Effects of Water/Rock Interaction upon Crack-like Reservoir

Abstract

In order to evaluate the effects of water/rock interaction upon a long term reservoir performance in HDR, it is essential to understand the mechanisms and kinetics of dissolution and precipitation of rocks in high temperature water. In this work, these dissolution and scaling kinetics of granite in pressurized high temperature water at 200°C-300°C is investigated experimentally by use of a flow system with once-through type autoclave. Special emphasis is focused upon the effects of temperature upon dissolution rate of minerals composing granite, say quartz, K-feldspar and plagioclase in relation to leaching behavior of elements. Results show that, at a temperature range of 200°C to 300°C, quartz is most stable and shows the lowest dissolution rate. However, at temperature range above 300°C, quartz has the highest dissolution rate and those of K-feldspar and plagioclase show a negative temperature dependence above 300°C. SEM and EMPA analysis reveals that this trend results from the formation of fibrous Xonotlite (Ca₆ Si₆O17(OH)₂) on the surface of K-feldspar and plagioclase. This Xonotlite can be expected as a diffusion barrier and result in rapid dicrease in dissolution rate. On the other hand, at the lower temperature range with rapid temperature down by cooling, amorphous SiO₂ is found in a test tube as has been found in actual geothermal plant pipings. Based upon above evidences, scaling behavior can be classified into two categories. One is scaling at high temperature, so called high temperature scaling and the other is one at low temperature or with temperature dicrease. They can be considered as a kind of scaling observed at actual geothermal plant. The reproduction of these scale would be expected to give more effective countermeasure against the scaling problem in existing or future geothermal plant.