Subsurface Voids and their Sedimentology – Implications for Drilling into Mines for Heat

Abstract

Flooded mine workings across County Durham and the Northumberland coalfields represent potential shallow geothermal aquifers, where groundwater has returned to pre-mining levels to flood the abandoned workings. These provide potentially large aquifers to extract low-enthalpy geothermal heat from, using ground source heat pumps. Heat pumps tapping flooded mine waters have significant potential in the decarbonisation and regeneration of densely populated ex-coal mining areas. Coal extraction largely took place through room and pillar mining, with the majority of flow occurring through residual ‘rooms’, possessing very high permeabilities and porosities. Pillars left to support the overburden can fail overtime, giving rise to various anthropogenic coal deposits within the workings, which are important to appraise as the heterogeneity of the Carboniferous coal measures will impact the flow properties of the workings. The structure of collapsed workings has been studied previously, but no attention has been paid to the lithologies and highly variable permeabilities and porosities (over several orders of magnitude). This research has investigated anthropogenic coal deposits in the former mine workings at Whitley Bay, and integrated findings with borehole datasets from Bayfield in the local region. Detailed permeabilities, porosities and petrography were collected through extensive fieldwork, focused on the Upper Carboniferous High Main seam. This research demonstrates the negligible permeabilities (<0.01 mD) and low porosities (<8%) of the anthropogenic coal deposits compared to the surrounding lithologies, and may act as flow barriers. Closely associated and laterally connected sandstones and siltstones have much higher permeabilities (~100-300 mD) and porosities (~15-30%). Integration with biostratigraphy and detailed chemostratigraphy from Whitley Bay and Bayfield datasets ~10 km inland enables the sedimentological observations to be more widely correlated, allowing for identification of coal seams. The findings presented establish one of the first data sets that can be used for modelling of geothermal prospects in former shallow coal mine workings.