Influence of Viscous and Capillary Forces on Residual Water in Anthracite Fracture Networks

A deeper understanding of the role of residual water and its influence on gas flow behavior in coal seams is a key factor to achieve enhanced coalbed methane (CBM) production. In this study, numerical simulations were performed to study the gas–water drainage displacement in different coal lithotypes (bright coal, semi-bright coal, and semi-dull coal) at various viscosity ratios ( M ) and capillary numbers (Ca). The results indicate that capillary and viscous forces influence the displacement stability as well as the residual water saturation. The residual water saturation remains higher in unstable displacement vs. stable displacement conditions. The residual water saturation in bright coal, semi-bright coal, and semi-dull coal ranged at 21.5–36.4%, 29.9–36.8%, and 46.4–49.6%, respectively. The performed numerical simulations showed four residual water formation mechanisms in the modeled coal fracture networks, including nonsimultaneous displacement, deflection flowrate, dead-end fractures, and by-passing. From unstable displacement to stable displacement, nonsimultaneous displacement, deflection flowrate, dead-end fracture, and by-passing accounted for 3.0%, 2.1%, 13.1%, and 20.3% of the reduction of residual water saturation, respectively. The residual water in the by-passing mechanism is easily remobilized in coal. Our study suggests that the daily gas and water production should be controlled to reach stable displacement, which is beneficial to CBM production.