Molecular Simulation of CO₂/CH₄ Competitive Adsorption and Enhanced Gas Recovery in Low-Rank Coal

Injection of carbon dioxide (CO₂) into coal layers can be a viable strategy for underground CO₂ sequestration and enhanced gas recovery (CS-ECBM) in coalbed methane (CBM) reservoirs. Although the CS-ECBM technique have been drawn to research attentions, the microscopic competitive adsorption mechanism of methane (CH₄) and CO₂ remains to be determined. A better understanding of CO₂/CH₄ competitive adsorption behaviors in coal systems can help provide useful guidelines for the design of the CS-ECBM project.

In this work, the simulation model was established by low-rank coal (LRC) matrix, which was built by coal realistic molecular model. Then the slit-shaped pore was built by a width of ∼20 Å was constructed by two LRC matrices. The adsorption and diffusion properties of CH₄ and CO₂ in the LRC slit nanopore were investigated through grand canonical Monte Carlo (GCMC) simulation. The affinity between adsorbates and atoms in the LRC surface was analyzed by computing the radial distribution function (RDF). At last, the molecular dynamics (MD) simulation method was used to explore the displacement efficiency of the adsorbed CH₄ displaced by CO₂ in LRC slit nanopore at the subsurface condition.

Through simulation, the adsorption capacity, adsorption selectivity and displacement efficiency of CO₂ and CH₄ were discussed to elaborate the competitive adsorption behavior of CO₂ over CH₄ in LRC slit nanopore. It was found that gas molecules adsorbed in LRC slit nanopore can be divided into three parts: N_p-matrix, N_p-surface and N_p-central parts. The adsorption density distribution and the mean squared displacement (MSD) of CH₄ and CO₂ in LRC slit nanopore indicated that gas molecules can adsorb more steadily in the matrix of the nanopore than other parts. The RDF results showed that between adsorbates and atoms in the LRC surface, CO₂ had high affinity with the oxygen-containing groups. By analyzing the micro-behaviors of the adsorbed CH₄ displaced by CO₂ in LRC slit nanopores, it was found that the displacement efficiency was enhanced with the enlarged bulk pressures, accompanied by the sequestration amount of CO₂ in LRC slit nanopore during the displacement process.

Our findings and related analyses attempt to provide useful guidance for enhancing CBM extraction by injecting CO₂ and can shed light on the details of transport and storage processes at the atomistic level. The methods proposed in this work can assist the future design in the CS-ECBM engineering and the development of shale gas reservoirs.