Pore system, microstructure and porosity characterization of Gondwana shale of Eastern India using laboratory experiment and watershed image segmentation algorithm

Porosity is the most crucial parameter to assess the potential of fine grain rocks like shale. Shale has very low connected porosity and permeability, which controls the fluid flow and migration. Therefore, detection of connected and closed pore network in digital images can help to evaluate porosity and permeability of rock. The objective of this paper is to characterize Gondwana shale of eastern India in micro-scale, that is, in terms of its porosity, pore-structure and pore size distribution for its shale gas potential. Watershed-transform has been performed to segment pores, throat and mineral grains by using 2-D Scan Electron Microscopy (SEM) images of Gondwana shale of Barren-Measures Formation. The watershed transform, when coupled with morphological operators and a customized disk shape structuring element, segments the pores and throats efficiently and estimates the porosity of Gondwana shale for the very first time. Three Gondwana shale samples from different borehole depth is used for this study. Finally, porosity measured in a laboratory using Mercury Injection Capillary Pressure (MICP) test, is compared with the numerically modeled watershed porosity values, which shows good agreement between the two sets of results with precisely segmented pore bodies and throat bodies at appropriate locations. Most of the pores in the sample are found to be in a mesopore category (2-50 nm). The porosity values measured using MICP ranges from 5.01% to 6.53% whereas the porosity estimated from watershed image segmentation ranges from 5.21% to 6.91%.