HEAT PRODUCTION FROM SEDIMENTARY BASINS: A MODELING STUDY OF THE BECANCOUR AREA IN THE ST LAWRENCE LOWLANDS, QUEBEC, CANADA

Heating represents an important expense in Northern countries. Using geothermal heat from 1 to 2 km depth in widely present sedimentary basins is a promising option to fulfill this heat demand in an eco-friendly and sustainable way. Despite its potential, geothermal heat pump systems that use these intermediate depths have rarely been used so far in Canada. One of the main reasons is that drilling of deep geothermal wells involves major investments and if productivity of the targeted unit is not adequate or water temperature is too low, the large investment required will not be profitable. Our research project aims to assess the feasibility of such systems in a sedimentary basin having a moderate heat flux of about 55 mW m^-2 and a geothermal gradient of ~23°C km^-1 using numerical simulations. This project specifically focuses on the Bécancour industrial park, where several exploration wells were drilled and for which access to industrial data was obtained. Three of these wells were used to extract brines over nine years, showing a permeability of 1.5x10^-14 m² (15 mD) to 4x10^-13 m² (405 mD) for the Trenton unit.

Geothermal doublets are the most efficient ground heat exchangers, but they require the presence of a deep aquifer with sufficient porosity, permeability and extent. Numerical simulations based on available data showed that the Trenton unit can be advantageously exploited for heat with well doublets using heat pumps. Indeed, results indicated that a geothermal doublet (960 m depth, 200 m spacing between injector and producer) can supply a flow rate of 5 L s^-1, generating a drawdown of 45 m after 30 years based on mean hydraulic properties. With a re-injection at 8°C, 442 kW of heat can thus be produced consistently over the first 11 years with a downhole temperature of 30.1°C. After 11 years of production, production is expected to decline, reaching 394 kW with a downhole temperature of 27.7°C after 30 years of production.

Deep coaxial borehole heat exchangers (DBHE) were modelled for less permeable units. Results showed that a 1500 m deep DBHE with an input temperature of -5°C can offer a heating power of 106 kW during 30 years. This is achieved using borehole, outer pipe and inner pipe diameters of 125 mm, 123 mm and 90 mm respectively. Simulations have also been initiated to obtain representative heat demands for facilities in the Bécancour area, to eventually compare costs of shallow (< 400 m) and deep systems (up to 2 km), including the use of currently abandoned gas wells.