Experimental Study of Nonlinear Flow Behaviors Through Fractured Rock Samples After High-Temperature Exposure

Abstract

This study focuses on the transport properties and permeability evolution characteristics of fluid flow through thermally treated rock samples containing single fractures. First, splitting fractures were generated in cylindrical granite samples after high-temperature exposure (25–800 °C). Then a series of water flow tests through both intact and fractured samples were conducted in a triaxial cell under different confining pressures (10–30 MPa) and varying inlet hydraulic pressures (0.4–6 MPa). The results show that as the temperature increases from 25 to 800 °C, the standard deviations of the 3D spatial distribution parameters, including the asperity height, slope angle, and aspect direction of the fracture surface mesh element planes, all increase, indicating gradually increasing fracture surface roughness. The relationships between the pressure gradient and flow rate of intact samples, fractured rock samples, and the fractures themselves can all be well fitted using the Forchheimer’s law. Both linear and nonlinear coefficients in the Forchheimer’s law increase with increasing confining pressure. An exponential function is used to evaluate the equivalent permeability of intact samples based on temperature levels. The permeability undergoes an increasing trend as the temperature increases due to thermally induced defects, but undergoes a decreasing trend as the confining pressure increases due to defect closure. Two representative types of flow characteristics through the fractured rock samples, dominated by either the rock matrix or fracture flow, are identified. In the temperature range of 25–800 °C, the critical Reynolds number of the fractures declines, which first remains generally constant for temperatures of 25–400 °C and then experiences a dramatic decrease for temperatures of 400–800 °C. The nonlinear coefficient bf in Forchheimer’s law versus the hydraulic aperture eh curves displays a decreasing trend following a power-law relationship. The Forchheimer’s law results are evaluated by plotting the normalized transmissivity against the pressure gradient. An increase in the confining pressure shifts the fitted curves downward. As the temperature increases, the contribution of the matrix to the overall discharge capacity of the fractured rock samples gradually enhances, while that for the fractures weakens. The reduction extent in permeability of the rough-walled fractures is more remarkable than that of the matrix under an applied confining pressure.