Effect of permeants on fracture permeability in granite under hydrothermal conditions

Abstract

Flow-through experiments were conducted with three permeants to determine the effect of pH, temperature, and cation concentrations on changes in permeability. Granite with a single fracture was used for each sample. Changes in the permeant concentrations due to pressure dissolution, free-face dissolution, and precipitation were identified by measuring the element concentrations before and after the experiments. In addition, the mineral transformation was analyzed by SEM-EDX. The results of the flow-through experiments showed a reduction in permeability in almost all the samples. This decrease in permeability may have been caused by the interaction between pressure dissolution and free-face dissolution, which occurred in the high pH water experiment, or between pressure dissolution and precipitation, which occurred in the saturated mineral water and simulated seawater experiments. When pressure dissolution and free-face dissolution occurred in the samples, the pH and temperature were seen to greatly affect the decrease in permeability, namely, the permeability decreased significantly with increasing pH and temperature. This remarkable decrease in permeability could have taken place because the dissolution rate constant of the mineral increased with the increasing pH and temperature. Moreover, when pressure dissolution and precipitation occurred in the samples, the cation concentrations and temperature were seen to greatly affect the changes in permeability, namely, the permeability decreased significantly with increasing cation concentrations and decreasing temperature.