Pressure solution and permeability evolution in fractured granite at elastic and plastic deformation regimes

A new and economically attractive type of geothermal resource consisting of supercritical water was recently discovered in the Krafla volcanic system, Iceland. Moreover, a recent study on permeability of fractured granite at temperatures exceeding the brittle-ductile transition temperature has suggested that potentially exploitable supercritical geothermal resources may form even in the ductile granitic crust. This is because high permeability may be created by tensile fracturing such as hydraulic fracturing in the ductile crust, and the created permeability may not be fully destroyed even in a case of transition from elastic to plastic deformation of fracture, where permeability is strongly stress-dependent. However, there is a concern with viability of permeability, since the permeability reduction with time is significant at high temperature/high effective confining stress environment due to pressure solution at bridging asperities within the fracture. Here, permeability reduction rate is expected to be affected by the elastic-plastic transition. We show results of hydrothermal flow-through experiments on 400 °C fractured granite samples at effective confining stresses of 18 MPa, 30 MPa (elastic conditions) and 50 MPa (plastic condition). Fracture permeability decreased at 30 MPa and 50 MPa, whereas permeability increased slightly at 18 MPa. Positive correlation between permeability reduction rate and stress level is observed, and such a correlation is expected theoretically. However, the increasing rate in the permeability reduction rate from 30 MPa to 50 MPa was larger than that from 18 MPa to 30 MPa. These results imply an acceleration of permeability reduction by the elastic-plastic transition, perhaps due to onset of strain solution driven mainly by plastic strain energy in addition to stress solution driven by stress level. The permeability reduction rate at the plastic condition seems to be only several times larger than that at the elastic condition, suggesting that permeability may not be destroyed so rapidly even in supercritical geothermal reservoirs in the ductile crust. In producing supercritical resources, it is desirable to minimize the influences of pressure solution by maintaining the fluid pressure as high as possible, in turn, the effective confining stress as low as possible.