Investigation on the Nanopore Heterogeneity in Coals and its Influence on Methane Adsorption: A Multifractal Theory Study

Abstract

Coalbed methane (CBM) reservoirs constitute a distinct class of dense organic rocks characterized by extremely low porosity and permeability. Conducting an in-depth investigation into pore heterogeneity assumes paramount importance for the exploration and development of CBM. This study focuses on the multifractal analysis of the pores with diameters below 300 nm in six coal samples sourced from the Junggar Basin and the Qinshui Basin in China. The analysis is based on a series of experiments involving CO 2 adsorption, low-temperature N 2 adsorption/desorption, and CH 4 isothermal adsorption. This work delves into the influence of pore heterogeneity on gas adsorption capacity by linking the structural parameters to CH 4 adsorption properties. The results indicate that both the micropores, as assessed through CO 2 adsorption, and mesopores to macropores, measured via N 2 adsorption, exhibit multifractal behavior. In contrast to micropores, the mesopores and macropores display stronger heterogeneity and lower connectivity. Generally, uniform and well-connected nanopores are anticipated to positively contribute to gas adsorption. However, there is a positive correlation between the Langmuir volume and the heterogeneity degree of micropores. This phenomenon is ascribed to the fact that the greater surface complexity in micropores involves a larger specific surface area and a higher abundance of adsorption sites. This research contributes to a more profound and precise comprehension of the heterogeneous pore structure within CBM reservoirs, thereby establishing a theoretical foundation for the sustainable exploitation of CBM.