In coalbed methane recovery, oil exploitation and geothermal resources utilization, the exploitation is always accompanied with a two-phase flow process in the fracture network. To quantitatively describe the two-phase flow hydraulic properties in intersecting-fractures, in this study, a gas-water two-phase flow experiment was conducted in a smooth 3D intersecting fracture model. The results show that: at a certain water flow rate, the two-phase pressure drop increases nonlinearly with respect to the gas flow rate, which is different from previous results of the two-phase flow in rough single fractures. It is believed that this nonlinearity is induced by the strong inertial effect of water in the intersecting fracture. The Martinelli-Lockhart model is not only effective for describing the two-phase flow in single fractures, but also for the two-phase flow in intersecting fracture. Since the Martinelli-Lockhart model considers the inertial forces, which is quite significant in the intersecting fracture, the good fitting results are obtained. This study provides basis for further investigation on the two-phase flow characteristics in the fracture network.
Two-phase flow in fractures is a key issue in many engineering applications such as gas-oil flow in the oil exploitation, the gas-water flow in the coalbed methane recovery and the steam-water flow in the geothermal energy development. Consequently, the two-phase flow in fractures are required to be better understood. Existing studies on two-phase flow include the displacement mechanisms and simultaneous flow of two phases in the fractures or porous media. For the simultaneous two-phase flow, the flow characteristics in a single fracture are well investigated by many researchers. Different from the single-phase flow, the two-phase flow in the fracture is not only influenced by the coherent fracture properties like the fracture roughness, but the gas-water interactions (Corey, 1954; Dana, 1999; Dong, 2008). However, the two-phase flow characteristics in fracture networks or the intersecting fracture still remain to be deeply investigated. Due to the inertial effect and the turbulence induced energy loss, fluid flow in the intersecting fracture shows quite different properties to that in the single fracture. At a fracture intersection, the inertial effect usually induces backflow regions, and correspondingly induces nonlinearity of flow. Even in laminar flow state, the flow's nonlinearity exists due to the influence of the fracture intersection (Kosakowski and Berkowitz, 1999). On the other hand, available studies on two-phase flow in intersections mainly focus on the separation of two phases, which is induced by the inertia difference of two phases (Seeger et al, 1986; Li et al, 2017); but less studies are conducted on the hydraulic properties of two-phase flow in intersecting fractures. This is because two-phase flow is turbulent in most cases, and the mechanism of the flow nonlinearity induced by the intersection, such as the backflow, cannot be directly studied by Navier-Stokes equation (NS equation). About the two-phase flow in the fracture network, though the NS equation and VOF method can simulate the two-phase flow in the fracture network (Zhang et al, 2018), the NS equation cannot account for the nonlinearity and additional energy loss induced by the interactions between two phases in the turbulent state, especially at the fracture intersections. Consequently, the influence mechanisms of fracture intersections on the hydraulic characteristics of two-phase flow are still not well understood. As a basis for studying the gas-water flow in the fracture networks, an experiment was conducted in a 3D smooth intersecting fracture model to obtain the pressure-drop characteristics. This experiment provides a basis for further studies to understand the flow behavior in the fracture network.