Shale lamina thickness study based on micro-scale image processing of thin sections

Abstract

The mechanical properties of shale play an important role in unconventional gas exploration, and macro-mechanical tests on the Longmaxi shale show significant anisotropy, which is closely related to lamination. According to experimental and numerical studies on shale, the lamina thickness has an important influence on the mechanical properties of shale. Therefore, this study investigates the actual lamina thicknesses of the Longmaxi shale outcrop. Photomicrographs of seven thin sections showed that the shale can be viewed as a composite of two phases: the clay matrix and inclusions. The lamination of the Longmaxi shale is considerable, although the lamina lack sharp boundaries. After image processing (binary segmentation, image denoising and lamina segmentation), auto-segmentation was performed, and the lamina thicknesses of both the clay matrix and inclusions were estimated. The results show that for both phases, most lamina thicknesses are narrow, and no representative thickness was found for the samples. Additionally, the clay matrix was much thicker than the inclusions. The impact of the inclusion thickness ratio on the shale properties was evaluated based on the Mori-Tanaka model, which confirmed the important impact of lamina thickness on the mechanical properties of shale.