Stabilizing and enhancing permeability for sustainable and profitable energy extraction from superhot geothermal environments

Superhot geothermal environments in granitic crusts of ca. 400–500 °C and depths of 2–4 km are recognized as a frontier of geothermal energy. In developing such environments, hydraulic fracturing is a promising way to create or recreate permeable fracture networks to effectively access the energy through enhanced geothermal systems (EGS). However, there is a concern about the possibility of stabilizing or enhancing the permeability created by hydraulic fracturing, required for sustainable and profitable energy production, because pressure solution of the fracture surfaces may reduce permeability. On the other hand, permeability may be enhanced by free-face dissolution of the fracture surfaces even if pressure solution occurs. However, the rates of permeability reduction and enhancement are not fully understood, and the possibility of stabilizing/enhancing permeability is therefore unclear. We have conducted hydrothermal flow-through experiments on 400 °C fractured granite samples to clarify the influences of stress level and plasticity of the fracture on the rate of permeability reduction by pressure solution and the influences of pore water pressure and corresponding mineral solubility on the rate of permeability enhancement by free-face dissolution. Results suggest that permeability may be either stabilized or enhanced in superhot EGS even when pressure solution can occur by keeping the difference between the concentration of the pore water and the solubility of quartz higher than the stress-dependent permeability stabilization criterion.