Hydraulic Fracturing and Permeability Enhancement in Granite at Supercritical Temperatures

Supercritical or 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). Previous studies on hydraulic fracturing of granite from near-critical to supercritical temperatures under conventional and true triaxial stress state suggested that creation of fractures densely distributed in a large volume of rock at relatively low water pressures may be possible due to stimulation of preexisting microfractures by low-viscosity water at such high temperatures. Moreover, the previous studies showed that fractured granite samples had high permeabilities of 10 m at room temperature and atmospheric pressure. However, the maximum permeability created by the fracturing, and the criterion and process for creating such complex fracture pattern have not been clear yet. Thus, the present study has conducted additional hydraulic fracturing experiments on granite samples at 450C under conventional triaxial stress state. As a result, it has been found that much higher permeability of >10 m (measured at room temperature and atmospheric pressure) may be possible if water pressure does not become smaller than the pressure required for fracturing, immediately after creation of some fractures that provide flow paths to outside of the sample which usually occurs in laboratory experiments. Additionally, all results provided by the present and previous studies have suggested that Griffith (or modified Griffith) criterion that assumes initiation of fracture from preexisting microfractures (i.e., Griffith cracks) is largely appropriate for the hydraulic fracturing of granite with low-viscosity water. Based on this, it is inferred that the complex fracture pattern is caused by initiation of fractures from preexisting microfractures distributed within granite by infiltrated low-viscosity water and interaction of propagating fractures, and perhaps initiation and propagation of fractures at various directions due to some variation of tensile strength within the rock.