Experimental study of fluid-rock interaction and permeability evolution in granite: applications to geothermal energy reservoirs in Cornwall

Fluid-rock interactions affect the evolution of permeability in geothermal reservoirs. Precipitation of secondary minerals within the fracture network can significantly decrease the permeability of the system and limit its lifetime. Yet dissolution of existing minerals can enhance permeability and ensure durable operations. In this study, we characterise the physico-chemical properties of the Carnmenellis formation, target unit of geothermal operations in Cornwall (UK), and perform reactive transport experiments to evaluate the response of the system to varying fluid chemistry.

Intact granite samples show porosity in the range of 1.2% to 0.6% and permeabilities ranging from 1x10^-20 to 1x10^-21 with effective pressures of 5 to 55 MPa. Samples were subject to thermal cracking as a proxy for hydraulic fracturing resulting in an isotropically distributed fracture system and an increase of surface area of the granite cores. The thermally cracked cores were inserted into a hydrostatic pressure vessel set at a constant temperature of 180 °C and a pressure of 40 bar.

Results show that basic undersaturated solutions (pH⁺ 10-10.5) promote small scale dissolution within the pre-existing fractures enhancing the permeability of the cores. Geomechanical results show pressure dependence of the permeability of reacted cores up to 30 MPa, and insensitivity at pressures higher than 30 MPa. We interpret the dissolved fractures as unmated at the microscale, allowing for the dissolved fluid paths to remain open and maintain permeability at higher confining pressures. Consequently, we suggest that creating chemical dissolution in the early stages of geothermal operations could generate permeable paths that are less sensitive to effective stress and will remain open at higher pressures. Conversely, supersaturated solutions (pH⁺ 9-9.5) promote the precipitation of clay minerals and decrease the permeability of the thermally cracked cores. However, the permeability of cores containing authigenic clay minerals remains higher than that of intact granite and higher than thermally cracked cores that have been subject to hydrostatic pressures of 50 MPa. These results highlight the importance of fluid chemistry in geothermal systems where the interplay between dissolution and precipitation define reservoir productivity.