Pore-scale experiments reveal appearance of water blocking phenomenon and its distribution characteristics in coal pores

The water blocking phenomenon formed by the retention of fracturing fluid in the pores leads to an increase in the resistances of gas flow, thereby inhibiting the efficient extraction of coalbed methane (CBM) reservoirs. The opaque of the gas and water two-phase migration process and the ambiguity of its distribution characteristics jointly hinders the release of water blocking phenomenon. A visualization experiment for the gas–water two-phase displacement in the pores was implemented at ambient condition, while the porous media was divided into five flow regions based on the distribution characteristics of the seepage velocity field. The results showed there were “Y,” “I”, “L,” and “H” four primary types of water blocking phenomenon depending on the retention morphology of gas and water phases in different pores. The amount of “I” type water blocking phenomenon accounted for the majority in five regions, and the whole amount was five times than that of “H” type. The “Y” type water blocking phenomenon was created under the dual influence of pore radius and intersection angle, while the “I”, “L,” and “H” types manifest the corresponding morphology under the restriction of pore radius. The ratio of the amount of water blocking phenomenon in the main stream region to the upper and lower of transition region and boundary region exhibited a symmetrical distribution of approximately 3:2:1, mainly caused by the seepage velocity field induced by inhomogeneous pore structures. This research can provide a fundamental guidance for elimination of the water blocking damage in the extraction of CBM reservoirs.