Pore structure characteristics and full-scale characterization of coal reservoirs in the Laochang mining area

Chinese coal reservoirs are characterized by “Three High and One Low”, and the pore structure of coal is an important factor affecting the permeability, the degree of drainage difficulty and the subsequent drainage mode. In order to accurately characterize the full scale pore size structure of coal, three methods of high pressure mercury injection, low temperature liquid nitrogen and carbon dioxide adsorption are used. Based on the compressibility correction of the high pressure mercury injection experimental data, combined with the fractal theory, the dominant pore size characterization sections of different experimental methods are analyzed, and the full scale joint characterization of 10 groups of coal samples in Yunnan Laochang mining area is realized. The results show that: The compression coefficient of coal matrix ranges from 7.06179×10⁻¹¹ to 1.23531×10⁻¹⁰ m²/N, and the cumulative mercury intake after correction is reduced by 15.89% ～ 27.82% compared with that before correction, with an average of 21.65%. The coal matrix begins to be compressed when the mercury injection pressure reaches 0.7 MPa. Irreversible damage occurs when the mercury injection pressure reaches 14.3 MPa. In the liquid nitrogen adsorption experiment, the pore volume increases gradually with the increase of the pore size on the whole, the pore area tends to be stable and the adsorption volume is small, and the pore adsorption rate is the fastest. The overall rate decreases with the increase of the pore size. The adsorption volume curve and differential curve of carbon dioxide adsorption experiment stage are basically consistent, and the pore volume decreases with the increase of pore size. The mercury injection fractal curve can be divided into three sections. When lg P_i > 1.1, the fractal dimension reaches the maximum, and the coal matrix suffers irreversible compression failure. The low temperature liquid nitrogen adsorption fractal curve takes P_j/P_s =0.5 as the boundary line. The fractal dimension of 2−5 nm pore size is between 1.93−2.92, with an average value of 2.57. The fractal dimension of 5−100 nm pore diameter is 2.16−2.76, with an average value of 2.49, indicating strong heterogeneity. Based on the three experimental results, with 1.5nm and 100nm as the splicing points, the coal samples are dominated by micropores with pore sizes ranging from 0.41nm to 0.86nm, with the largest proportion of pore volume.