The University of Minnesota, the Minnesota Geological Survey, and the U.S. Geological Survey are studying the feasibility of storing water at a temperature of 150 degrees Celsius in the Franconia-Ironton-Galesville aquifer. The Aquifer Thermal-Energy Storage project has a doublet-well design with a well spacing of approximately 250 meters. One well will be used for cool-water supply, and, the other, for hot-water injection. The U.S. Geological Survey is constructing a model of ground-water flow and thermal-energy transport to aid in determining the efficiency of the Aquifer Thermal Energy Storage system. A preliminary model of radial flow and thermal-energy transport was constructed, based on hydraulic and thermal properties of the Franconia-Ironton-Galesville aquifer determined in previous studies.
The model was used to investigate the sensitivity of model results to various hydraulic and thermal properties and to study the potential for buoyancy flow within the aquifer and the effect of various cyclic injection-withdrawal schemes on the relative thermal efficiency of the aquifer.
Sensitivity analysis was performed assuming 8 days of injection of 150-degree-Celsius water at 18.9 liters per second, 8 days of storage, and 8 days of withdrawal of hot water at 18.9 liters per second. The analysis indicates that, for practical ranges of hydraulic and thermal properties, rock-heat capacity is the least important property and thermal dispersivity is the most important property used to compute temperature and aquifer thermal efficiency.
The amount of buoyancy flow was examined for several values of hydraulic conductivity and ratios of horizontal to vertical hydraulic conductivities. For the assumed base values of hydraulic and thermal properties, buoyancy flow was negligible. The greatest simulated buoyancy flow resulted from simulations in which horizontal hydraulic conductivity was increased to 10 times the base value, and the vertical hydraulic conductivity was set equal to the horizontal hydraulic conductivity.
The effects of various injection-withdrawal rates and durations on computed values of aquifer relative thermal efficiency and final well-bore temperature were studied for five 1-year hypothetical test cycles of injection and withdrawal. The least efficient scheme was 8 months injection of 150-degree-Celsius water and 4 months of withdrawal of hot water at 18.9 liters per second. The most efficient scheme was obtained with 6 months of injection of 150-degree-Celsius water at 18.9 liters per second and 6 months of withdrawal of hot water at 37.8 liters per second. The hypothetical simulations indicate that the subsequent calibrated model of the doublet-well system will be a valuable tool in determining the most efficient system operation.