The Dinantian carbonate reservoir (DCR) is a proven geothermal play in Northwest Europe. In the UK, evidence points towards it being an important thermal spring source and a major aquifer in many basins. In the Midlands region, the carbonates have been buried to below 4000 m while they are below 2000 m in the south of England. Available data suggests a temperature range from 30 to in excess of 100 °C.
In the UK, DCR have very low primary porosity and permeability which may increase locally due to karstification, dolomitization and/or faulting. However, the influence on reservoir permeability of depositional environment, diagenesis, dolomitisation, connectivity between fault zones and high porosity zones is not yet fully understood. The current uncertainties about the physical properties of DCR have a direct impact on the sustainability of deep geothermal systems as they affect costs of exploration and drilling, accuracy and reliability of microseismic monitoring (e.g., microseismicity location) and of pore pressure distribution models. To reduce the level of uncertainties about reservoir properties, we integrated new microstructural and experimental (e.g., density, porosity, sonic velocity, permeability) datasets from pre-cut cylindrical samples (limestones, fractured limestones and karstified dolomites) collected from DCR sections of Roddlesworth-1 (1288-1298m) and Grove-3 (2308-2315m) boreholes in the UK.
Measured average bulk densities show high variation in the dolomites (2.39-2.75 g/cm3) and a much more constant value in limestones (2.58-2.62 g/cm3). The variability of bulk density within the dolomites can be attributed to the high variability of total porosity (averages ranging 3.2-15.9%) which also affects their sonic velocity. In contrast, the limestone samples showed uniform porosities of less than 5%. All limestone samples showed small permeability (>10-20 m2) as expected from their low porosities. Dolomitised samples show relatively low permeability compared to their high total porosity, ranging from 10-17 m2 to 4*10-19 m2. These results suggest that permeability of dolomitised samples are instead controlled by their effective (1.7-2.9%) rather than total porosity (3.2-15.9%). These interpretations are consistent with microstructural observations showing a rather complex pore network, consisting of intercrystalline pores and vugs.
The results improve our understanding of the porosity-permeability relationship within the DCR. The porous vuggy nature of dolomitised lithology means that an EGS hydrothermal type reservoir concept is possible, where reservoir stimulation would increase connectivity of the pores and improve permeability in order to circulate reservoir fluids saturating the pore network.
