Nanopore structure characterization for organic-rich shale using the non-local-density functional theory by a combination of N2 and CO2 adsorption

Abstract

Low-pressure gas adsorption is widely used for pore size analysis of porous materials, and has been employed to characterize pore systems in shale. However, the complexity of shale pore structures means that different methods and models may lead to distinct interprets for adsorption data. Non-local-density functional theory (NLDFT) analysis based on N2 and CO2 composited adsorption isotherms is used here to investigate the pore structure of nanopores in marine organic-rich shale and compare with the results from some conventional methods in this paper. The results indicate that (1) The N2 adsorption isotherms of organic-rich shale are a composite of Types I(b), II, and IV(a), according to the IUPAC (2015) classification of physisorption isotherms. The hysteresis loops show similar shapes to Type H2(a). Delayed capillary condensation is observed in the adsorption isotherms, and the desorption step is shifted to the lower relative pressure of ∼0.45 characteristic of the cavitation mechanism, indicating ink-bottle pores with narrow necks. The CO2 adsorption isotherms are similar to Type I(b), but appear to increase without limit when p/p0 = 0.03 because of the occurrence of meso- and macropores in the shales. (2) NLDFT method based on N2 and CO2 composited adsorption isotherms is the most suitable and accurate method for using gas physisorption when considering the entire size distribution of nanopores, which allows a suitable range of critical pore sizes (∼0.33–100 nm) to be explored.