Evolution of fracture permeability through reactive flow at elevated temperatures

Flow-through experiments are conducted on a natural fracture in novaculite at a constant effective stress of 1.38 MPa and at staged temperatures of 20, 40, 80, and 120°C. Fluid and mineral effluxes are measured throughout the 3150-hr experiment, together with post-experiment imaging by X-ray CT and destructive fracture-casting by Wood's metal impregnation. These measurements are used to constrain the evolution of dissolution-driven changes in fracture structure, and related permeability under hydrothermal conditions. At 20°C the fracture aperture monotonically decreases from an initial aperture of 18.5 μm to 7.5 μm during the first 1494 hrs. This reduction is attributed to removal of mineral mass from bridging asperities. Subsequently, fracture aperture increases to 13 μm and is interpreted as a switching of dominant dissolution processes to free-face etching of the fracture void surfaces. The resulting rate of gaping increases with an increase in temperature. Post-experiment imaging by X-ray CT and by using the fracture cast independently constrains the resulting architecture of the evolved fracture porosity. No localized flow channel is apparent, despite the evolving hydraulic response which is suggestive of an evolving dissolution channel.