[SCG64-P05] Analysis of hydro fracturing behaviors of supercritical geothermal rock mass under true triaxial stress by elastic wave measurement
The greatest, intermediate and least principal stresses can be set independently in the resin melt type true triaxial testing equipment, and the confining pressure is controlled by melted resin. In this experiments, we used cube-shaped Inada granite (taken from Ibaraki prefecture) with a borehole for hydraulic stimulation. We conducted hydraulic fracturing by raising water pressure in the borehole (BP) after setting the temperature inside the equipment to the experimental conditions and injecting water into the borehole with load applied to the specimen in the pressure vessel. Exuded water to each surface after fracture propagate to each surface by fracturing was recorded by using the hydraulic gauge. Also, elastic wave velocity before and after fracturing was measured by using ultrasonic probes attached to the pistons to apply load to the specimen.
BP turned to decrease at the time when the water pressure increased to some extent during raising BP, which was thought to be due to fracture propagation from the vicinity of the borehole by reaching fracturing water pressure. As BP decreased, the value of the hydraulic gauge increased on each side of the specimen. In addition, as a result of measurement of elastic wave velocity before and after hydraulic fracture, it is considered that cracks are propagating in all directions, not only in the specific direction. Elastic wave velocity was declined in each direction.
Furthermore, it was confirmed that the elastic wave velocity decreased by about 40 to 50% in all directions, so that regardless of the direction of the greatest, intermediate and least principal stress, fractures due to hydraulic fracturing were considered to progress uniformly in all directions. However, extent of fracture propagation was enhanced in the parts where there were relatively high pre-existing fractures densities compared to the other parts in the specimen. Based on the experimental results, fracture networks were thought to progress through existing microfractures.
In addition, the permeability increased to the order of 10-15 m2 from 10-18 m2 of the intact specimen and it was considered that the permeability in the Earth’s crust can be sufficiently increased by hydraulic fracturing.
Watanabe, N. et al. Hydraulic fracturing and permeability enhancement in granite from subcritical/brittle to supercritical/ductile conditions. Geophys. Res. Let. 44, 5468-5475(2017).