Competitive adsorption characteristics and diffusion process of CO₂−CH₄ on mineral surface: A case study of the 2^nd Section of Shanxi Formationin Yan 'an Gas Field

This paper is the result of environmental geological survey engineering.

Objective The CO₂−Enhanced Gas Recovery (CO₂−EGR) technology significantly augments natural gas extraction efficiency while concurrently facilitating the permanent subsurface sequestration of CO₂. This dual benefit substantially aids in achieving carbon neutrality goals. The mechanisms pivotal to enhanced recovery and storage include the competitive adsorption and diffusion of CO₂−CH₄ within nanopores.

Methods This study focuses on the 2^ndsection of Shanxi formation in the Yan'an Gas Field located in the Ordos Basin. Using molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) methods, a model was established toinvestigate the competitive adsorption behaviors of CO₂−CH₄ mixed gases in the nanoporous matrices of key minerals, specifically quartz and illite, under reservoir−specific temperature and pressure conditions. Additionally, the study analyzes the correlation between the self−diffusion coefficient of CO₂ and the variabilities in temperature and pressure.

Results The study yields several findings: (1) At an isothermal condition of 353.15 K and varying pressures from 5.9 to 17.7 MPa, both quartz and illite exhibit heightened adsorptive capacities for CO₂ in comparison to CH₄. Furthermore, the competitive adsorption selectivity for CO₂−CH₄ is found to be greater in quartz pores than in illite pores. (2) Under similar isothermal conditions and at a constant pressure of 11.8 MPa with temperatures ranging from 313.15 K to 373.15 K, the competitive adsorption selectivity for CO₂−CH₄ in both quartz and illite pores is observed to diminish with increasing pressure and temperature. (3) Under conditions of low pressure (5.9 MPa) and high temperature (373.15 K), there is an enhancement in the mobility and diffusion efficiency of CO₂ within both CO₂−CH₄−quartz and CO₂−CH₄−illite systems.

Conclusion Quartz and illite have higher CO₂ adsorption capacity, greaterCH₄displacement capacity, and better CO₂ storage effect.