Modified Method for Calculating Saturation Gas Content in Deep Coal and the Pore Size Effect of Methane Adsorption on Guizhou Middle- and High-Rank Coals

Comprehensive development of deep coalbed methane is the basis of sustainable coalbed methane development in China. An accurate assessment of deep coal gas-bearing characteristics is of great significance for the efficient development of coalbed methane. The Guizhou middle- and high-rank coals were taken as examples. The quantitative characterization of full pore size distribution and isothermal adsorption experiments were first conducted. Then, the calculation of saturated gas content in deep coal was modified, and various methane adsorptions in middle- and high-rank coal were elaborated. The difference in saturated gas content calculated by two methods and their pore size effect was then discussed. The results showed that (1) the absolute adsorption amounts of WJZ-11 and LT-6 coals have different increases as a function of depth. The gap between the excess adsorption and absolute adsorption amount showed a positive relationship with depth. (2) Adsorbed gas is still the main constituent of saturated gas content in deep coal. The difference calculated by the two methods showed significant increase in depth, similar to high-rank coal below 2000 m. The relative size between micropore volume and adsorption space determines this difference. (3) The average number of layers of the adsorbed molecule (ANLAM) ranged from 1.25 to 1.66 in WJZ-11 coal, indicating the coexistence of micropore filling and multilayer adsorption. The ANLAM ranged from 0.45 to 0.56 in LT-6 coal, suggesting unsaturated monolayer adsorption with minor volume filling in smaller micropores. (4) The pore size effect of methane adsorption as a function of depth is the fundamental reason for the different calculated results of saturated gas content in deep coal. However, it shows different effects on middle- and high-rank coals. This study aimed to provide a new idea and method for distinguishing methane adsorption behaviors of middle- and high-rank coals and their applicability for prediction of saturation gas content in deep coal.