temperature recovery is approximately 10 years. The model of a geothermal doublet producing water at a temperature of 23?C from abandoned workings presents the rate of cooling of the reservoir at production rates of 10 l/s (thermal output power ~800
kWt) and 20 l/s (thermal output power ~1600 kWt).