Abstract High-temperature thermal energy storage in subsurface reservoirs (HT-RTES) can help balance seasonal heat demand and supply by storing excess thermal energy in subsurface reservoirs. Mature hydrocarbon fields are promising storage targets because their reservoir properties are relatively well constrained; however, many legacy fields lack modern petrophysical data. In the Leopoldshafen oil field (northern Upper Rhine Graben), complete log coverage is limited to legacy self-potential (SP) and short-normal (SN) resistivity logs, whereas neutron porosity (NPHI) logs are available only in two wells (LH-5 A and Stutensee-1). Here, we present an integrated workflow to reconstruct NPHI in legacy wells using machine-learning methods. SP and SN logs from 24 boreholes were digitized, standardized, and cleaned to remove acquisition artefacts. Lithological classes were derived from SP–SN cross-plot clustering and used as a rock-physics-guided lithology flag. Five supervised algorithms were tested using two input configurations: Model 1, based on scaled SP and SN logs, and Model 2, which additionally included the lithology flag. Random Forest with Model 2 yielded the best performance in the training well Stutensee-1 and generalized well to the independent validation well LH-5 A. The main prediction errors occurred in gas-bearing intervals, but a lithology-specific post-processing correction markedly improved agreement with measured NPHI. The results show that lithological information substantially improves porosity prediction from sparse legacy logs and enables geologically consistent NPHI reconstruction for reservoir characterization at Leopoldshafen.
Freezing of central drainage ditches constitutes a significant cause of freeze damage in tunnels located in frigid climates. Traditional passive insulation methods, such as thermal insulation layers, are inadequate to prevent the freezing of central drainage ditches in these harsh environments. This study investigates the effectiveness of ground-source heat pump-based active heating systems in enhancing frost resistance for tunnel central drainage ditches in cold regions through laboratory tests and numerical simulations. The impacts of various parameters, including the volumetric flow rate of the circulating medium, heating system supply water temperature, heating pipe linear density, and heating pipe layout, on the temperature differential between the supply and return water of the heating system, heat exchange power, and the temperature field of the central drainage ditch, were analyzed. A novel sectional layout method for heating pipes within the central drainage ditch was proposed. Furthermore, the study explored how the wind speed inside the tunnel affects the temperature field of the central drainage ditch, the annual running time of the sectional heating system, the heating power of the system, and its energy consumption under active heating conditions. The results indicate that the temperature differential between the supply and return water of the heating system diminishes as the volumetric flow rate of the circulating medium increases, yet it escalates with higher supply water temperatures and increased linear density of heating pipe. The heating power of the ground-source heat pump system initially rises before declining as the volumetric flow rate of the circulating medium grows, while it consistently increases with elevated supply water temperatures and greater linear density of heating pipe. Results also highlight that helical heating pipes in the ground-source heat pump system outperform U-shaped heating pipes. The annual operating time of the heating system lengthens with higher tunnel wind speeds and shortens with increased distance from the tunnel inlet. Lastly, the average annual energy consumption of the heating system rises with increased tunnel wind speeds and declines as the distance from the tunnel inlet increases.
Abstract 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.
The medium-deep U-tube borehole heat exchanger (MDUBHE) extracts geothermal heat via a closed loop without groundwater withdrawal, thereby preserving geothermal resources, with its performance jointly governed by horizontal-section length and operational, geological, and structural parameters. A field test was conducted on an MDUBHE with a 1100 m horizontal section in the Weihe Basin. Circulation test results gave an average outlet water temperature of 21.2 ℃ and an average inlet–outlet temperature difference of 10.8 ℃; the heat extraction rate ranged from 760.4 to 1034.7 kW, governed primarily by circulation flow rate and inlet water temperature. A three-dimensional heat transfer model was developed and validated, with predicted outlet temperature deviation below 6
This study focuses on a building in Xining, Qinghai Province, China. Based on TRNSYS, two heating systems were modeled: a medium-deep borehole heat exchanger (DBHE) heating system and a solar thermal–medium-deep borehole heat exchanger coupled heating system (ST-DBHE). Their heating performance, energy-consumption characteristics, and environmental benefits were comparatively evaluated. The results showed that: (1) For the DBHE system, the average supply and return water temperatures at the geothermal well were 37.62 °C and 36.49 °C, the cumulative heat extraction from the geothermal well was 141,133.48 kWh, the average system COP was 2.30, and the annual energy consumption was 70,299.82 kWh; (2) For the ST-DBHE system, the average supply and return water temperatures at the geothermal well were 38.68 °C and 37.76 °C, the cumulative heat extraction from the geothermal well was 110,756.04 kWh, the cumulative solar thermal recharge was 47,410.96 kWh, the average system COP was 2.26, and the annual energy consumption was 75,992.99 kWh. (3) Compared with the DBHE system, the solar preheating subsystem in the ST-DBHE system increased the COP of the heat pump unit from 3.77 to 3.79, reduced geothermal heat extraction by 21.52
