The characterization and quantitative analysis of nanopores in unconventional gas reservoirs utilizing FESEM-FIB and image processing: An example from the lower Silurian Longmaxi Shale, upper Yangtze region, China

In contrast to the millimeter-to-micron scale of pore systems found in conventional oil and gas reservoirs, unconventional oil and gas reservoirs have nanoscale pore systems. However, the methods used to characterize conventional reservoirs are not normally effective for studying nanopores. In the present study, a dual-beam field emission scanning microscope-focused ion beam (FESEM-FIB) device was used to investigate nanopores in a core plug sample of the Longmaxi Shale from Pengye Well #1, Chongqing, China. A large number of nanopores were observed, which can be divided into three types: intraparticle pores within mineral particles (intraP pores), interparticle pores between mineral particles (interP pores), and organic-matter pores (OM pores). The latter type is the most abundant. The Pores (Particles) and Cracks Analysis System (PCAS) was used to identify and analyze pores in high-resolution SEM images. The results show differences in pore size, pore area, probability entropy, form factor, and fractal dimension between the three pore types, especially between organic-matter pores and the other two pore types. The pore size of organic-matter pores ranges from 4 to 483 nm (average 37 nm), whereas the pore sizes of intraP pores and interP pores are 57-1136 nm (average 210 nm) and 31-1976 nm (average 219 nm), respectively. Results for pore area also reflect the smaller pore sizes of organic-matter pores. The probability entropy, mean form factor, and fractal dimension of organic-matter pores are 0.90, 0.72, and 1.08 respectively, suggesting disordered pore directions and limited morphological complexity. The form factor and fractal dimension of intraP and interP pores indicate more complex pore shapes for these pore types. InterP pores have more ordered alignments, as indicated by their lower probability entropy. Differences in matrix composition and/or the mode of occurrence of pores give rise to differences in pore size and shape. (C) 2014 Elsevier B.V. All rights reserved.