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During the process of gas and water production in coal reservoirs, the interface effect between gas and water should not be ignored. In this study, based on nuclear magnetic resonance technology, the unsteady displacement experiment was carried out with coal samples to monitor the interface effect, and a new relative permeability model (with a specific benchmark) is established and verified which considering the interface effect. The results show that the NMR method can be used to monitor the interface effect effectively, and in the experiment movable water controlled by the pressure difference is limited (11% and 20%), and that controlled by the interface effect is dominant (89% and 80%), which means the interface effect plays a dominant role in the two-phase flow. In the flow, the gas phase drives the water phase, and this additional driving effect results in that the relative permeability of water phase is greater …