[SCG64-P01] Mechanical and hydrological properties of fractured granite under supercritical conditions
Our purpose of this series of experiments is to clarify a relationship between the rock strength and the fracture density under hydrothermally supercritical conditions. We conducted triaxial deformation test on intact granite rock strength under high-temperature (250 – 750°C), high-pressure (104 MPa) condition at a constant load velocity (0.1 μm/sec) using a gas-rig at GSJ, AIST. We used Oshima granite, which has initially <0.2 % of the porosity, 4.26±0.42 km/s in Vp at dry condition (dry). All experimental products showed brittle feature having several oblique fracture surfaces, but both value of peak stress and amount of stress drop became smaller at higher temperature. Additionally, Young’s modulus decreases with increasing the temperature from 57.4 GPa at 250°C to 32.3 GPa at 750°C . At 400 °C, the stress drop accelerated the deformation with ~98 times faster velocity than that at load-point. In contrast, at 650 °C and 750 °C, the velocity during stress drop kept the same order of the load-point velocity. Therefore, the deformation feature may start to be of ductile when the temperature exceeds 650 °C.
Highly dense cracked granite specimens were formed by a rapid decompression test (RDT) using an autoclave settled at Tohoku University (Takagi et al., 2017GRC Trans.). We reduced fluid pressure within 2 seconds from vapor/supercritical state (10 – 48 MPa, 550 °C) to ambient pressure. X-ray CT scanning on the specimens after RDT showed that numerous microcracks were generated in them. The RDT imposed the porosity increasing towards 3.75 % and Vp decreasing towards 1.46±0.22 km/s (dry). We conducted the permeability measurement on fractured granite after RDT with the porosity of 1.06% under a combined condition of 400 °C in temperature and 104 MPa in the confining pressure. Permeability measured by pore pressure oscillation method decreases from 4.3×10-18 m2 at pore fluid pressure (Pp) of 70 MPa to 9.0×10-19 m2 at Pp of 38 MPa. In the meeting, we will present results of triaxial deformation test on such cracked granites and show the relationship between strength and crack density under supercritical conditions.