Occurrence mechanism, environment and dynamic evolution of gas and water in deep coal seams

Accurately understanding of the occurrence states, relative content, and distribution characteristics of gas - water under deep conditions has important guiding significance for efficient exploration and development of coalbed methane (CBM). Based on the theoretical model, molecular simulation and systematic analysis of gas-water, the occurrence states of gas-water in coal seam is clarified, and the boundary and dynamic evolution process of gas-water dynamic migration and accumulation are revealed. Considering the coal - water interface interaction, the mobility and occurrence states of water, the coalbed water can be divided into movable water (gravity water and capillary water), bound water (adsorbed water, zeolite water, crystallization water and interlayer water) and structural water. The adsorbed water, capillary water and gravity water are dominated by pores, and zeolite water, structural water, crystallization water and interlayer water are dominated by minerals. The molecular simulation results show that water molecules are saturated and filled in 0.7 nm pores, with consistent adsorption and desorption processes, while weak adsorption layers and free states appear in larger pores. The adsorption process of water molecules is manifested in stages: single molecule oxygen-containing group adsorption, monolayer strong adsorption, multi-layer weak adsorption, water clusters formation, and pore filling. Methane molecules stably fill pores (1.5 nm) with 3 layers of adsorption, and coexist in monolayer adsorption and free state in the larger pores(>1.5 nm), resulting in the widespread presence of free state in pores above mesopores. According to the boundary between adsorption and free gas mentioned above, the theoretical calculation formulas for free gas and adsorption gas have been improved to provide new ideas for gas content calculation. Deep thermogenic CBM is the residual gas generated after large-scale hydrocarbon generation and expulsion from coal. During the hydrocarbon expulsion process, the water is driven by methane and the evaporation diffusion process results in a small amount of residual water (bound water and structural water) remaining in the pores, which cannot be changed in the later stage. Assuming a static water pressure of 20 MPa, under reservoir pressures of 0, 5, 10, 15, and 20 MPa, the maximum pore size that external water can invade is 7, 9, 13, 27 nm and non-invasive. Controlled by differential preservation conditions, coal-derived gas not only forms overpressure and under pressure differential gas bearing systems, but also forms multiple types of gas bearing modes in coal measures. The above work has clarified the micro-occurrence mechanism and evolution mode of CBM and water, which has guiding significance for the enrichment characteristics and efficient development design of deep CBM.