Quantitative characterization of microscopic pore structure for shales using scanning electron microscopy

Key words:  scanning electron microscopy  fractal dimension  pore size distribution  pore structure  shale

Abstract:

The pore distribution of shales was extracted using the pores gray-level thresholds of the 2-dimensional high resolution scanning electron microscopy (2D-SEM) images. The fractal dimension of pore morphology and radius were calculated using the fractal theory. The impacts of pore size distribution (PSD) model, radial step increase and resolution on the PSD determined by 2D-SEM were investigated, and the comparisons between the PSD determined by 2D-SEM, mercury intrusion capillary porosimetry (MICP), and low-temperature nitrogen adsorption/desorption (LNA/D) were made. The results show that the complexity of shale pore structure can be quantitatively characterized by the pore distribution extracted from 2D-SEM images. In general, the higher the fractal dimension of pore morphology is, the more complex the pores become; and a higher fractal dimension of pore radius usually results in a more complex PSD and a higher proportion of micropores. The PSD of shale can be precisely characterized by the continuous pore size distribution (C-PSD) method. The PSD determined from the 2D-SEM has a good consistency with that determined from the MICP and LNA/D, if the radial step increase is set to half of the average radius of pore throat. The PSD determined from the 2D-SEM is affected by the resolution and the heterogeneity of the pore distribution, so that the PSD should be determined from the high-resolution region representing the pore structure of shales (or the patchwork spliced by multiple high-resolution SEM images). The pore structure of shales can be precisely characterized by 2D-SEM images, providing a new method for investigating the pore structure of coring, and for shales that are difficult to prepare samples.