Analysis of permeability reduction during flow of heated, aqueous fluid through westerly granite

The permeability of intact granite is known to be controlled by low aspect ratio cracks. A substantial reduction of permeability occurred in experiments in which a heated, aqueous fluid was passed through Westerly Granite down a temperature gradient. Scanning electron microscopy (SEM) of the sample from this experiment indicated that deposition of material in small cracks (1 μm wide) was the probable cause of the permeability reduction. Energy dispersive x-ray analysis indicated that much of the deposited material was very Si-rich, suggesting that these deposits were silica. The fluid discharged from the sample was supersaturated with respect to quartz but undersaturated with respect to other silica polymorphs, which implies that the silica deposits were quartz. Rate equations for dissolution and precipitation of quartz have been integrated to calculate the concentration of silica in the pore fluid moving through the sample. These calculations also predict the concentration of silica in the discharged fluid for comparison with measured values