Permeability Enhancement by Selective Mineral Dissolution of Rocks in Geothermal Environments

Enhanced geothermal system (EGS) technologies have been investigated and developed to generate electric power using high-temperature rocks which have however relatively low initial permeabilities. One of the important techniques in EGS is therefore stimulation to enhance the rock permeability, where the traditional methods are hydraulic and chemical stimulations. However, hydraulic stimulation gives rise to a concern of induced seismicity because it may induce seismic shear slip of a sizable natural fracture. On the other hand, chemical stimulation, which uses strong mineral acids such as hydrochloric acid, has challenges regarding achieving mineral dissolution over long distances and highly variable fluid chemistry. Therefore, we have proposed and investigated a new chemical stimulation method to enhance the rock permeability using a chelating agent that dramatically enhances the dissolution rates of specific minerals to create voids that are sustained under crustal stress without the challenges in the traditional methods. In the present study, we have conducted chelating agent flooding experiments on various types of rocks, such as granite and andesite, at 200℃ under confining stress using aqueous solutions of environmentally friendly chelating agents (GLDA and HEDTA) at pH 4. As a result, it has been observed that the rock permeability increases quickly and significantly and the expected selective mineral dissolution creates voids within the rock. The present study has demonstrated the possibility of the new chemical stimulation using environmentally friendly chelating agents.