DISCRETE-DEPTH HEAT DISSIPATION TESTS AS A POTENTIAL TOOL FOR CHARACTERIZING SUBSURFACE THERMAL CONDUCTIVITY VARIATIONS

Variations in formation thermal conductivity and groundwater flow velocity with depth can significantly affect the performance of ground-source heating and cooling systems. However, traditional thermal response tests (TRTs) do not allow for evaluation of the variability of formation thermal conductivity and groundwater velocity with depth. We investigate the potential for using discrete-depth heat dissipation tests in an open, water-filled borehole to characterize formation heat exchange capacity with depth.

We conducted discrete-depth heat dissipation tests in a test well in south-central Wisconsin. Heat dissipation tests were initiated at target depths in the test well using an electrical resistance heater. Heat dissipation was monitored by measuring borehole water temperature with time using a fiber optic distributed temperature sensing system. Temperature data were then used to compare the thermal response at different depths in the well. To evaluate both conduction and convection as heat transfer mechanisms we used a numerical groundwater flow model (MODFLOW) and contaminant transport model (MT3DMS) to simulate heat dissipation tests.

Simulation results indicate that the measured response to a heat dissipation test in an open borehole is strongly dependent on the horizontal measurement location within the borehole, making it difficult to evaluate thermal conductivity and groundwater flow velocity if the measurement position is unknown. However, modeling results indicate that the thermal response of a heat dissipation test as measured at the center of the borehole is sensitive to variations in thermal conductivity and groundwater flow velocity, suggesting that heat dissipation tests are a potentially useful method for characterizing depth-variability in thermal properties if a centralized temperature measurement method is used to monitor the tests.