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

Abstract

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.

Pore structures and fractal dimensions in Early Permian Barakar shales of Rajmahal Basin, Jharkhand, India

“…This observation is in accordance with the earlier reports of Permian shales from the adjacent Raniganj and Jharia coal fields of Damodar Basin (Hazra et al, 2018c;Mendhe et al, 2017c). Furthermore, several researchers have emphasized on the pore structures and fractal characterizations of shale beds for the gas adsorption/storage/transport (Curtis et al, 2012;Mishra et al, 2018a;Hazra et al, 2020;Wang et al, 2021). In the studied shales, the existence of slit-shaped pores indicates ease flow of gas through interconnected pore network (Kuila and Prasad, 2013).…”

“…13f). A negative trend has also been observed among the estimated surface areas and organic contents for the shales/coals from India and Turkey, whereas positive correlation with the inorganic content is suggesting more influence of minerals and clays for hoisting pores than the organic matters (Hazra et al, 2020;and references therein).…”

Multiscale Pore Structure Characteristics and Crack Propagation Behavior of Coal Samples from High Gas Seam

“…The mean pore volume of the samples with diameter less than 2 nm is 0.0083 cm 3 /g acquired from LT-CA tests, which is higher than that results of LT-NGA. This may be as the coal samples in the LT-NGA experiment will shrink at the experimental temperature of 77 K, resulting in a decrease in pore volume; while in the LT-CA experiment, adsorption expansion occurred in the coal samples [16]. After decreasing the intrusion pressure, mercury exits from the pores and micro-fractures.…”

“…Admittedly, gas adsorbed in coal samples, will result in matrix expansion effects, and quadruple moment [16,17]. Some scholars have pointed out that using gas adsorption data to modify MIP results can obtain more accurate pore volume [12,13,15].…”

Quantitative Characterization and Differences of the Pore Structure in Lacustrine Siliceous Shale and Argillaceous Shale: a Case Study of the Upper Triassic Yanchang Formation Shales in the Southern Ordos Basin, China

“…The shape, size, and location of the pores can be obtained by light microscopy (LM) and scanning electron microscopy (SEM). , It is difficult to obtain the volume data of all pores by the limited view field and two-dimensional plane of LM and SEM. The quantitative information of the pore size, pore volume, and pore surface area can be obtained by high-pressure mercury injection (HPMI), carbon-dioxide adsorption (CA), and nitrogen adsorption (NA) experiments. ,− CA and NA experiments indirectly obtain the data of micropore and mesopore structure characteristics by using gas to adsorb and fill spaces of pores in rocks. − HPMI analysis is a good way to characterize the pore structure of macropores and partial mesopores. ,, Some scholars believe that the data of micropores can be obtained from CA, while mesopore and partial macropore information can be obtained by the NA experiment . Due to the influence of compressibility effects, the micropore and mesopore structures cannot be accurately characterized by the HPMI experiment, while HPMI is an effective method to obtain the feature of macropore structures. , It is impossible to quantitatively characterize all pore structures, especially the full pore size distribution (PSD), by one single experimental method.…”