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Enhanced Shale Gas Recovery by the Injections of CO₂, N₂, and CO₂/N₂ Mixture Gases

Du, Xidong, Gu, Min, Liu, Zhenjian, Zhao, Yuan, Sun, Fulong, Wu, Tengfei
Energy & fuels 2019 v.33 no.6 pp. 5091-5101
carbon dioxide, carbon sequestration, dispersibility, fuels, hydrodynamics, mass transfer, methane, models, nitrogen, shale, shale gas
In this paper, the experiments of enhanced shale gas recovery by the injections of CO₂, N₂, and CO₂/N₂ mixture gases were carried out in a fixed bed setup to investigate the influence of the types of displacing fluid on CH₄ recovery and gas flow dynamics. Investigation results show that when taking CO₂ or N₂ as displacement agent, the Coats–Smith dispersion–capacitance model can give an excellent simulated result to the breakthrough curves of CO₂ and N₂. The injection of N₂ leads to the shortest breakthrough time (tb) of injected gas and the lowest recovery of CH₄ product (RCH₄₋ₚᵣₒdᵤcₜ), while injecting CO₂ into shale formations results in the longest tb of injected gas and the highest RCH₄₋ₚᵣₒdᵤcₜ with a relatively sharp displacement front. The differences of dispersion coefficient (KD) and the flowing fraction of pore space (Fᵥ) in the Coats–Smith dispersion–capacitance model are the underlying reasons for the distinct behaviors of CO₂ injection and N₂ injection. With increasing CO₂ mole fraction in CO₂/N₂ mixture gases, RCH₄₋ₚᵣₒdᵤcₜ rises. The injection of 50:50/N₂:CO₂ mixture gases exhibits the biggest enhancement degree of N₂ concentration during the displacement process. The injection of a N₂-rich mixture can significantly prolong tb of CO₂ and help to sequestrate injected CO₂ over a long-term. For the transport of CO₂ in reservoir, Fᵥ increases and KD and the mass transfer coefficient between mobile and immobile regions (Kₘ) decreases with increasing N₂ concentration in binary gas mixture, revealing that N₂ can hinder the diffusion of CO₂ into the micropore system to displace CH₄. The fluctuation range of flow rate of injected gas (Fᵢₙⱼₑcₜₑd₋gₐₛ) and the CO₂ storage amount (Vₛₜₒᵣₐgₑ₋CO₂) enhance as CO₂ mole fraction in mixture raises. In order to optimize RCH₄₋ₚᵣₒdᵤcₜ, Vₛₜₒᵣₐgₑ₋CO₂, and CO₂ sequestration time, the selection of displacing fluid and the ratio of CO₂/N₂ mixture gases should be taken into consideration.