Experimental investigation of nonlinear flow for single rock fractures

The hydraulic behavior of rocks is significantly controlled by fracture geometries, such as aperture, contact areas, roughness, interconnections, and so on. However, these characteristics are strongly influenced by the confining pressure conditions. This paper experimentally investigated the nonlinear flow behaviors through single rock fractures subjected to a wide range confining pressures. A series of transient pulse tests were conducted on three fractured limestone samples by MTS815 Rock Mechanics Test System. The experimental results show that the pulse decay curves diverge from the classical exponential model due to nonlinearity, thus an empirical relationship between differential pressure and time is developed with consideration of nonlinearity. Subsequently, the nonlinear flow coefficient and permeability were calculated based on the Forchheimer equation. The calculated results show that nonlinear flow coefficient increases with confining pressure, and rougher fracture surface helps to stronger nonlinearity. As the confining pressure increases, the permeability first experiences a dramatic decrease and then behaviors a much slow-down drop. The critical confining pressure for climb-rush shifts increases with the fracture rouhghness.