Pore structure characterization of Carboniferous shales from the eastern Qaidam Basin, China: Combining helium expansion with low-pressure adsorption and mercury intrusion

Pore characteristics such as pore size distribution, pore geometry, and connectivity are key factors for evaluating shale reservoir capacities. In this study, multiple methods are used to characterize the pore structure of Carboniferous shales from the eastern Qaidam Basin, China. Low-pressure nitrogen and carbon dioxide adsorption (LPA) and high-pressure mercury intrusion (MICP) were applied to obtain pore size distributions (PSDs) of shale samples. X-ray diffraction (XRD) was used to obtain the mineral composition of the shale, while helium expansion using cylindrical shale samples was performed to obtain total porosity. In addition, field emission scanning electron microscopy (FESEM) was used to verify the results of the PSD analysis. Pores in the shale samples are generally under 10 µm in size. These micro- and mesopores, especially the mesopores, play a dominant role in PSDs. Bimodal PSDs are observed for micro- and mesopores, while unimodal or bimodal PSDs emerge in the macropore range. Total porosity was obtained by helium expansion, which provides relatively accurate porosity values when compared with the results of low-pressure adsorption and mercury intrusion. Total porosity and microporosity show an increasing trend with total organic carbon (TOC). Micro- and mesoporosities are found to significantly increase with total clay content, while macroporosity does not show any correlation with the latter. No correlation is found between porosity and quartz content. Surface area increases with total clay content as well as the proportions of micro- and mesopores, but no correlation between surface area and macroporosity is observed. This study suggests that a simple helium expansion experiment is sufficient to obtain reliable total porosity of shale samples. Pore characteristics obtained in this study provide useful information for the evaluation of shale reservoir capacity in the eastern Qaidam Basin.