Rock Properties at Depth Shape Performance of Deep Borehole Heat Exchangers in Pannonian Basin Study

Deep borehole heat exchangers, in which heat carrier fluid is warmed purely by conductive heat transfer from the surrounding rock, are considered low-risk options for geothermal energy exploitation across a wide range of geological settings. Because no fluid is circulated through the rock itself, the thermal conductivity of the geological environment is the primary control on how well such a system performs. A new modelling study focused on siliciclastic sedimentary formations in the Pannonian Basin examines how this key property changes with depth, temperature and pressure, and what those changes mean for performance predictions.

The researchers built bulk thermal conductivity profiles in two ways: from porosity-depth trends combined with thermal conductivity measurements, and from estimates derived from well logs, allowing them to represent realistic cases with complex lithologies. Comparing models that used a constant thermal conductivity against models in which conductivity varied with depth, temperature and pressure revealed substantial differences. Performance estimates for the deep borehole heat exchangers differed by up to 35 percent between constant and depth-dependent conductivity profiles in sand and shale lithologies.

According to the study, models with depth-dependent thermal conductivity performed better because conductivity increases with depth as porosity decreases, enhancing heat transfer in the deeper sections of the borehole.

Neglecting the temperature and pressure dependency of thermal conductivity also skewed predictions. Models that ignored this dependency overestimated the borehole outlet temperature by up to 1.75 degrees Celsius. The effect becomes more relevant as borehole depth increases and as thermal conductivities rise, for example in sandstones. The authors therefore recommend that temperature and pressure dependency be considered in performance modelling for borehole heat exchangers deeper than roughly two kilometres, with the exact threshold depending on local conditions.

The study’s central conclusion is that adequate performance evaluation of deep borehole heat exchanger projects requires carefully selected input thermal conductivity profiles, together with an assessment of the related uncertainties.

Subject of Research: How depth-, temperature- and pressure-dependent rock thermal conductivity affects deep borehole heat exchanger performance in the Pannonian Basin

Article Title: 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

Article References: Békési, E., Porkoláb, K., & Lenkey, L. (2026). 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. Geothermal Energy. https://doi.org/10.1186/s40517-026-00410-8

Image Credits: AI Generated

DOI: 10.1186/s40517-026-00410-8

Keywords: deep borehole heat exchangers, geothermal energy, thermal conductivity, Pannonian Basin, porosity, sandstone, shale, heat transfer, numerical modelling, sedimentary rocks, temperature dependence, pressure dependence