Mapping the geoexchange potential of the St. Lawrence Lowlands from thermal conductivity measurements of rock samples

English Abstract:

The objective of this project was to determine the geothermal heating and cooling potential of the St. Lawrence Lowlands sedimentary basin. This region encloses major cities such as Montreal and Quebec City and is the most important market for ground source heat pumps in the province of Quebec because of the population density. Tables and maps were developed from a thermostratigraphic assessment of rock samples to illustrate the geothermal potential associated to vertical closed-loop systems.

Sizing calculations were first performed for 45 locations to evaluate the drilling length needed for the installation of a ground heat exchanger at a typical home. The building design parameters were kept constant for each location and the subsurface properties were varied according to laboratory measurements of samples collected in surface outcrops. The subsurface thermal conductivity was measured with a needle probe and the subsurface heat capacity was calculated according to the mineralogy estimated with thin section descriptions. Thus, for all rock samples, sizing calculations revealed that bore length should vary between 101 and 213 m.

Thermostratigraphic classifications of the geological units were subsequently created according to their geothermal potential (high, medium or low) based on drilling depth. Results highlight high potential for units such as Theresa, Covey Hill and Cairnside formations having depth of drilling varying from 101 to 129 m and thermal conductivity ranging from 4 to 6,9 W/mK.

Thermal conductivity measurements and sizing calculations were finally mapped using ArcGIS software. Two separate maps showing results with points over the surface distribution of the geological units were designed to simplify visualization. The maps shows that thermostratigraphic units with high geothermal potential are mostly found in the western part of the sedimentary basin.