Study on evolution of granite permeability under stress after high temperature exposure

With the vigorous development of nuclear waste disposal, underground coal-bed methane, the exploitation and utilization of geothermal resources, the evolution of physical and mechanical properties and permeability characteristics of rocks after high temperatures has attracted much attention.In order to test the permeability characteristics of granite samples after different high temperatures, the laboratory test method was used to set the confining pressure of 10～30 MPa, and the inlet hydraulic pressure is lower than the corresponding confining pressure interval and gradually increases.The GWD-02A high-temperature furnace is used to heat the processed granite samples at high temperature at 200, 300, 400, 500, 600 and 800 ℃,respectively. A series of penetration tests were carried out on the granite samples after high temperature using the fully automatic rock permeability test system. By measuring the diameter, height and mass of the granite samples after different temperatures, the density of the granite at different temperatures was obtained. The PDS-SW acoustic wave detector was used to test the longitudinal wave velocity of granite after different high temperatures, and finally the change of the wave velocity and density with temperature was obtained. On this basis, the influence of high temperature on granite density and wave velocity was analyzed, and the relationship between volume flow velocity and pressure gradient of granite sample after high temperature was obtained. Finally, the relationship between permeability coefficient and water conductivity coefficient and confining pressure and temperature was obtained. The results show that with the increase of temperature, micro-cracks in the granite gradually develop, and the longitudinal wave velocity and density gradually decrease，the relationship between volume flow rate and pressure gradient demonstrates to be linear and fits very well with the Darcy’s law. As the temperature increases, the equivalent permeability coefficient shows a gradual increasing trend, and the higher the temperature, the more significant the increase is. The test results can fit well with exponential function. For the same kind of samples after high temperature, as the confining pressure increases, the water conductivity shows a decreasing trend, and the rate of its decrease diminishes gradually.