The effect of depth, temperature and pressure dependency of rock thermal properties on the performance of deep borehole heat exchangers: example from the Pannonian Basin

Abstract Deep borehole heat exchangers (DBHEs) are low-risk options for geothermal energy utilization that can be installed in a wide range of geological environments. In DBHEs, the heat carrier fluid is heated through heat conduction from the surrounding rock, thus system performance is primarily controlled by the thermal conductivity of the geological environment. In this study we investigated the effect of depth- temperature- and pressure- (Tp) dependent rock thermal conductivity on the performance of DBHEs, through the case of siliciclastic sedimentary formations in the Pannonian Basin. Bulk thermal conductivity profiles were calculated using porosity-depth trends and thermal conductivity measurements and were estimated from well logs to demonstrate realistic cases with complex lithologies. Results show significant (up to 35%) differences in DBHE performance estimates between models with constant and depth (~ porosity)-dependent thermal conductivity profiles and sand/shale lithologies. Depth-dependent models perform better due to increasing thermal conductivity (governed by decreasing porosity) with depth, enhancing heat transfer in the deeper part of the DBHE and highlighting favourable settings in wells penetrating basement rocks with high thermal conductivity. Models neglecting Tp dependency overestimate DBHE outlet temperature with up to 1.75 ºC. Tp dependency becomes more relevant with increasing DBHE depth and high thermal conductivities (e.g. sandstones), thus should be considered in performance modelling for DBHE depth > ~2 km, further depending on local conditions. This study demonstrates that the adequate performance evaluation of DBHE projects requires carefully selected input thermal conductivity profiles, with the assessment of related uncertainties.