Nanopore structure and nanomechanical properties of organic-rich terrestrial shale: An insight into technical issues for hydrocarbon production

Abstract Hydrocarbons in terrestrial shale are prospective unconventional resources, which have not yet been successfully produced worldwide despite strong interest. The unique microscopic characteristics of terrestrial shale and their implications for oil and gas development are demonstrated. Ultra-high-resolution imaging is performed based on a state-of-the-art focused ion beam and helium ion microscope (FIB/HIM) system. The results reveal that the nanopore structure and connectivity in terrestrial shale possess significant anisotropy and heterogeneity. Terrestrial shale presents much less micro- and mesopores than does successfully developed marine shale, which is the primary reason for the extremely low permeability in the former. Quasi-static indentation and modulus mapping based on a nanoindenter find significant modulus reduction due to water imbibition, which provides direct microscopic evidence for the ineffectiveness of hydraulic fracturing in terrestrial shale. The resources in terrestrial shale have the potential to be profitably developed. However, unfavorable factors, including high clay content, extremely low permeability, significant heterogeneity, strong anisotropy, and high water sensitivity, can lead to technical challenges for reservoir stimulation, which may be solved promisingly by waterless or water-free fracturing.