Heat potential of the Upper Jurassic–Lower Cretaceous aquifer in Central Northern Bulgaria: conditions and prospective use

The regional hydrogeological conditions in Northern Bulgaria determine the existence of a large artesian basin consisting of layered aquifers. The groundwater temperature increase with depth and the conditions in the deeper layers are favourable for the formation of stratified hydrothermal reservoirs with potential for heat extraction. Although it was initiated in the 1980s, the utilization of this green energy resource is still in a pre-feasibility phase. The Upper Jurassic–Lower Cretaceous aquifer has the highest yield and it is the most prospective geothermal source in the multi-layered sedimentary complex in Central Northern Bulgaria. This study provides new understanding of the formation and potential for extraction of thermal energy from this stratum, which has all characteristics of a low temperature hydrothermal reservoir. The study includes delineation of its boundaries, evaluation of hydrogeological and thermal parameters and data on hydraulic connectivity. The flow and temperature fields, the elements of the water and heat balance, as well as the groundwater quality, have been assessed. The groundwater and thermal energy resources in the hydrothermal reservoir have been estimated on a regional scale, including a scenario with reinjection. Different options for potential utilization of the thermal energy using reinjection wells have been put forward.