THE THERMOFACIES CONCEPT AS APPLIED TO DISTRICT-SCALE GROUND-COUPLED HEAT PUMPS – CORRELATION OF TEMPERATURE DATA TO THERMAL AND HYDROSTRATIGRAPHY AT BALL STATE UNIVERSITY

Standard thermal response tests (TRTs) are the industry standard for borehole heat exchanger (BHE) design in ground coupled heat pump systems (GCHPs). Two such TRTs conducted in Phase 2 of the district-scale GCHPs at Ball State University (BSU) measured a bulk ‘formation’ thermal conductivity (K_T) between 2.6 and 3.0 Wm^-1K^-1. Meanwhile, K_T from a core recovered near BSU averages 2.2 and 3.5 Wm^-1K^-1 for dry and water-saturated samples, respectively. Importantly, these data do not include the shale-rich Dillsboro and Kope Formations, which comprise a significant proportion of lower K_T media in each BHE at BSU. The range in K_T data from saturated samples (1.8–7.2 Wm^-1K^-1) leads to the logical conclusion that TRTs do not capture the vertical and horizontal heterogeneity of thermal flux in layered sedimentary aquifers. Characterization of the hydrogeologic environment can therefore be a tool to tune district-scale GCHPs to the specific on-site conditions that may influence the magnitude and mode of heat transfer.

At BSU, temperature (T) changes in the groundwater environment at the active Phase 1 field through October 2013 support this hypothesis. After constant thermal loading, a T increase of 14–18 ºC has been observed in the central monitoring well. Vertical structure in the T profile of this well may correlate to ‘thermofacies’. For example, a T ‘spike’ between 14–19.5 m in depth may correspond to a sand and gravel zone in the surficial glacial till and a T ‘dip’ at a depth of 70 m agrees with the position of the Brainard shale, zones of higher permeability and lower measured K_T (2.0 Wm^-1K^-1), respectively. Higher measured K_T zones, such as the low siliciclastic Silurian Salamonie Limestone and the Ordovician Whitewater Formation may be target thermofacies for heat deposition and extraction. In contrast, sand and gravel zones within the glacial till may allow for significant thermal loading; however, groundwater advection may reduce the fraction of recoverable heat.