Characteristics, mechanism and prediction model of water saturation variation during gas and water interaction in deep coal reservoirs

Deep coalbed methane (CBM) exhibits substantial production potential, yet gas-water interaction mechanisms require elucidation for production optimization. This study employs integrated high-pressure mercury intrusion porosimetry (HMIP), gas-water displacement experiments and coupled nuclear magnetic resonance (NMR) to characterize water saturation processes in deep coals. Results identify multi-scale pore systems: fractures (8000–200000 nm) and pore-throat (1–8000 nm) networks. The pore sizes smaller than 30 nm (pore-throat volume ratio >1) contain narrow throats and wider pores, while larger than 30 nm (ratio <1) feature extended throats with marginally wider pores. The gas displacement pressure displaced adsorbed and total water saturation in different pore sizes by Haynes jump and decreased water saturation according to the power law, whereas the capillary water saturation decreased in four stages and bulk water saturation decreased in two stages by piston propulsion. Under higher displacement temperature, the evaporation-condensation synergism leads to an overall linear decrease and replace on different apertures segments in adsorbed water saturation. The predicting results of water saturation in different depth coalbeds show that the water saturation variation with burial depth corresponds to the deep CBM definition depth, thus providing a theoretical basis for the enrichment of free gas in deep CBM.