Adsorption behaviors of near-critical carbon dioxide on organic-rich shales: Modeling, multifractality, and kinetics

Abstract Studies on the near-critical carbon dioxide (NC-CO2) adsorption behavior of organic-rich shales are important for shallow shale gas exploration and hydrocarbon recovery enhancement of liquid CO2 fracturing, which further contributes to geological CO2 sequestration and global carbon neutrality. We performed CO2 isothermal adsorption and kinetic experiments on six organic-rich shale samples at a temperature of 303.15 K and pressures of up to 5 MPa to investigate the modeling, multifractality, and kinetics of NC-CO2 adsorption behavior on shales. The equilibrium data were fitted to the Langmuir, Freundlich, BET, modified BET (M−BET), DA, DR, and OK adsorption models; the multifractality of the isotherms was analyzed; and pseudo-first-order, pseudo-second-order, Elovich, and Avrami's fractional-order models were adopted to analyze the adsorption kinetics. Furthermore, an error analysis technique was employed to compare the accuracies of various models for fitting the experimental data. The results showed that various adsorption models presented different fitting effects; however, the M−BET, Freundlich, and DA adsorption models performed better over the entire studied pressure range. The fitting results of the M−BET model indicated that the number of adsorption layers ranged from 2.528 to 4.663 and had a multilayer adsorption mechanism. The multifractality of the adsorption behavior was described by the generalized fractal dimension and multifractal singularity spectra; we demonstrated that some multifractal parameters characterize the NC-CO2 adsorption behavior on shales well. Additionally, the TOC, clay, and quartz contents show close covariations with the isothermal adsorption and multifractal parameters. Moreover, the adsorption process was TOC-dependent and subject to Avrami's fractional-order model. Finally, we propose conceptual three-stage adsorption and geological storage models to understand the nature of the NC-CO2 adsorption behavior of shales.