Abstract: Injecting CO₂ into deep coal seams to strengthen CBM extraction has both environmental and economic benefits, and has broad development prospects. In order to explore the changes of coal structure of different types of coal after CO₂ injection, five samples with different maximum vitrinite reflectance (R_{o, max}) were selected to carry out supercritical CO₂ injection experiments under the condition of simulated coal seam burial depth of 1 500 m. The characteristics of pore and fracture structure of coal samples before and after injection were tested by low temperature nitrogen adsorption and mercury porosity method, and the degree of change was quantitatively compared by fractal theory. The results of nitrogen adsorption experiments show that the pore volume of coal samples before and after supercritical CO₂-H₂O reaction decreases first and then increases with the increase of coal rank. An inflection point is formed at coking coal, and the pore volume increases the most in the micropore stage (pore diameter 0~2 nm). The change of pore volume in mercury intrusion experiment is more complicated, and the pore volume increases obviously in the transition pore (pore diameter 2~50 nm) and fracture (pore diameter 1 000 nm) stages. This is because the supercritical CO₂-H₂O reaction increases the non-effective connected pores in coal and improves the local connectivity of coal samples. The total pore volume of some samples even showed a decreasing trend after reaction, which may be related to the blockage of pore cracks by shedding minerals. The results of fractal analysis of pore parameters of samples before and after reaction show that the change of pore and fracture structure of different samples depends on the characteristic parameters of coal body. The pore volume of low rank coal and high rank coal changes more after reaction, and the higher the mineral content, the greater the change. This study is helpful to understand the effect of CO₂ injection in deep coal seam on the pore structure of coal seam, and can provide reference for the site selection of CO₂ geological storage and enhanced coalbed methane development (CO₂-ECBM) project.

Key words: supercritical CO₂, CO₂-ECBM, coal rank, pore change, fractal