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Adsorption/Desorption Behavior of CH₄ on Shale during the CO₂ Huff-and-Puff Process

Li, Ligong, Li, Chao, Kang, Tianhe
Energy & fuels 2019 v.33 no.6 pp. 5147-5152
adsorption, carbon dioxide, desorption, fuels, methane, nuclear magnetic resonance spectroscopy, phase transition, shale
The CO₂ huff-and-puff method has been widely adopted for enhancing CH₄ recovery in shale reservoirs. Revealing the behavior of CH₄ adsorption/desorption in shale during the CO₂ huff-and-puff process clarifies the recovery mechanisms of CH₄ from shale reservoirs. In our work, the question of how CO₂ plays a role in affecting the adsorption/desorption of CH₄ is investigated using the low-field nuclear magnetic resonance technique. In addition, phase transition of CH₄ is also analyzed to investigate how the existing states of CH₄ transform in shale during this process. Specifically, the states in which CH₄ resides in shale are first recognized by analyzing the measured T₂ spectrum of shale after injecting CH₄. CO₂ huff-and-puff tests are then conducted to investigate how CO₂ impacts the adsorption/desorption behavior of CH₄ on shale samples. Furthermore, the T₂ signals of shale during depressurization are measured to investigate the state transformation of CH₄ in shale during the CO₂ huff-and-puff process. Test results show that three states are observed for CH₄ storage in shale samples, that is, bulk CH₄, free CH₄ at the pore center, and adsorbed CH₄ on the pore surface. After injecting CO₂, the adsorbed CH₄ will be desorbed from the shale surface, which thus increases the free CH₄ at the pore center. During depressurization, the free CH₄ is more readily produced from the shale samples, whereas the adsorbed CH₄ is hard to be recovered; more advanced technology should thereby be proposed for enhancing the adsorbed CH₄ from shale reservoirs. This work is expected to inspire new understanding of the mechanisms of CH₄ recovery using CO₂ huff-and-puff methods.