Applicability of Low-Pressure CO2 and N2 Adsorption in Determining Pore Attributes of Organic-Rich Shales and Coals

Low-pressure gas adsorption (LPGA) using N-2 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 N-2-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 N-2 at -196 degrees C to penetrate complex microporous structures in coals and thus yielding a low SSA_ Likewise, the limitations of N-2 to decipher complex porous structures in coals will hold for shales as well. An overall trend of decreased N-2-SSA with increasing TOC content, especially for shales with TOC >10 wt %, and higher N-2-SSA at lower TOC levels indicates that N-2 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.