Gas desorption characteristics in different stages for retained water infiltration gas-bearing coal and its influence mechanism

Coal seam water injection (CSWI) can introduce additional water to form retained water. The migration of retained water will go through the stage from the end of CSWI to water migration equilibrium and the stage of water re-migration caused by the connection between the wetting area and the atmosphere. However, the gas desorption characteristics in two water migration stages reflect the effect of CSWI for gas control. To clarify the evolution characteristics of gas desorption, an experimental platform for simulating gas desorption characteristics of different stages was developed. The following results are obtained through the platform test and theoretical analysis. When the retained water transport equilibrium, the maximum replacement gas efficiency was inversely proportional to the gas pressure and proportional to the retained water content, respectively. The retained water infiltration gas-bearing coal can reduce the degree of water lock in the fractures and increase the wetting degree of pores. The interaction energy between coal molecules and water molecules was proportional to the number of water molecules added and inversely proportional to the gas pressure, respectively. The methane molecules interfered with the interaction between coal molecules and water molecules. After the water infiltration equilibrium was broken, the maximum gas desorption efficiency was inversely proportional to gas pressure and the retained water content, respectively. In a word, retained water can promote gas desorption and reduce coalbed methane content. The CSWI should be employed in advance to increase the wetting time, improve displacement gas efficiency, and delay gas emission when the working face mines. The research result provides some theoretical foundation for the rational use of CSWI for the gas control of coal mine.