Process and optimum pH for permeability enhancement of fractured granite through selective mineral dissolution by chelating agent flooding

Abstract

Chemical stimulation of geothermal reservoirs via selective mineral dissolution with eco-friendly chelating agents has been recently proposed as a novel method complementary to hydraulic stimulation. In our previous study, we demonstrated rapid and significant permeability enhancement accompanied by the creation of voids due to the selective dissolution of biotite in fractured granite at 200 °C under confining stress. However, the process of permeability enhancement and its optimum pH have not been investigated yet. In this study, we present the results of the stimulation experiments conducted on fractured granite at 200 °C under confining stress using a 20 wt% aqueous solution of the sodium salt of the chelating agent, N, N-bis(carboxymethyl)-l-glutamic acid (GLDA), at pH 1–8. The permeability enhancement was the highest at pH 4, at which preferential flow paths connecting the voids caused by biotite dissolution were observed. With decreasing and increasing pH, silica precipitation and suppressed selective dissolution of biotite, respectively, became more significant, accompanied by a decrease in permeability enhancement. Our findings suggested that the optimum pH was a combination of two pH values suitable for the creation of stress-resistant preferential flow paths by selective mineral dissolution of biotite and for the accelerated dissolution of quartz, which could not be achieved by the chelating agent. The successive use of first pH 4 and then pH 8 resulted in a more than 2-fold permeability enhancement in 4 h. Therefore, our study has strengthened the possibility and effectiveness of the new chemical stimulation method to facilitate the extensive use of enhanced geothermal systems worldwide in future.