Experimental investigation on spontaneous imbibition of water in coal: Implications for methane desorption and diffusion

During the process of coal mining and coalbed methane extraction, the application of hydraulic techniques generally leads to the spontaneous imbibition of water in coal seams. To study spontaneous imbibition of water in coal and evaluate its influence on methane desorption and diffusion, experiments of spontaneous imbibition in coal containing methane were conducted under different adsorption equilibrium pressure and water imbibition ratios. The displacement effect of water during imbibition was investigated, and methane desorption was also tested under atmospheric-pressure conditions. Imbibition test results show that adsorbed methane can be displaced by water. The displacement amount and its efficiency increase with water imbibition ratio, but an upper limit on the displacement amount exists because of the limited imbibition capacity. Desorption test results show an inhibitory effect of water blocking on methane desorption and diffusion. The desorption time increases and diffusivity decreases with the imbibition ratio. However, the time-dependent characteristics of desorption ratio indicate that the desorption process is temporarily delayed rather than completely stopped by water blocking. Moreover, the residual methane amount decreases with the water imbibition ratio based on long-time-scale desorption, suggesting that spontaneous imbibition of water can promote methane desorption on the whole. The scale effects of coal samples and actual conditions on methane adsorption, desorption and imbibition are also discussed. These findings show the multiple effects of spontaneous imbibition on methane desorption in coal and help in better understanding the overall effects of water-based treatment techniques on coalbed methane extraction and coal seam gas control for mining safety.