Dependence of the Rate of Granite Dissolution on Temperature and Fluid Velocity under Simulated Geothermal Reservoir Environments

In a Hot Dry Rock (HDR) geothermal reservoir, water/rock interactions such as dissolution and precipitation of rocks, may significantly influence the long-term reservoir performance of the artificial water circulation system. In order to predict the long-term reservoir performance of the HDR system, it is first essential to understand the kinetics of the water/rock interactions at various fluid velocities, in addition to their temperature dependence. In this work, the dissolution kinetics of a granite, in pressurized high temperature (250~350°C) water at 0.05×10^-3-2.0×10^-3 m/s, has been investigated experimentally using an once-through type tubular flow reactor in which hot water flows through the passage between the granite samples and the inner wall of the reactor. It has been shown that the velocity of hot water significantly affects the dissolution rate of the granite as well as temperature. The dissolution rate of the granite is shown to increase with temperature and fluid velocity. It is suggested that the apparent dissolution rate constant of the granite is affected by both the surface reaction and diffusion in the boundary layer, within the range of temperature and fluid velocity used for this study. The apparent dissolution rate constant of the granite has been determined as a function of temperature and Reynolds number. Based on analysis of the apparent dissolution rate constant, the rate constant obtained from this study, can also be regarded as the mass transfer coefficient.