Experimental study on influence of water with different salinity on methane desorption performance of coal seam

In order to study the influence of moisture and salinity on methane desorption of coal samples, taking the coal samples from the hex16-17 coal seam in Pingdingshan mining area as an example，an isothermal desorption experiment was carried out on coal samples with different salinity (0,2,5，15 g/L) under the condition of dry and saturated water, and the influence of water with different salinity on coalLMdesorption performance was discussed. The results show that the existence of water with different salinities greatly reduces the initial rate of coal methane desorption, the total desorption time, and the total amount of methane desorption; it increases the proportion of methane desorption in each time period to the total desorption to varying degrees, but the increase of this ratio decreases with the increase of equilibrium pressure; however, it has little effect on the fitting law of coal desorption of methane. For saturated coal watersamples with different salinities, the desorption rate of each coal sample in the early stage decays rapidly under different balance pressure. Under same equilibrium pressure, with the increase of salinity, the initial methane desorption velocities in early stage and total desorption times and desorption amounts of the different coal sample increase first and then decrease. When the total desorption is at its maximum, the salinity is about 2.5 g/L; After that, the total desorption is gradually reduced, and the degree of decrease amplitude is gradually reduced. When the salinity is at 15 g/L, coal sample methane desorption amount is even lower than that of the coal sample saturated pure water, however, the total amount of methane desorption in saturated coal samples with different salinity will not always decrease; when the salinity is greater than 20 g/L, the total amount of methane desorption will be less than 0 g/L. The change in the amount has no obvious effect. The reason is that it is different from previous cognition that in addition to competing to occupy the adsorption sites of methane, water molecules also adsorb on the surface of coal in multiple layers to occupy methane adsorption channels, which further reduces the amount of methane adsorption by coal. When it reaches saturation, excess water will be stored in the large pores between coal particles in a free state; under certain temperature and pressure conditions, methane can be dissolved in this part of the water, increasing the amount of coal′s "adsorption" of methane.Therefore, the amount of methane adsorbed by coal isotherm mainly includes two parts: the limit dissolved amount of methane in coal pores and fissure water and the limit adsorption amount in remaining pores. Inorganic salt ions in the salinity water affect the former, and through hydration and gap filling these two competition mechanismaffect the dissolution and diffusion of methane. In general, when desorption of saturated coal with different salinity was carried out, the hydration of inorganic salt ions in the water was conducive to enhancing the diffusion ability of methane molecules in the free water of coal holes and cracks, while gap filling was the opposite. Under low temperature (less than 85 ℃), hydration dominates,therefore, when the salinity is low, it promotes the diffusion and transport of methane in the free water of coal holes and cracks. When the salinity is high, the effective clearance degree of free water in pores and crevasses will be reduced and the gas diffusion ability will be reduced. However, there is a limit that the excess of the salinity has little influence on the total desorption amount of coal samples.