Experimental study on nonlinear seepage characteristics of porous–fracture composite sandstone

Accurate characterization of non-Darcy flow in pore - fractured rock mass is essential for geothermal mining, hydraulic fracturing, and tunnel engineering. Currently, most studies focus on the characteristics of fractures or the confining pressure effect of porous media, while the influence of porous-fracture composite media parameters has not been clarified. In this paper, the non-Darcy flow characteristics of porous-fracture composite sandstone (PFCS) with different water injection hole diameters under different confining pressures and pore water pressures are systematically studied by triaxial seepage test. The experimental results show that the seepage characteristics of PFCS are significantly affected by the diameter of the water injection hole and the confining pressure. The Forchheimer coefficient b changes positively and negatively when the pore water pressure exceeds the critical threshold related to the confining pressure, which indicates that the flow mechanism changes from the inertia-dominated non-Darcy flow to the hydraulic-mechanical coupling-induced fracture expansion flow. Based on the Forchheimer number F 0 , the pressure gradient ratio and the discharge ratio are defined, and the quantitative model of seepage characteristics in PFCS is established. The water injection hole significantly changes the seepage path of the fluid in the pore-fracture composite medium and weakens the influence of confining pressure on the seepage characteristics. When the diameter of the water injection hole is 4 mm, the seepage path is more uniform and the energy dissipation is smaller, and the mechanical-geometric coupling balance is realized. Therefore, a 4-mm- diameter water-injection hole is recommended as the preferred geometric parameter for laboratory-scale pore-fracture medium seepage research. • The competition mechanism of “inertia effect-crack expansion” in pore-fractured composite rock mass is revealed. • The criterion of fluid flow pattern in pore-fractured composite rock mass which can be popularized on site is proposed. • The main control effect of water injection hole diameter on nonlinear seepage is clarified. • The remodeling mechanism of water injection hole-confining pressure-pore pressure coupling on seepage path is clarified.