Abstract
Addition of water to the coal is actually an isobaric imbibition process. To study the dynamic response characteristics of water and methane, the isobaric imbibition process was stimulated by a self-designed experimental device which can eliminate the reabsorbing phenomenon. The results indicate that adding water can displace absorbed gas. The displacement mechanism is attributed to the capillary effect and competitive adsorption during isobaric imbibition process. A competitive adsorption phenomenon exists between gas molecules and water molecules. Since oxygen-containing functional groups in coal and the hydrogen bond of water, water can easily occupy high-energy sites and only the low-energy sites are available for methane. The imbibition quantity increases with increasing water content or adsorption equilibrium pressure. Moreover, the imbibition quantity would reach a maximum value. The relationship between water content and maximum imbibition quantity or the maximum imbibition rate can be described by a Langmuir function under the same adsorption equilibrium pressure. The maximum imbibition quantity increases with increasing adsorption equilibrium pressure under the same water content, which also can be described by a Langmuir function. However, the maximum imbibition rate decreases with increasing adsorption equilibrium pressures under the same water content, which can be described by an exponential function. Compared to the adsorption equilibrium pressure, the water content has a greater effect on the imbibition quantity and imbibition rate. This study revealed the mechanisms of the dynamic response characteristics of water and methane during isobaric imbibition process based on the transformation form of Hagen–Poiseuille equation, adsorption force of coal and gas and adsorption force of coal and water, which can provide a new method to control gas in deep coal seams.
Highlights
With the progressive increase of mining depth, most Chinese mines have the characteristics of high gas pressure, low permeability, and high in-situ stress (Cao et al, 2017; Chen et al, 2017)
The free gas is released through the isobaric pressure relief system, and the rubber hose will be closed until the free gas pressure in the coal sample tank is equal to initial adsorption equilibrium pressure
Under the same adsorption equilibrium pressure, the larger imbibition quantity increases with increasing water content
Summary
With the progressive increase of mining depth, most Chinese mines have the characteristics of high gas pressure, low permeability, and high in-situ stress (Cao et al, 2017; Chen et al, 2017). The first insight is that high pressure water can increase the permeability of the coal seam, displace the free gas in the coal body and enhance the gas production for CBM (Cheng and Chen, 2014; Teng et al, 2016; Wei and Zhang, 2005). The third insight is that water invades into the coal matrix pores under the action of the capillary force, and water can inhibit gas emission and reduce the methane diffusion rate (Xiao and Wang, 2011; Zhao et al, 2011). The adsorbed gas will increase the gas pressure in the pores and prevent the movement of water in the coal It cannot accurately simulate the imbibition effect of water in CBM. Laboratory studies on the dynamic response characteristics of water and methane during isobaric imbibition process remain to be reported
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