IDENTIFICATION OF VARIATIONS IN HEAT EXCHANGE CAPACITY WITH DEPTH IN A BOREHOLE USING HEAT DILUTION TESTS

The ability of a geologic medium to transmit heat to or away from a ground-source energy well is dependent on the thermal properties of the saturated formation as well as the rate of groundwater advection through the formation. Geologic variation of the thermal and aquifer properties with depth results in depth-variable capacity for heat exchange.

We assessed the depth-variability of heat exchange of geologic formations adjacent to a well by conducting dilution tests at multiple depths using heat as the tracer. The test well was installed in south-central Wisconsin to investigate the geothermal gradient. The well is 300 meters deep and is open to 186 meters of Cambrian sandstone and 40 m of underlying Precambrian crystalline bedrock. Dilution tests were initiated by using a downhole resistance heater to warm the water at twenty selected depths in the well. In-well temperatures were monitored with depth and time using a fiber optic distributed temperature sensing (DTS) system. We used the dilution test data to estimate the apparent horizontal groundwater flow velocity at each tested depth.

Tests results show variation with depth in the induced temperature change and the rate of heat dissipation for equal amounts of heating, indicating variation with depth in heat exchange between the well and the formation. Comparison of dilution test results to borehole geophysical data and image logs allowed for correlation of heat exchange variation with lithological variation. We found the sandstone to be more efficient in heat exchange than the crystalline bedrock due to higher groundwater velocities. Our results indicate that evaluation of heat exchange through dilution tests can provide valuable results to aid in the design of ground-source heat exchange systems.