Direct imaging of micropores in shale kerogen

Abstract

Although micropores (<2 nm) play an important role in shale gas storage, these pores have not been imaged previously. Aberration-corrected scanning transmission electron microscope (AC-STEM) with a secondary electron detector (SED) has been used here to study such pores in kerogen isolated from Longmaxi shale. In addition, a three-dimensional (3D) kerogen molecular model was constructed based on 13C Nuclear Magnetic Resonance (13C NMR), Fourier transform infrared spectroscopy (FT-IR) and CO2 adsorption experiments, to further study the relationship between micropores and kerogen molecule. The results show that AC-TEM with SED can directly image pores smaller than 2 nm in the kerogen particle by operating in the scanning transmission electron microscopy (STEM) mode. These micropores can occur everywhere on the kerogen surface, which is consistent with the large micropore volume and surface area determined by gas (CO2 and N2) adsorption experiments. The surface of the 3D kerogen model is similar to that observed in the SED-STEM images in terms of surface morphology and scale. This suggests that the 3D model of the kerogen molecule is a good representation of the kerogen sample. This study links the direct images of micropores in kerogen sample and the kerogen molecular models, which is important for further understanding of shale gas storage, as these micropores and molecules provide the sites and surface energy for methane storage.