Methane as probes for characterizing pore volume in coalbed gas reservoir: Expanding the pore analysis method

Molecular probes are indispensable tools for structure analysis of porous media. Notably, methane, the primary component of coalbed gas, provides a more direct insight into coal pore properties compared to traditional molecular probes like nitrogen and carbon dioxide. This study develops a methodology for determining the pore volume of coal, utilizing methane as a probe. Two Dubinin–Radushkevich (DR) adsorption model-based methods, the virtual saturated vapor pressure modified method (DR-P${}_0$) and the adsorbed phase density modified method (DR-$\rho$), were discussed. To select a superior precision method for pore volume determination, we proposed an evaluation method that leverages molecular simulation, comparing direct-model pore volumes with fitted-calculated pore volumes obtained by fitting the DR modified models to simulated adsorption isotherms. The results demonstrate that the DR-$\rho$ methods exhibit high precision, with a discrepancy rate of only 2.45%. Compared with the pore volumes of traditional molecular probes with well-defined pore measurement ranges such as CO${}_2$ and N${}_2$, it was confirmed that the CH${}_4$-DR-$\rho$ methods are mainly aimed at micropores. Moreover, the CH${}_4$ probe performs better than the CO${}_2$ probe in reflecting adsorption characteristics. By conducting a correlation analysis between the pore volume data obtained from CH${}_4$ and CO${}_2$ probes on 20 coal samples and the methane adsorption amount at 5 MPa, we found that the Pearson correlation coefficient for the CH${}_4$ probe is significantly higher than that for CO${}_2$, which was more than 0.99. This study reveals the potential of methane probes in pore characterization and expands pore analysis methods.