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The technical feasibilityof high-temperature[>100°C (>212°F)]aquifer thermal energy storage (ATES) in a deep, confined aquifer was tested in a series of experimentalcycles at the Universityof Minnesota's St. Paul field test facility (FTF). ,_This report describesthe second long-termcycle (LT2), which was conducted from October 1986 through April 1987. Heat recovery_ operational experience;and thermal, chemical,hydrologic, and geologic effects are reported, Approximately614 of the 9.21 GWh of energy added to the 9.38 x 104 m3 of ground water stored during LT2 was recovered. Temperaturesof the water stored and recovered averaged 118°C (244°F) and 85°C (185°F),respectively. Results agreed with previous cycles conductedat the FTF. System operation during LT2 was nearly as planned. Operationalexperience from previous cycles at the FTF was extremely helpful. Ion-exchangesofteningof the heated and stored aquiferwater prevented scaling in the system heat exchangersand the storage weil, and changed the major-ion chemistry of the stored water. Sodium bicarbonatereplaced magnesium and calcium bicarbonate as primary ions in the softenedwater. Water recovered from storage was approximatelyat equilibriumwith respect to dissolved ions. Silica, calcium, and magnesiumwere significantlyhigher in recoveredwater than in injectedwater. Sodium was significantlylower in water recovered than in water stored. Temperaturesat Ironton-Galesville horizons in storage site monitoring wells reached~118°C (~244°F) during LT2. Following heat recovery, temperatureswere~40°C (~I04°F) at the same locations. Slow and slight thermal responseswere observed in low permeabilityzones. No thermal or chemical effects were observed at the remote monitoring site. The softener lowered hardness of the source water from 160 mg/L to less than I0 mg/L as calcium carbonate. Sodium concentrations in the source water increased from~44 mg/L to~122 mg/L in the water softener. 6. Groundwater chemistry results from LT2 suggest that the water from the source well was at equilibrium with respect to major ions. Water recovered from storage had reached equilibrium with respect to hardness and silica at the recovered water temperature. Mass balances calculated for different ports of LT2 show effects of ion-exchange softening, changes in equilibrium concentration or water temperature, and mixing. viii ACKNOWLEDGMENTS This work, supportedby the U.S. Departmentof Energy, was performed by the University of Minnesota as part of the UndergroundEnergy Storage Program at Pacific NorthwestLaboratory. Pacific Northwest Laboratoryis operated by Battelle Memorial Instituteunder contract DE-ACO6-76RLO1830. Special thanks go to program managers L.D. Kannberg and J.R. Raymond for their suggestions, encouragement,and guidance.

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