Relationship between rate of aperture reduction and contact pressure of fracture in granite under hydrothermal condition

A coupled experimental and numerical analysis was performed on artificial fractures of granite in order to evaluate a relationship between a rate of aperture reduction and a contact pressure under hydrothermal conditions. In the experimental work, distilled water was injected into the fractures of 150 °C at the different effective confining pressures of 16 and 25 MPa during several tens of hours. At both the effective confining pressures, the fracture permeabilities decreased non-monotonically with time. Since Si concentrations of effluents were far from saturation over the range of experiments, the predominant reaction within the fractures was dissolution of rock-forming minerals. In addition, the surface mapping did not show significant changes in the fracture surface roughness between before and after the experiment, suggesting that the permeability reductions originated mainly from the aperture reductions by pressure solution of contacting asperities. The rates of aperture reduction were therefore evaluated on the basis of the numerical modeling of the aperture structures during the experiment. The numerically determined changes in the aperture reduction and the contact pressure of the fractures provided the rates of aperture reduction of 1.0-6.4 μm/hour for the contact pressures of 35-54 MPa. Although the relationship between the aperture reduction and the contact pressure was far from one-to-one, the rate of aperture reduction seemed to grow exponentially with an increase of contact pressure.