Pore structure and multifractal characteristics of deep coal-rock in the Ordos Basin: insights from LPCA, LTNA, HPMI, and MIA

This study investigates medium to high rank coal-rock from the Shanxi Fm. and Benxi Fm. in Well S5-3, Ordos Basin. Multiple experimental methods such as low-pressure CO 2 adsorption (LPCA), low-temperature N 2 adsorption (LTNA), high-pressure mercury injection (HPMI), and methane isothermal adsorption (MIA) were employed to systematically analyze the multi-scale pore structure of coal-rock. Multifractal analysis based on the box counting method was conducted to elucidate the pore heterogeneity characteristics of coal-rock and its main controlling factors. The results demonstrate that micropores are more developed in coal-rock in the Benxi Fm., with micropore volumes ranging from 1.3 cm 3 /g to 1.8 cm 3 /g and specific surface areas from 117.4 m 2 /g to 158.2 m 2 /g. Furthermore, coal-rock from the Benxi Fm. exhibits higher methane adsorption capacity, with values at the maximum experimental pressure (28.2 MPa) ranging from 12.40 to 15.66 cm 3 /g, significantly higher than those of the Shanxi Fm. (6.90–10.47 cm 3 /g). Multifractal parameters reveal that all samples exhibit distinct multifractal characteristics. The pore structure of coal-rock in the Shanxi Fm. exhibits stronger overall heterogeneity than that from the Benxi Fm. (the average value of ΔD in Shanxi Fm. is 0.67, and Benxi Fm. is 0.62), primarily originating from high-probability regions. Correlation analysis further reveals a positive relationship between micropore volume and adsorption capacity (R 2 = 0.94), which contributes to reducing the heterogeneity of the pore structure. In contrast, mesopores and macropores primarily enhance the heterogeneity of the pore structure, with particularly significant effects on low-probability regions. Among coal-rock components, liptinite and ash content are key factors enhancing the heterogeneity of the pore structure, while inertinite and fixed carbon contribute to the homogenization of pore structure. This study provides a theoretical basis for understanding the pore structure and heterogeneity of deep coal-rock gas (CRG) reservoirs, offering guidance for coal-rock methane development in the Ordos Basin.