Quantitative Evaluation of the Critical Fracture Size for Migration of Mobile Water during Slug Flows in Coalbed Methane Wells

The distribution of mobile water during slug flows in coalbed methane (CBM) wells directly affects the water pressure propagation path. In this article, the distribution characteristics of gas and water in fractures during slug flow are characterized by gas–liquid microscopic flow experiments. Fluid–structure interaction was adopted to analyze the fracture morphology after deformation under stress. A mathematical model of the critical fracture size for migration of mobile water during slug flows was established through nuclear magnetic resonance tests, contact-angle tests, and the theory of the gas–water migration equilibrium. The results show that the flow rate of the gas and liquid affects the length and period of the gas plug and slug. The gas–liquid–solid three-phase properties affect the shape of the gas–liquid boundary. When the mobile water during slug flows is transformed into bound water, the fractures are deformed to an hourglass shape. The fracture size for migration of mobile water is negatively correlated with the reservoir pressure and contact angle with a power exponent while linearly positively correlated with the surface tension. Using fracturing fluids with low surface tension and high liquid–solid contact angles can promote the expulsion of liquids from reservoir fractures, thereby achieving higher resource productivity. Mathematical statistical methods have been employed to establish a rapid calculation model for the movable water transport fracture size. In summary, the research provides an effective and accurate quantitative method of evaluation for the critical fracture size for the migration of mobile water.