Feasibility and application of an adsorption model for coal pore structure analysis through N2 adsorption

The pore structure of coal critically influences the occurrence and migration of coalbed methane. Low-temperature nitrogen adsorption (LTNA) is a key method for characterizing the properties of coal pores. However, the accuracy of experimental data directly determines model precision. In this study, LTNA experiments were conducted on six coal samples with varying degrees of metamorphism. The adsorption models were evaluated using residual and root-mean-square error analyses. The results showed that low- and medium-rank coals exhibited type-IV isotherms, indicating bottleneck-shaped mesopores. By contrast, higher-rank coal yielded type-II isotherms, suggesting a predominance of micropores. Unlike traditional methods that assume ideal smooth pore walls, this study systematically applies the Quenched Solid Density Functional Theory (QSDFT) to address the inherent heterogeneity and multiscale nature of coal reservoirs. Among the models, the QSDFT model showed the best accuracy. Crucially, we established a distinctive rank-dependent pore geometry selection framework: for low-and medium-rank coal, cylindrical-pore-based QSDFT is recommended, while for high-rank coal, a hybrid slit-cylindrical model is essential. This approach effectively eliminates artifacts caused by surface roughness, providing a more rigorous basis for coalbed methane migration studies. Under QSDFT, the low-and medium-rank coals showed a bimodal distribution (1–2 and 5-35 nm). In high-rank coal, micropores (2-5 nm) formed the primary peak, whereas mesopores were broadly distributed. QSDFT provides a reliable theoretical and quantitative basis for coalbed methane migration studies.