Pore characteristics and nonlinear flow behaviors of granite exposed to high temperature

This paper experimentally investigates the role of high temperature treatment on pore characteristics, permeability, critical Reynolds number, and flow nonlinearity of granite exposed to temperatures of 100~800 °C. First, variations in pore characteristics, porosity, and pore fractal dimension of granite were analyzed using the mercury intrusion method. Then, a number of water flow tests were conducted on the samples with confining pressures varying from 10 to 30 MPa. The results show that (i) the cumulative pore volume of granite shows an increase with temperatures, which first increases gradually in the range of 100~400 °C, and increases significantly in the range of 400~800 °C. The pore structure becomes developed gradually; (ii) as the temperature increases from 100 to 800 °C, the pore fractal dimension decreases by 12.09%, indicating gradually weaken complexity of pore distributions. An exponential function is used to evaluate the increasing porosity of rock based on the temperature levels; (iii) the relations between volume flow rate and pressure gradient can be well described using the Forchheimer’s law. Both linear and nonlinear coefficients increase with the confining pressure. The equivalent permeability experiences an exponential increase with the temperature due to thermally induced defects; (iv) by defining a critical nonlinear effect factor of 10%, the critical pressure gradient was calculated, which increases with the confining pressure. The critical Reynolds number shows a decrease of 25.49%~67.74% with confining pressure due to more isolated contact areas and tortuous flow paths but increases by a factor of 5.97~17.24 with the temperature. A three-order polynomial function is used to analyze the decrease in transmissivity versus the pressure gradient. The Forchheimer’s law results are evaluated by plotting the normalized transmissivity against the pressure gradient, and an increase in the temperature generally shifts the fitted curves downward.