Fracture permeability evolution as a result of long-term geochemical granite alteration in geothermal systems: A combined experiment and modelling approach

Geothermal systems in crystalline basement rocks require fractures and faults to allow economic heat production. Sufficient permeability of these flow paths is vital and affects the lifetime of such systems. As fluids are produced and reinjected, the resulting flow, fluid mixing as well as related pressure and temperature changes affect the geochemical equilibria between fluids and host rock. Disequilibria of fluids and rock then potentially drive geochemical alteration (e.g., dissolution, precipitation, illitisation). Such changes can affect the hydraulic properties of the major flow paths of the fault zones. Cornwall in SW England hosts several granitic plutons that are subject of current geothermal projects (United Downs Deep Geothermal Power and Eden Projects). These projects target fault zones in crystalline rock as pathways for fluid flow.

To study the effects of geochemical alterations on fracture permeability in granites, we conducted a series of long-term reactive transport experiments in our unique flow-through reactor setup. Carnmenellis granite samples from central Cornwall have been collected of which small, artificially fractured plugs (15 mm length, 10 mm width) and powders (< 125 μm grain size) were prepared. We injected water with different fluid composition, representing brines encountered around geothermal systems, into the fractured granite plugs and pulverized gouges at 80 °C and 20 MPa confining pressure. The development of sample permeability and effluent composition are analysed. CT-scans are used to analyse changes in fracture and gouge structure. To complement the experiments, we model our system with GeoChemFoam [Maes and Menke, 2021], an OpenFOAM-based reactive-transport modelling code.