Pore morphology characterization and its effect on methane desorption in water-containing coal: An exploratory study on the mechanism of gas migration in water-injected coal seam

Water-injected technique was once considered as an effective way to solve the gas-induced problems in the mining industry, but has been stagnant due to its unstable conditions. A systematic knowledge on the micro-mechanism of water injection into coal seams is beneficial to better understand water-based techniques. However, the laboratory investigations considering the engineering background of water injection as well as the pore morphology of targeted coal seam have been rarely studied. In this paper, characterization of pore morphology and the effect of injected water on gas desorption characteristic were carried out using pore structure analyzers (N-2 adsorption and mercury intrusion methods), high-resolution scanning electron microscopy and a home-made instrument (water-injecting desorption test). The pore morphology results from pore size distribution, fractal dimension, pore shape and connectivity indicate that the essential configuration of pore structure is well-developed larger pores containing abundant smaller pores with extensive distribution of constricted pores that are inaccessible to fluid migration. The influence of pore morphology on desorption process may be attributed to the microporous constrictions with non-effective pores, which are geometrically interpreted by pore blocking mechanism. The desorption test results show the total desorption volumes and initial effective diffusion coefficient have the reduce rates of 26.65% and 38% with the moisture content increasing from dry to 1.8% while have a little change as moisture increases (1.8%-11.2%), demonstrating the obstruction of water molecule on gas desorption pathway and the existence of extremity moisture content. Water injection has a remarkable effect on the average desorption rates in the initial period of 10 min; however, the ultimate desorption volume of 0.81 mL/ g with higher moisture content of 11.2% is not sensitive to adsorption equilibrium pressure. Moreover, combined with the mature water-injected technique in the actual coal seam, a conceptual design was summarized to consider the effect of the constricted pore morphology on the interactions of water-gas-coal, which may demonstrate the micro-mechanism of gas migration in the far water-injected coal seam. Meanwhile, in the near water injection zone, due to more energetic gas molecules forming gas bubble in the aqueous condition, nucleation appears to be imagined to explain the pressure-insensitive phenomenon. These findings are of great guiding significance to the theoretical studies and field applications of actual water-injected coal seam.