THERMAL CONDUCTIVITY OF HARTFORD BASIN STRATA: A PILOT STUDY COMPARING OUTCROP K VALUES TO TEST BORINGS

A pilot study examining the thermal conductivity of select bedrock units within the Hartford Basin is presented. Specifically, the thermal conductivity of bedrock samples collected from outcrops within the basin are compared to thermal conductivity measurements we made of test borings taken from similar units. Outcrop data was downloaded from the National Geothermal Data System (NGDS; U. S. Department of Energy). Bedrock core data was obtained from test borings provided by the City of Hartford (CT), Metropolitan District Commission.  

Thermal conductivity (K) is a key parameter in the design of efficient and cost effective ground source heat pumps (GSHPs). Limited space often forces GSHP proponents to install vertical ground heat exchangers. This demands that the length of the heat exchanger be properly sized to meet heating/cooling loads. Thus, determining the thermal conductivity of the formation in which the GSHP will be installed is key; because, the steeper the thermal gradient needed to dissipate heat to the ground, the longer the heat exchanger will need to be. Since drilling is a primary contributor to the high initial expense of GSHP installation, balancing exchanger length (i.e., drilling costs) with formation thermal gradient/conductivity (i.e., system efficiency) is critical.  

K values of representative samples were measured from test borings collected along an east-west transect across the Hartford Basin as it intersects Hartford. The stratigraphic units found along the transect and studied here, are the Holyoke Basalt, the East Berlin Fm, the Hampden Basalt, and the Portland Fm. A C-Therm TCi Thermal Conductivity Analyzer was used for all measurements. Replicate measurements (8 to 12) are made on a smooth slab of the core. Replicate average is reported and the error (1 sigma) is given as the standard error of the mean.  

Holyoke Basalt K values of the test borings range slightly above those listed in the NGDS database. East Berlin Fm. test boring K values are comparable to those listed in the database, while Hampden Basalt K values are higher. Portland Fm test boring K values are narrower in range than the NGDS. Also noteworthy is the broad range of K values for the units within the Hartford Basin test borings. Initial findings underscore the need for GSHP system designers to carefully consider formation K values.