The influences of composition and pore structure on the adsorption behavior of CH4 and CO2 on shale

CO2 enhanced shale gas recovery (CO2-ESGR) has attracted extensive attention as it can improve the shale gas recovery efficiency and sequestrate CO2 simultaneously. In this study, the relationship between mineral composition, pore structure, CH4 and CO2 adsorption behavior as well as selective adsorption coefficient of CO2 over CH4 (αCO2/CH4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\alpha _{{\rm{C}}{{\rm{O}}_2}/{\rm{C}}{{\rm{H}}_4}}}$$\end{document}) in marine and continental shales at different temperatures was investigated. The results illustrated that shale with higher total organic carbon (TOC), higher clay minerals and lower brittle mineral contents has a larger micropores and mesopores volume and specific surface area. TOC content was positively correlated with fractal dimension Df. Both CH4 and CO2 adsorption capacity in shale have positive correlations with TOC and clay mineral content. CO2 adsorption capacity of the all the tested shale samples were greater than CH4, and the αCO2/CH4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\alpha _{{\rm{C}}{{\rm{O}}_2}/{\rm{C}}{{\rm{H}}_4}}}$$\end{document} of shale were larger than 1.00, which indicated that using CO2-ESGR technology to improve the gas recovery is feasible in these shale gas reservoirs. A higher TOC content and in shale corresponding to a lower αCO2/CH4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\alpha _{{\rm{C}}{{\rm{O}}_2}/{\rm{C}}{{\rm{H}}_4}}}$$\end{document} due to the organic matters show stronger affinity on CH4 than that on CO2. Shale with a higher brittle mineral content corresponding to a higher αCO2/CH4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\alpha _{{\rm{C}}{{\rm{O}}_2}/{\rm{C}}{{\rm{H}}_4}}}$$\end{document}, and no obvious correlation between αCO2/CH4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\alpha _{{\rm{C}}{{\rm{O}}_2}/{\rm{C}}{{\rm{H}}_4}}}$$\end{document} and clay mineral content in shale was observed due to the complexity of the clay minerals. The αCO2/CH4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\alpha _{{\rm{C}}{{\rm{O}}_2}/{\rm{C}}{{\rm{H}}_4}}}$$\end{document} of shale were decreased with increasing temperature for most cases, which indicated that a lower temperature is more favorable for the application of CO2-ESGR technique.