Microstructural controls on geothermal reservoir host rock responses to elevated pressures and temperatures

Rock microstructure controls the nature of geothermal fluid flow within reservoirs hosted in active volcanic environments and as such it is necessary to constrain the microstructural evolution of rocks at elevated pressure and temperature conditions. Despite Andean geothermal systems hosting numerous high enthalpy geothermal plays, there remains a paucity of experimentally derived seismic velocity and porosity data at crustal relevant in-situ conditions. Here, we provide novel constraints on the evolution of rock physical properties of five main representative lithologies of the Nevados de Chillán Geothermal System under dry conditions with confining pressures up to 150 MPa and following heat treatment up to 600°C. The variability of P-wave velocity changes with elevated confinement reveals the presence of both different densities of pre-existing cracks and different distributions of pore aspect ratios within the tested lithologies. Two target units, crystalline granodiorite and diorite, were further subjected to heat treatment to recreate potential temperature conditions within and around the geothermal reservoir. The heat treatment generated new populations of low aspect ratio cracks in both rocks, but this pore space was apparently more difficult to close in the diorite than in the granodiorite when resubjected to confinement. This difference is explained in a conceptual model whereby a combination of crack realignment and mineral alteration in the diorite allows pore space to remain open and potentially connected at greater levels of confining pressure, at greater depths in the geothermal reservoir. These results have implications for the propensity for fluid flow in crystalline geothermal reservoir host rocks.