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Influencing Factors and Selection of CH4 and CO2 Adsorption on Silurian Shale in Yibin, Sichuan Province of China

Niu, Yue, Yue, Changtao, Li, Shuyuan, Ma, Yue, Xu, Xinyi
Energy & fuels 2018 v.32 no.3 pp. 3202-3210
Silurian period, adsorption, carbon dioxide, compressibility, equations, methane, micropores, microspores, mineral content, natural gas, nitrogen, organic carbon, reflectance, scanning electron microscopy, shale, shale gas, sorption isotherms, temperature, China
Shale gas is a very promising natural gas with substantial development potential. In this study, Longmaxi Formation shale samples from the Silurian system in Yibin, Sichuan province of China, were selected to characterize the geological parameters, such as total organic carbon, clay mineral content, and vitrinite reflectance. The pore structure of shale was analyzed by field-emission scanning electron microscopy and a low-temperature N₂ adsorption–desorption method. Isothermal adsorption experiments for CH₄ and CO₂ single components and mixtures were performed using a volumetric method. The second virial coefficient was introduced to calculate for the compressibility factor of the gas mixture. Then, the influencing factors and selection of CH₄ and CO₂ adsorption capacity of shale were investigated. According to the experimental results, the selected shale samples are black mud shale with high maturity, and they possessed diversified surface morphologies, complicated structures, and various pore types. Calculated pore parameter results showed that microspores comprised the majority of developed pores in shale samples and play a major role in the adsorption process. Isothermal adsorption experimental results for single CH₄ and CO₂ show that adsorptions of CH₄ and CO₂ follow similar rules. The amount of CO₂ adsorption was higher than that of CH₄ adsorption, and a high pressure was required to reach CO₂ adsorption saturation. The Langmuir–Freundlich equation can be used to fit the isothermal adsorption experimental results. Isothermal adsorption experimental results for the CH₄ and CO₂ gas mixture followed trends similar to those of a single-gas component. The adsorption isotherms were all I-type adsorption isotherms, indicating that micropores play a major role. The temperature, pressure, pore structure, organic carbon content, and clay mineral composition of shale are important factors that influence the gas adsorption capacity of shale. During the adsorption of the same gas, the adsorption capacity of shale samples is more sensitive to unit pressure changes under lower temperature and pressure. In competitive adsorption, shale prefers to adsorb CO₂. Therefore, CO₂ is easier to be adsorbed by shale and causes CH₄ to be released from the adsorption site. This occurrence holds guiding significance to CO₂ driving CH₄.