Field-scale analysis of local thermal non-equilibrium in sedimentary aquifers using two-phase heat transport modeling

Field-scale simulations of heat transport in sedimentary aquifers are commonly based on a single-phase temperature approach that assumes local thermal equilibrium (LTE) between the solid matrix and the pore fluid. However, the validity of this assumption under strong hydraulic heterogeneity and fast flow regimes has been questioned. In such settings, delayed interphase heat exchange may lead to local thermal non-equilibrium (LTNE) effects, resulting in temperature differences between the solid matrix and the fluid phase that cannot be captured by standard modeling approaches.

To investigate the potential influence of pore-scale LTNE effects at the field scale, a two-phase heat transport model was applied, resolving separate temperature fields within the fluid and solid phases and coupling them through an interphase heat transfer coefficient at the scale of a sedimentary aquifer. The model was implemented within the Multiphysics Object-Oriented Simulation Environment (MOOSE) and it numerically solves the coupled groundwater flow and heat transport equations. Simulations focused on the evolution of the thermal plume generated by a borehole heat exchanger. A systematic parameter study was carried out to assess the impact of homogeneous and heterogeneous hydraulic conductivity distributions, grain sizes, mean groundwater flow velocity, porosity, and injection temperature on the transient temperature differences between the fluid and solid phases.

For homogeneously distributed hydraulic conductivities, simulation results indicate that temperature differences between the fluid and solid phases remain below 10⁻³ K for most of the investigated cases and parameter combinations. Given the typical measurement accuracy of temperature sensors, differences of this magnitude are negligible. Preliminary results for heterogeneous hydraulic conductivity fields show that local temperature differences can exceed those observed in the corresponding homogeneous case. However, when averaged over an ensemble of 100 heterogeneous realizations with identical log-conductivity statistics, the mean temperature difference between the solid and fluid phases remains generally very close to that of the homogeneous case. Upstream of the borehole heat exchanger, ensemble-averaged temperature differences in the heterogeneous case are higher than those in the homogeneous case, whereas downstream the opposite trend is observed. Overall, the study quantifies the magnitude and spatiotemporal variability of LTNE effects under field-scale conditions, providing a basis for assessing the relevance of two-phase LTNE modeling for underground thermal energy storage.