Estimation of thermal conductivity of rocks from their mineralogical composition (Asturian Coal Basin, NW Spain) for modelling purposes

The Carboniferous Coal Basin (CCB) in NW Spain has been extensively mined for a long time. At present, abandoned and flooded mines constitute “mining reservoirs” which act as storage facilities containing mine water susceptible to being exploited as a geothermal resource to supply heating and cooling using heat pumps. The development of numerical models capable of predicting the geothermal behaviour of the reservoir after a certain number of years of use can be useful and applicable to this and other sedimentary mining basins. A procedure for predicting in situ thermal conductivity (k) profiles using lithological and mineralogical information for modelling purposes was applied to a mining reservoir in the CCB. First, all the involved geological units were sampled in unaltered outcrops, assuming that weathering have not significantly biased thermal properties. The matrix thermal conductivity of each sample was estimated from the thermal conductivities of the respective mineralogical components, whose proportion were determined by optical microscopy. A final k was acquired considering the effects of porosity, temperature and mineralogical composition of the rock matrix and assuming water as the pore-filling fluid. Finally, the obtained parameters were extended to the deep geological profile, affected by the mining works. Values ranging from 1.42 W m−1 K−1 (lutite) to 4.89 W m−1 K−1 (sublitarenite) were found in the studied reservoir (average of 2.86 W m−1 K−1).