Hydraulic fracturing in granite at subcritical/brittle to supercritical/ductile conditions

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 (BDT) 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 in case of transition from elastic to plastic deformation of fracture, where permeability is strongly stress-dependent. For success of developing such new resources, a methodology to enhance or create permeability by artificial fracturing is required. This is because of concerns about lack or long-term viability in permeability. Moreover, clarifying the possibilities and characteristics of hydraulic fracturing and resultant permeability gain in the ductile crust has been a big issue, which possibly has an influence on the reliability of productive reservoirs in the ductile crust. In the present study, hydraulic fracturing experiments at temperatures of 200-450 °C have been conducted by injecting water into cylindrical granite samples having a borehole at an initial effective confining pressure of 40 MPa. Formation of fractures was observed at all temperatures, but fracturing characteristics varied depending on temperature, perhaps due to difference in water viscosities. At the lowest temperature, a small number of fractures propagated linearly from the borehole, and the borehole pressure required for the fracturing (breakdown pressure) was much larger than the initial stress level, similarly to those at room temperature. However, these fracturing characteristics disappeared with increasing temperature. Fracturing pattern shifted to formation of a larger number of shorter fractures over the entire body of the sample, and the breakdown pressure became much smaller, with increasing temperature. Permeability increased significantly by hydraulic fracturing at all temperatures, where the permeability gain was high enough to form a productive reservoir even at the highest temperature that exceeded both BDT and critical temperatures.