Microscopic mechanism of gas-water two-phase flow based on low-field nuclear magnetic resonance

To investigate gas-H2O two-phase flow during H2O injection into gas-containing coal, this study evaluated the nuclear magnetic resonance T-2 spectra of coal under different injection pressures and times using a triaxially constrained low-field nuclear magnetic resonance experimental platform. During H2O injection, the capillary force increases rapidly with a decrease in the pore size, which is the main factor hindering the intrusion of the aqueous phase into the pores of coal, whereas the expansion force and dissolution of the gas phase somewhat control the gas-liquid equilibrium within the pore structure of coal. A model of gas-H2O two-phase transport within the microporous structure of coal was constructed, and the equilibrium of gas-H2O two-phase transport within the pores was observed via the relative concentration and change in energy of the system, and thus, the dissolution effect breaks the equilibrium of gas-H2O two-phase transport. The four mechanisms of gas-H2O two-phase transport in the multiscale pores of coal were summarized. The results obtained are crucial in recognizing the characteristics of gas-H2O transport within the microstructure of coal.