Analysis of the Gas-Bearing Structure and Carbon Isotope Fractionation Characteristics in Deep Coal: Insights from In Situ Gas Desorption and Molecular Simulations

Abstract

Carbon isotope fractionation plays a key role in understanding the occurrence state, migration mechanism, and resource potential assessment of deep coalbed methane (CBM). The gas-bearing characteristics and carbon isotope fractionation behaviors of deep coal in the eastern margin of the Ordos Basin were systematically investigated using in situ gas desorption and molecular simulations. In situ gas desorption experiments were conducted on coal samples to determine the gas content, gas components, and isotopes. Molecular simulations were conducted by constructing a coal molecular model to simulate methane adsorption and diffusion in pores under varying nanoscales, temperatures, and pressures, thereby revealing the microscopic mechanisms governing gas-bearing properties of deep coal. The results show that the free/adsorbed gas ratio in the desorption process of deep coal ranges from 0.94 to 2.22. The measured δ13CH4 values are generally heavy and exhibit a gradual enrichment trend at different temperature stages. The microscopic theoretical model of methane molecular adsorption and diffusion is innovatively proposed, which reveals that methane in deep coal mainly exists in the adsorbed state, and the free state content is gradually increasing. Meanwhile, during methane migration, there are differences in the diffusion rates of isotopes. The macroscopic experiments reflect the overall trend of methane desorption under field conditions, while the microscopic simulations reveal the microscopic mechanism of methane adsorption/desorption at the pore scale. The two complement each other from different perspectives and jointly support the research conclusion, providing a new perspective for the study of the enrichment laws of deep coalbed methane.