3 / 2025-11-04 20:25:47

Dynamic characterization of CH4 transformation and migration during CO₂-enhanced shale gas recovery: real-time NMR-a directly monitoring method

CO₂-ESGR; NMR; dynamic characteristic; competitive adsorption; desorption

CO₂ injection into shale reservoirs has emerged as a promising technique for both enhanced shale gas recovery (CO₂-ESGR) and CO₂ geological sequestration. Understanding the effects of CO₂ on CH₄ desorption and migration is essential to elucidating the mechanisms of CO₂-ESGR. Traditional testing methods often struggle to directly observe the dynamic transformation and migration of CH₄ during the CO₂-ESGR process. In this study, real-time nuclear magnetic resonance (NMR) technology is employed to investigate the dynamic characteristics of CH₄ transformation and migration in CO₂-ESGR, with a focus on quantitatively evaluating the influence of CO₂ on the adsorption-desorption behavior of CH₄. The results show that free CH₄ is produced rapidly during the process of depressurization, while the release of adsorbed CH₄ from micropores is restricted, leading to incomplete desorption. CO₂ injection facilitates the transformation of CH₄, with spectral variations indicating a sequential displacement process: CO₂ first occupies macropores and fractures, then gradually diffuses into micropores, promoting the transformation of CH₄ from adsorbed to free states until competitive adsorption equilibrium is established. Meanwhile, CO₂ drives the migration of CH₄ into larger pores, significantly reducing the CH₄ content in micropores. The desorption efficiency of adsorbed CH₄ increases by 29.7%, while the production efficiency of free CH₄ improves by 24.1%, resulting in an overall recovery enhancement of 14.5%. This study provides experimental evidence and theoretical support for understanding the dynamic transformation and migration of CH₄ following CO₂ injection, offering valuable insights into CO₂-ESGR processes.