Low-pressure gas adsorption (LPGA) using N2 and CO2 has been widely used by researchers to evaluate the porous structures present within shales and coals. For a suite of shale and coal samples from India, a drop in the N2-BET specific surface area (SSA) was observed with an increase in total organic carbon content (TOC), with low-TOC shales showing a higher SSA than high-TOC shales and coals. Previous research works have demonstrated the limitations of using N2 at −196 °C to penetrate complex microporous structures in coals and thus yielding a low SSA. Likewise, the limitations of N2 to decipher complex porous structures in coals will hold for shales as well. An overall trend of decreased N2-SSA with increasing TOC content, especially for shales with TOC >10 wt %, and higher N2-SSA at lower TOC levels indicates that N2 does not completely detect the porous structures in organic-rich rocks. It mostly accesses the porous structures in minerals, thereby yielding a generally high SSA for low-TOC shales. In light of these facts, correlating and evaluating SSA in shales based on organic richness and thermal maturity levels can be misleading. On the other hand, while LPGA studies using CO2 are also debated, we propose an improved relationship between organic matter abundance and CO2-SSA in coals and shales.
The influence of degassing time and temperature on low-pressure gas adsorption (LPGA) behavior of shales was examined in this study. Two organic-rich shales of contrasting maturity, reactivity and organic matter type, were crushed to <1 mm and <212 μm grain-sizes and degassed at 110, 200, and 300 °C for 3 and 12 h, respectively. Our results indicate that degassing duration has a minimal influence on pore-character interpretations from LPGA experiments, while the degassing temperature shows a strong influence on the pore attributes. For both shales, reliable porosity estimates were obtained when the samples were degassed at 110 °C. When the degassing temperature was increased to 200 and further to 300 °C, distinct changes in adsorption isotherms and other pore structural features were observed. For the mesoporous low-mature shale (collected from a lignite mine) when the degassing temperature was kept at 200 °C, a macroporous character was induced with a manifold increase in pore diameter. Results from thermogravimetry and Rock-Eval indicate abundance of reactive kerogen, which undergoes alteration when degassed at higher temperatures. When the degassing temperature was kept at 300 °C, the organic matter underwent further alteration and showed an isotherm similar to the shales degassed at 110 °C. Similarly, for the oil-window mature shale sample, a transition towards macroporous structure was observed when the sample was degassed at 200 and 300 °C, compared to a mesoporous structure observed when degassed at 110 °C. The results from fractal dimensions also support the above inferences, indicating the presence of simpler structures at higher degassing temperatures. Reduction in pore volume (110–200 °C) and its further rise (200–300 °C) are also evident in the micropore domain, more distinctly in the oil window mature shale. Our results strongly indicate that degassing temperature should be kept at around 110 °C for reliable shale pore character estimation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.