Reliability analysis and compressibility correction of mercury intrusion porosimetry experiments for low-medium-rank coals

Low-medium-rank coals are soft and exhibit significant compressibility under high pressure. Therefore, it is debatable whether the pore structure of low-medium-rank coals can be reliably determined using mercury intrusion porosimetry (MIP) experiments. In this study, MIP experiments were conducted on four samples of low-medium-rank coals. By comparing the mercury injection curves of samples with different particle sizes, as well as the macroscopic and microscopic characteristics of the samples before and after mercury injection, this study established a correction method for the mercury injection volume at the high-pressure stage and investigated the reliability of MIP experiments in determining the pore structure of low-medium-rank coals. The results showed that: (1) Columnar samples can retain more primary fissures and effectively avoid intergranular-pore and pockmark effects in the low-pressure stage, making them more suitable for MIP experiments on low-medium-rank coals; (2) No microscopic and macroscopic destructive effects of mercury on coal matrix and pores were observed, and the high pore volume in the MIP experiments was caused primarily by the coal matrix compressibility. The porosity determined using the MIP experiments after compressibility correction was roughly consistent with that derived from low-temperature nitrogen adsorption (LTNA) experiments, with differences of 29.87%‒55.49% within the pore volume range of 6‒100 nm. This result indicates that MIP can be used to determine the pore structure of low-medium-rank coals; (3) To correct the coal matrix compressibility at the high-pressure stage, this study established a cumulative correction method. After the data corresponding to a pressure above 20 MPa were corrected using this method, the obtained mercury injection volume decreased by 0.0170‒0.0323 mL/g. Accordingly, the nano-scale porosity obtained was more consistent with that determined using LTNA experiments; (4) The pore volume of low-medium-rank coals is attributed mainly to macropores. Lignite and long-flame coals show significantly different degrees of pore development, yielding pore volumes of 0.1687 cm³/g and 0.0272‒0.0720 cm³/g, respectively in the MIP experiments. For the samples with well-developed pores, their pore volume is attributed primarily from plant tissue pores.