Response Relationship Between Ash Content and Nanopore Structure of Deep Coal Seams in North China Craton

In this paper, coal samples from multiple regions on the eastern margin of the Ordos Basin in the North China Craton were selected as the subject of study. All the samples are selected based on their ash content. Then, micropore volume and heterogeneity of the pore size distribution of deep coal seams are studied by using low-temperature liquid nitrogen and high-pressure methane adsorption experiments. Then the factors influencing methane adsorption capacity as constrained by ash content are studied. The results are as follows: (1) The coal samples can be divided into two categories based on ash content: high-ash and medium-to-low-ash. The micropore structure distribution curve for medium-to-low-ash coal exhibited a four-peak distribution, and that of high-ash coal also showed a four-peak distribution. (2) Micropore volume and heterogeneity of micropore distribution are controlled by thermal evolution. Conversely, for high-ash coal, the micropore volume and heterogeneity are constrained by ash content; the micropore volume decreases and heterogeneity increases with rising ash content. (3) The methane adsorption capacity of medium-to-low-ash coal was higher than that of high-ash coal. A critical pore size range of 0.3~0.6 nm for controlling methane adsorption was found. Within this range, an increase in pore size distribution heterogeneity leads to a decrease in adsorption capacity.