Impact of spatial heterogeneity on flow and heat transfers at the scale of a deep well doublet in the breccia layer of the Hainaut limestone geothermal reservoir, Belgium

In the South-West of Belgium, the Hainaut limestone geothermal reservoir is exploited since the eighties. The geothermal reservoir is mainly composed of limestone from the Carboniferous period, with highly permeable breccia evaporites levels. Three single wells currently extract warm groundwater from this transmissive breccia layer and provide energy for the heat production to two hospitals, schools, station, housings and an economic area. The depth of the exploited layers is around 2000 meters and the pumping groundwater temperature is about 70°C.
This paper focusses on a new geothermal doublet to be drilled in Mons city (Belgium). A numerical model has been developed at the scale of the future geothermal doublet. Numerical models are implemented using HydroGeoSphere to simulate fluid flow and heat transport in the geothermal reservoir. Simulations aim at analysing the conditions of the exploitation regarding pressure and temperature variations at the scale of the doublet and to assess the longevity of geothermal operations according to hydrogeological conditions.
Results analyse the sensitivity of the pressure and temperature fields as a function of the hydraulic and thermal parameters and their spatial variability in the breccia layer. To account for uncertainties related to the reservoir geology, different patterns and types of heterogeneity are studied, using Gaussian distributions and multiple-point geostatistics. Multiple-point geostatistics uses training images to characterize the geological heterogeneity. Training images are representations of the expected geology. Multiple-point geostatistics simulations allow reproducing the occurrence of permeable channels within the reservoir. Indeed, the dissolution of the anhydrite layers in the Carboniferous is not contiguous. Heterogeneity results in preferential flowpaths inside the breccia layer. They show the importance of the permeability distribution in the life expectancy of the geothermal well doublet. The life expectancy corresponds to the period between the start of the pumping and the start of decrease of temperature at the pumping well (>2°C). This spatial heterogeneity increases or affects the life expectancy of the doublet by isolating the extraction and injections wells, or conversely by inducing thermal shortcuts between the two wells.