Macroscopic and mesoscopic characteristics of liquid-gas two-phase flow in a single fracture

The liquid-gas two-phase flow in rock fractures is of great significance for oil and gas field development, groundwater pollution control and underground nuclear waste disposal. In this study, liquid-gas two-phase flow experiments under different liquid and gas flow rates were carried out on a self-developed transparent-plate fracture apparatus with liquid-gas two-phase flow. Typical liquid-gas two-phase flow patterns, i.e., bubble flow, fingering flow and annular flow, were obtained. The relationship between the relative permeability and phase saturation indicates obvious inter-phase interference between the liquid and gas phases. The mesoscopic flow characteristics of bubbles under different flow patterns are obtained. With the change of flow pattern, the phenomena of bubble merging and dispersion occur, resulting in area fluctuation, and the actual flow velocity of the gas phase in the fracture is greater than the "nominal" flow velocity. Based on the generalized Darcy's Law, a surface tension model for liquid-gas two-phase flow in a smooth parallel-plate fracture was proposed, which can well reflect the effect of surface tension between the liquid and gas and the inter-phase interference. Based on the experimental data, it is verified that the surface tension model has a better description of the relative permeability variation compared with other classical two-phase flow models. Finally, the connection between mesoscopic and macroscopic flow characteristics is analyzed and discussed. Furthermore, surface tension correction factors are proposed to represent the effect of surface tension on the inter-phase interference in the fracture.