Multiphysical Monitoring of a Thermal Plume during Mine Water Geothermal Experimentation

The rate and magnitude of conductive and advective thermal processes that control the efficiency and sustainability of mine water geothermal and thermal energy storage are poorly quantified. We present results of multiphysical observations collected during a 25-day heat extraction experiment performed on an abstraction-reinjection mine water well doublet at the UK Geoenergy Observatory in Glasgow. The results showed how the thermal plume generated by injection of cool water developed from the injection borehole, mainly driven by advection at the level of mine workings. Heat conduction around the cased part of the injection borehole resulted in different cooling rates related to lithological variations, clearly shown in the distribute temperature sensing (DTS) data. The electrical resistivity tomography (ERT) monitoring provided a time-lapse image of the evolution of the cool plume, that extended laterally within the mine workings to pass a borehole located at 10 m away in less than 2 days, and later above the mine workings into the bedrock. It also showed how the effects of cooling remained for at least 18 days after the injection stopped, providing new insight into thermal storage behaviour of mine water systems. The results provide an unprecedented opportunity to visualize the effects of geothermal operation in mine water settings and are useful guidance for project developers and regulators.