| Title | Permeability Evolution of Fractured Granite Due toPressure Solution and Free-face Dissolution in Superhot Geothermal Environments |
|---|---|
| Authors | Noriaki WATANABE, Kohei SAITO, Atsushi OKAMOTO, Noriyoshi TSUCHIYA, Takeshi KOMAI, Takuya ISHIBASHI, Hanae SAISHU, Norihiro WATANABE |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | permeability, fractured granite, pressure solution, free-face dissolutin, superhot geothermal environment |
| Abstract | Superhot geothermal environments in high temperature ductile granitic crusts of ca. 400-500°C and 2-4 km depths 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 (i.e., geothermal reservoirs) to effectively access the geothermal energy frontier with the concept of enhanced geothermal systems (EGS). However, there is a concern about the possibility to stabilize/enhance the permeability created by hydraulic fracturing because pressure solution may reduce permeability after pore water pressure/effective confining stress returns to the original lower/higher level. On the other hand, permeability may be enhanced by free-face dissolution even if pressure solution occurs. However, the rates of permeability reduction by pressure solution and permeability enhancement by free-face dissolution have not been fully understand for the superhot geothermal environments, and the possibility to stabilize/enhance permeability has therefore been unclear. We present the results of a set of hydrothermal flow-through experiments on 400°C fractured granite samples at various effective confining stresses and clarify influences of stress level and plasticity of the fracture on the rate of permeability change by pressure solution. Additionally, we present results of another set of hydrothermal flow-through experiments on 400°C fractured granite samples at various pore water pressures and clarify influences of pore water pressure and corresponding mineral solubility on the rate of permeability change by free-face dissolution. Finally, based on the experimental results, the rates of permeability changes by pressure solution and free-face dissolution are modeled respectively as functions of effective confining stress and quartz solubility. The present study suggests that permeability may be stabilized or enhanced in superhot EGS even under the presence of pressure solution by keeping a difference between a concentration of pore water and solubility for quartz higher than a stress-dependent permeability stabilization criterion. |