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

In need for alternative energy sources, deep borehole heat exchangers (DBHEs) are gaining increasing attention worldwide. Compared to deep open-loop geothermal systems, closed-loop systems generally provide low heat performance, while the advantage of DBHEs lies in their flexible installation (no reservoir needed), safer operation (low impact on the geological environment), and lower maintenance costs. Additionally, DBHEs may be installed in abandoned hydrocarbon wells or unsuccessful geothermal wells, significantly reducing installation costs. In DBHEs, the heat carrier fluid circulates inside the borehole, heated through heat conduction from the surrounding rock. The thermal conductivity and heat capacity of the geological environment therefore control system performance. In DBHE modelling studies, thermal properties of rocks are commonly averaged for lithological groups and the temperature- and pressure dependency of these properties are neglected. Bulk thermal properties are primarily controlled by porosity (i.e. pore fluid content) and the minerals constituting the rock matrix and can significantly change with increasing temperature and pressure conditions. Therefore, realistic estimates on in-situ thermal properties are key input for DBHE performance models. In this study we demonstrate and quantify the effect of depth- temperature- and pressure-dependent thermal properties on the performance of DBHEs in the siliciclastic sediments of the Pannonian Basin. Thermal conductivity and heat capacity profiles of typical lithotypes constituting the Neogene sedimentary succession of the Pannonian Basin, calculated using regional porosity-depth trends and literature-based correction formulas for temperature and pressure, are used as input for the numerical modelling. In addition to general thermal property profiles, we present DBHE models using well-log-based thermal conductivity estimates, showing the effect of local variations in thermal property profiles. DBHE models for an operational period of 1 year highlight significant differences in DBHE performance using constant vs. depth-dependent thermal properties. Models with well-log based thermal property profiles can improve DBHE performance estimates with 10 to 20 %. In general, models adopting temperature- and pressure-dependent thermal properties predict lower DBHE performance, governed by thermal conductivity decrease compared to non-dependent conductivity values. The effect of temperature- and pressure-dependent property variations on DBHE performance is dependent on lithotype and becomes relevant in the case of DBHE depth ~>2 km, further depending on local geothermal conditions. This study demonstrates that the adequate performance evaluation of DBHE projects requires modelling studies adopting carefully selected thermal properties representing the in-situ conditions of the geological environment.