Application of mercury injection correction in pore characterization of lignite reservoir

Mercury intrusion experiment is widely used in porous material characterization because of its simple model, wide testing range, high speed and high precision. However, lignite samples are easy to be affected by Hemp skin effect and matrix compression effect due to low metamorphic degree, weak compaction and soft coal quality, resulting in large error in test results. In order to improve the test accuracy, the mercury injection data of lignite samples in Erlian basin were corrected. Firstly, based on the cumulative mercury injection curve and fractal characteristics, the displacement pressure and hemp skin coefficient are determined. Secondly, aiming at the deficiency of traditional matrix compression correction method in low-pressure section and over correction in high-pressure section, a new method based on cubic spline interpolation and subsection to obtain matrix compression coefficient of coal sample is proposed to improve data utilization and accuracy of correction results. Finally, the pore development of lignite samples in this area was analyzed by combining mercury injection and liquid nitrogen adsorption experimental data. The results show that the hemp skin effect mainly affects the pressure range of < 0.0274 MPa, and the average hemp skin coefficient is 16.9%. Based on the new method, the average matrix compressibility of lignite samples in this area ranges from 1.73 to 3.48 × 10⁻⁴ MPa⁻¹, with an average of 2.28 × 10⁻⁴ MPa⁻¹. With the increase of pore fluid pressure, organic matter content and mineral based humic group content, the matrix compressibility increases. Before and after correction, the mercury intake decreases by 0.0383-0.079 2 mL/g, with an average decrease of 35.1%. The pore volume (> 1.7 nm) of lignite in the study area is between 0.03-0.25 cm³/g, with an average of 0.15 cm³/g. The average pore volume from micropore to macropore is 12.5%, 16.8%, 16.2% and 54.5%, respectively. The specific surface area of B-E-T is between 2.4-37.19 m²/g, with an average of 13.85 m²/g, which is mainly provided by micropores. The pore size distribution curve of coal sample can be divided into I (>5 μm), Ⅱ(0.1-5 μm), Ⅲ(<0.1 μm). Among them, the pores in section I are mainly tracheid and cell cavity pores, the pores in section Ⅱ are mainly striated pores and pores, and the pores in section Ⅲ are mainly interchain pores and molecular structure pores. The pore volume of the first and second sections is positively correlated with the content of  the content of inertinite, and the pore volume of the third section is positively correlated withR_o.