Effects of SEM image resolution on organic pore structure estimation of shale reservoirs

Abstract

SEM images on ion-polished shale samples can visualize organic pores and can be used to analyze the organic pore structures quantitatively. Compared to the routine fluid intrusion methods (e.g., helium porosity, N2 adsorption, mercury intrusion, etc.), the SEM image-based organic pore structure analysis has many unique benefits. However, image resolution can significantly influence the analysis results. This should be investigated carefully. The image resolution effects on the analysis of organic pore structures was confirmed and estimated through (1) imaging organic matters and their organic pores at different resolutions (0.7–29.2 nm/pixel), and (2) comparing the differences of 2D organic porosity, pore size distribution (PSD), and organic pore geometry calculated at different image resolutions. The two ways of image resolution effect on organic pore analysis was revealed. First, organic pores with a wall close to the sample surface were more challenging to recognize at low image resolution. Any incorrect recognition of these organic pores can significantly affect the calculated 2D organic porosity and the sectional area-based PSD. Second, the small size organic pores were also difficult to recognize at low image resolution, considerably affecting the estimation of pore amount-based PSD but only slightly affecting the calculated organic porosity and sectional area-based PSD due to their small pore size. When SEM image resolution is less than 10nm/pixel, organic porosity will be significantly underestimated, and the estimated PSD and geometry usually have high uncertainty. Therefore, SEM images with 5nm/pixel or higher resolution are recommended to assess organic pore structures.