Modeling for gas dissolution and diffusion in water-intrusion coal seam and its potential applications

In order to determine the gas pressure for water-intrusion borehole, a new method of gas pressure measurement in water-intrusion borehole based on gas solubility was proposed. A mathematical model of gas migration in water-intrusion borehole considering the effective diffusion coefficient was established. The influencing factors, such as water-intrusion radius, diffusion coefficient and solution convection, on the dissolution equilibrium of gas in borehole water were investigated. Finally, the feasibility of the new gas pressure measurement method was discussed. Results showed that the gas diffusion coefficient and water-intrusion radius of coal seam are the key factors influencing the equilibrium duration of gas dissolution. During the accumulation stage of formation water in borehole, the gas is able to quickly dissolve and diffuse in borehole water. The amount of dissolved gas can reach more than 90% of solubility in three days. After the formation water enters the coal seam, the equilibrium time grows sharply because the effective diffusion coefficient of gas in water-saturated coal is much lower than that in pure water. In addition, the water convection causes a higher amount of gas dissolved at the borehole bottom. Finally, the field applicability for the new method of pressure measurement were analyzed by combining the data from previous works. This work provided a new idea for the gas pressure measurement of waterintrusion borehole.