Experimental investigation of gas diffusion kinetics and pore-structure characteristics during coalbed methane desorption within a coal seam

Abstract

The desorption-diffusion properties of coalbed methane (CBM) are not only the theoretical basis for investigating gas outburst hazards in a coal mine, but also impose a non-negligible impact on CBM recovery performance. In this paper, the pore structure of coal with five different metamorphism degrees was qualitatively and quantitatively characterized with the scanning electron microscopy (SEM) and low-temperature nitrogen gas adsorption (LTNGA) measurements. The isothermal methane ad/desorption experiments were performed with four initial gas desorption pressure scales. Subsequently, the matching degree between eight empirical correlations (ECs) and the measured methane desorption data was determined, and the methane diffusion coefficient (Dfg) was numerically calculated combining with an FGDG (free gas density gradient) diffusion model. Importantly, we evaluated and discussed the sensitivity of pore structure parameters, initial gas desorption pressure, and volatile fraction to Dfg. The results show that pore density distribution differs among coal samples and the pore structure, especially the micropores, is developed better in high-rank coals than in low- and medium-rank ones. Initial gas release velocity increases with an increase in coal rank and decreases negatively with the increasing particle size in an exponential manner. Among the eight ECs, EC-VIII can be applied to more accurately describe gas desorption process during the whole time scale, and its correlation coefficient with the measured data is larger than 0.9900. Dfg increases with an increase in pore volume and specific surface area (SSA), and it is negatively correlated with the average pore diameter. Additionally, Dfg exhibits an asymmetric U-shaped pattern with an increase in the volatile fraction, which is mainly related to the switch between the dominant roles of mesopores and micropores during coal metamorphism. At a lower volatile fraction, the diffusion coefficient is more sensitive to micropores, while, at a higher volatile fraction, mesopores contribute more to the diffusion coefficient.