10:00 AM - 10:15 AM
[SCG58-05] New chemical stimulation for geothermal reservoirs: Selective mineral dissolution by environmentally friendly chelaging agents
★Invited Papers
Improving geothermal systems through hydraulic stimulation to create highly permeable fractured rocks can induce seismicity. Therefore, the technique must be applied at a moderate intensity; this has led to concerns of insufficient permeability enhancement. Adding chemical stimulation can mitigate these issues, but traditional methods using strong mineral acids have challenges in terms of achieving mineral dissolution over long distances and highly variable fluid chemistry. We proposed a new chemical stimulation method for improving the permeability of rock fractures using a chelating agent that substantially enhances the dissolution rate of specific minerals to create voids that are sustained under crustal stress without the challenges associated with the traditional methods (Watanabe et al., 2021, Scientific Reports). Applying this agent to fractured granite samples under confining stress at 200 °C in conjunction with 20 wt% aqueous solutions of sodium salts of environmentally friendly chelating agents (HEDTA and readily biodegradable GLDA) at pH 4 was assessed. A significant permeability enhancement of up to approximately sixfold was observed within 2 h, primarily due to the formation of voids based on the selective dissolution of biotite. These results demonstrate a new approach for chemical stimulation.
A subsequent study (Takahashi et al., 2023, Geothermics) investigated the process of such permeability enhancement and its optimum pH. In this study, we conducted stimulation experiments on fractured granite at 200 °C under confining stress using a 20 wt% aqueous solution of the sodium salt of 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.
In another study (Salala et al., 2023, Geothermics), we also examined the effectiveness of the new chemical stimulation for volcanic rocks through flooding experiments conducted with a pH 4 GLDA solution on fractured dacitic, andesitic, and basaltic rocks from geothermal fields in El Salvador at 200 °C under confining pressure. Results showed substantial permeability enhancement of up to 4.3-fold in 2 h, where the magnitude of enhancement depended mainly on the initial proportion of Fe-rich phenocrysts, which dissolved to form voids. Therefore, this 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.
A subsequent study (Takahashi et al., 2023, Geothermics) investigated the process of such permeability enhancement and its optimum pH. In this study, we conducted stimulation experiments on fractured granite at 200 °C under confining stress using a 20 wt% aqueous solution of the sodium salt of 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.
In another study (Salala et al., 2023, Geothermics), we also examined the effectiveness of the new chemical stimulation for volcanic rocks through flooding experiments conducted with a pH 4 GLDA solution on fractured dacitic, andesitic, and basaltic rocks from geothermal fields in El Salvador at 200 °C under confining pressure. Results showed substantial permeability enhancement of up to 4.3-fold in 2 h, where the magnitude of enhancement depended mainly on the initial proportion of Fe-rich phenocrysts, which dissolved to form voids. Therefore, this 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.