Chemical Stimulation for EGS: The Use of Chelating Agents at High pH for the Simultaneous Dissolution of Calcium Carbonate, Silicas, and Silicates

Abstract

Dissolution of silica, silicate and calcite minerals in the presence of a chelating agent (NTA) at a high pH has been successfully demonstrated in laboratory experiments using a high-temperature flow reactor. The mineral dissolution and associated porosity enhancement in the experiments has been reproduced by reactive transport simulation using TOUGHREACT. The chemical stimulation method has been applied by numerical modeling to a field geothermal injection well system, to investigate its effectiveness. Parameters applicable to the quartz monzodiorite unit at the Enhanced Geothermal Systems (EGS) site at Desert Peak (Nevada) were used. Results indicate that the injection of a high pH chelating solution results in dissolution of both calcite and plagioclase minerals, while avoids precipitation of calcite at high temperature conditions. Consequently reservoir porosity and permeability can be enhanced especially near the injection well. Injection at lower temperature results in a porosity increase that is smaller close to the injection point, but extends to larger radial distance. Slower kinetic rate results in less aggressive mineral dissolution close to injection point and larger extent along the flow path, which is favorable for chemical stimulation.