Pore distribution characteristics of various rank coals matrix and their influences on gas adsorption

Methane is primarily stored in coal matrix pores and pore size distribution has an important effect on gas adsorption/desorption. Investigation of the relationship between pore size distribution and adsorption performance of coal is of significance for the understanding of the evolution of coalbed methane reservoirs. In this study, a series of laboratory experiments (nitrogen, carbon dioxide and methane adsorption) were carried out to determine the pore size distribution and adsorption capacity of coal samples of various coal ranks. The relationship between pore size distribution, Langmuir volume and the metamorphic degree of coals were established. The factors influencing methane adsorption of coals were also analyzed. The results show that the matrix pores are mainly cylindrical, thin bottleneck-, ink bottleneck- and parallel plate-shaped. With the increase in coal rank, both the total pore volume and the specific surface area initially decrease and then increase. Matrix pores, including micro-, transitional- and mesopores, show a similar asymmetric U-shaped trend in the pore volume and the specific surface area with an increase in coal rank. The percentages by volume and by specific surface area increase for the micropores and declines for the transitional pores and mesopores. In the low-pressure zone, micro- and transitional pores play a dominant role in methane adsorption; however, in the high-pressure zone, the influence of mesopores on methane adsorption is pronounced.