Characterization on pore structure of tectonic coals based on the method of mercury intrusion,carbon dioxide adsorption and nitrogen adsorption

Tectonic coal is generally developed in China because of multi-stage tectonic movement. The pore size distri- bution scale of tectonic coal is wide (millimeter level-nanometer level),and the pore structure is more complex. The pore structure of different scales controls the process of adsorption-desorption ( pore surface),diffusion ( nanometer pores) and seepage (micron-millimeter pores) of coalbed methane. Pore structure of different scales is an important factor affecting the storage and migration of CBM. In order to study the distribution characteristics and evolution law of pore structure of tectonic coal with different scales,four kinds of destruction types of coal samples were collected in Lu’an mining area. The pore distribution characteristics of coal samples were tested by pressure mercury method,low temperature N2 adsorption method and CO2 adsorption method respectively. The dominant aperture segments of each test method are compared and analyzed. A method of multi-scale joint characterization of microporous structures ( 50 nm) by mercury injection method was proposed. The experimental results show that the pore capacity and pore specific surface area of coal are mainly distributed in the micro-pore stage,and the pore capaci- ty and pore specific surface area reach the maximum at about 0. 6 nm. Micropore volume accounts for more than 70%of the total pore volume,and micropore specific surface area accounts for more than 99% of the total pore specific sur- face area. The distribution of pore volume and pore specific surface area in coal has the rule that micropore is larger than macropore and macropore is larger than mesoporous. With the increase of coal damage degree,the pore volume and pore specific surface area gradually increase,the pore volume ratio of large pore and mesoporous pore gradually in- crease,and the pore volume ratio of micro pore gradually decrease. The increase of pore volume is mainly reflected in the macroporous stage,while the increase of surface area is mainly reflected in the microporous stage. Macropore evolu- tion is mainly controlled by exogenous pores,such as brectus pore,granular pore and friction pore. Mesoporous evolu- tion is controlled by the macromolecular stack structure and molecular spacing of coal. Micropore evolution is mainly controlled by the size and arrangement of aromatic plane in coal.