Coupled density–viscosity controls in low-temperature aquifer thermal energy storage under heterogeneous conditions

Aquifer heterogeneity and temperature-dependent fluid properties are key controls on heat transport in aquifer thermal energy storage systems, yet their combined influence on performance reliability in low-temperature doublets remains insufficiently quantified. We evaluate a three-dimensional confined-aquifer model using 50 stochastic hydraulic-conductivity realizations, four density–viscosity coupling schemes, and warm-well injection temperatures ranging from 14 to 30 °C, while the cold-well injection temperature is fixed at 6 °C. Performance is assessed using thermal-recovery percentiles, uncertainty range, and a robustness index that integrates mean performance, variability, and a minimum acceptable recovery threshold. Simulations reveal a pronounced asymmetry between the two wells. Warm-well recovery remains relatively stable across the investigated temperature range, whereas cold-well recovery deteriorates strongly with increasing warm-well injection temperature. The 10th-percentile cold-well thermal recovery decreases from approximately 61% at 14 °C to below 25% at 30 °C, reaching as low as 4.2% when both density and viscosity are treated as temperature dependent, while the corresponding robustness index decreases from approximately 0.8 to below 0.34. This reliability loss is associated with preferential inter-well heat transport in unfavorable heterogeneous realizations, amplified by increased fluid mobility. Within the investigated LT-ATES range, with warm-well injection temperatures not exceeding 30 °C and a maximum warm–cold injection-temperature contrast of 24 °C, temperature-dependent density has only a minor influence on the selected performance indicators. In contrast, neglecting temperature-dependent viscosity can overestimate lower-percentile cold-well recovery and robustness. Regional groundwater flow further reduces recovery at both wells, while increasing well distance mainly improves cold-well reliability by limiting inter-well thermal interference. These results identify the cold well as the reliability-limiting component and show that median recovery alone can mask unfavorable heterogeneous conditions. Lower-percentile and robustness-based metrics therefore provide useful complementary information for reliable low-temperature ATES design in heterogeneous aquifers.