Abstract
A prediction of host rock permeability in the deep seated geothermal reservoir is essential for the assessment of a potential geothermal energy extraction system. In order to test the permeability of granite under high temperature and high pressure conditions, experiments were undertaken using thick-walled cylindrical specimen of 45 mm outer diameter and 5 mm inner diameter. The permeability of the sample specimen was estimated from flow rate, and the pressure difference between borehole pressure (internal pressure) and confining pressure, based on Darcy's law. Permeability tests were conducted at temperatures of up to 600 ℃ and confining pressures of up to 100 MPa. Experimental results show that the permeability of granite increased with increasing temperature, particularly above 350 ℃. Examination of microcracks, by optical microscopy, showed that crack density in granite, after experiments at 600 ℃, and confining pressures of both 25 MPa and 100 MPa, was larger than at a confining pressure of 20 MPa. This indicates that water-rock interaction processes at supercritical conditions directly affect micro-cracking of granite. In addition, it was noted that hydrothermal cracking occurs at a temperature above 400 ℃ and a pore pressure above 25 MPa, This result indicates that it may be possible to increase granite permeability and to create a porous type reservoir in a supercritical rock mass, by hydrothermal cracking.
