Competitive Adsorption and Selective Diffusion of CH₄ and the Intruding Gases in Coal Vitrinite
2019
Tao, Honghua | Zhang, Liehui | Liu, Qiguo | Zhao, Yulong | Feng, Qing
Here, the adsorption and diffusion of CH₄ and the intruding gases in coal were systematically simulated via Monte Carlo. The adsorption selectivity of carbon dioxide over methane (SCO₂/CH₄, >1) decreases significantly at P < 6 MPa and is kept stable when P > 6 MPa. However, SN₂/CH₄ (<1) monotonously increases with the increasing pressure, temperature, and bulk mole fraction (BMF) of N₂. Both the cross exchange (Dᵢ,ⱼ) and diagonal diffusion (Dᵢ,ᵢ) coefficients in the nCO₂ + mCH₄ and nN₂ + mCH₄ systems gradually increase with the increasing temperature. Dᵢ,ⱼ is far higher than Dᵢ,ᵢ for these two systems, indicating the weaker coupling strength of gas–gas interactions than the gas–coal interactions. D₁₁¹ (or D₁₁²) increases while D₂₂¹ (or D₂₂²) decreases with the increase of CO₂ (or N₂) BMF. The swelling ratios of nCO₂ + mCH₄ and nN₂ + mCH₄ increase slightly at temperatures lower than 338 K and significantly at temperatures higher than 338 K, and both of them are positively related to BMFs of CO₂ and CH₄, respectively. Both SCO₂/CH₄ᵈ and SN₂/CH₄ᵈ increase with the increasing BMF of carbon dioxide and methane, respectively, indicating that the replacement effects of CO₂ and N₂ engineering are weightily related to the mole fractions of the invading gases. SCO₂/CH₄ᵈ first increases (≤398 K) and then decreases (398 < T < 438 K). Concerning the geological conditions, the optimization injection depths were 800–1100 m (7.94–10.88 MPa) and 600–900 m (5.98–8.92 MPa) for carbon dioxide and nitrogen, respectively.
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