Cyclic injection, storage, and withdrawal of heated water in a sandstone aquifer at St. Paul, Minnesota--Analysis of thermal data and nonisothermal modeling of long-term test cycles 1 and 2

Abstract

In May 1980, the University of Minnesota began a project to evaluate the feasibility of storing heated water (150 degrees Celsius) in the Franconia-Ironton-Galesville aquifer (183 to 245 meters below land surface) and later recovering it for space heating. The University's steam-generation facilities supplied hightemperature water for injection. This Aquifer Thermal-Energy Storage system had a doublet-well design in which the injection and withdrawal wells were spaced approximately 250 meters apart. Water was pumped from one of the wells through a heat exchanger, where heat was added or removed. This water was then injected back into the aquifer through the other well. Two long-term test cycles, consisting of approximately equal durations of injection, storage and withdrawal of about 59 days, were completed. Rates of injection and withdrawal of about 18 liters per second were maintained for each long-term test cycle. Injection temperatures averaged about 108.5 and 117.7 degrees Celsius for long-term test cycles 1 and 2, respectively. Withdrawal temperatures averaged about 75 and 85 degrees Celsius, respectively. Temperature graphs for selected depths at individual observation wells indicate that the Ironton and Galesville Sandstones received, stored, and yielded more thermal energy than the upper part of the Franconia Formation. Vertical-profile plots and timegraphs during storage indicate that the effects of buoyancy flow were minimal within the aquifer.