Flow structure transition and identification of two-phase fluid flow through rough rock fractures

To investigate the two-phase fluid flow structure transition, the two-phase fluid flow through rough rock fractures with different flow velocities, fracture surface wettability and fracture roughness was modeled with the level set method, and an advanced method for identifying the flow structure of two-phase fluids was proposed. Multiple flow structures of two-phase fluids were observed with the change of flow velocity and wall wettability, but only mist flow and long-bubble flow were observed in hydrophobic fracture. Increasing fracture contact angle gradually destabilized the flow structure of two-phase fluids, but such influence on flow structure stability weakened rapidly with the increase in fracture aperture. The fracture roughness plays a hindering role in the formation of continual gas flow channel. With the increase in fracture roughness, the proportion of annular flow decreased, making the proportion of bubble flow increase significantly. A flow structure identification method by comparing the two-phase fluid Reynolds number with the flow structure conversion boundary was proposed. To further improve the generalizability of the flow structure prediction method, the Hurst coefficient was introduced to evaluate the fracture surface roughness. Further exemplifying study shows that the inverse of the Hurst coefficient shows a linear relationship with the critical Reynolds number of the boundary equation for bubble flow. The proposed two-phase flow pattern identification template by introducing Hurst coefficient enables the prediction of the two-phase fluid flow pattern in rock fractures of different roughness.