Understanding the Thermal-induced Flow in the Carbonate and Sulphate Karstic Reservoir of Hainaut (South Belgium)

Abstract

The geothermal reservoir of Hainaut, South Belgium, produces geothermal energy for district heating for more than 25 years. Two wells give a 100 m³/h artesian flow rate of hot water at 70°C. Exploitation of a third well will start in the next future for a similar use. The reservoir is a fissured and karstic aquifer composed of thick Lower Carboniferous limestone and dolostone formations. In one of the wells, anhydrite was discovered in the Upper and Middle Visean carbonate, overlying a thick karstified and/or brecciated permeable horizon. The near outcrop show collapse breccias in coeval formations, probable result of the entire anhydrite/carbonate sequence karstification. In the other geothermal wells, permeability was met in these breccia layers, highlighting their importance for the reservoir productivity. Karst development in the deep Lower Carboniferous strata of Hainaut can be related to past tectonic activity, identified using several tectonic, karstic and sedimentological arguments, that caused the increase of permeability and the onset of thermal-induced convective flow in the aquifer. Anomalies in temperature and/or chemical and isotopic signatures of spring water from the outcrop attest moderate uprising flow of geothermal water, and show that convection is still occurring nowadays. Understanding the deep water flow at regional and local scale is crucial for the exploitation development of the Hainaut reservoir, as the thermal, lithological and hydrogeological heterogeneities of the aquifer are demonstrated. Failures due to lack of knowledge can be avoided by careful exploration in the future.