The Influence of Analytical Particle Size on the Pore System Measured by CO₂, N₂, and Ar Adsorption Experiments for Shales

Abstract

To investigate the analytical particle size impact on pore structure parameters of shale reservoirs measured by low-pressure gas adsorption (LPGA), shale samples with varying total organic carbon (TOC) contents and mineral compositions from the Lower Silurian Longmaxi Formation were selected and crushed into five different analytical particle size ranges. All samples with different analytical particle sizes were analyzed by CO2, N2, and Ar adsorption techniques, and the measured pore structure parameters were compared. Our results suggest that the analytical particle size has subtle influence on measured micropore structure parameters, whereas measured mesopore and macropore structure parameters are greatly affected by different particle sizes, especially the pores with a diameter greater than 10 nm. The average pore size (APS) and mesopore and macropore volume and surface area (V meso, S meso, V macro, and S macro) gradually increase with decreasing particle size, whereas the micropore volume and surface area (V micro and S micro) have subtle variation. With decreasing particle size, the contribution of V meso and V macro to the total pore volume (TPV) increases, while the contribution of V micro to TPV decreases. Meanwhile, the changing trend of fractal dimensions of the micropore (D micro), mesopore (D meso), and macropore (D macro) with decreasing particle size indicates that the micropore still maintains strong heterogeneity, whereas the mesopore and macropore gradually become homogeneous, and the pore surface gradually becomes smooth. The changing rates of APS (ΔAPS), V meso (ΔV meso), S meso (ΔS meso), V macro (ΔV macro), S macro (ΔS macro), D meso (dD meso), and D macro (dD macro) with decreasing particle size were defined, and the changing rates show negative correlations with TOC, quartz content, and positive relationships to clay content. The LPGA experiments should adopt unified particle size so as to eliminate the particle size impact on measured pore structure parameters. This study suggests that the particle size range of 250–180 μm (60–80 mesh) used by most previous studies is recommended for LPGA experiments in shales, and the finer particle size (e.g., 106–75 μm) should be used with caution.

Full-Scale Pore Structure and Fractal Characteristics of Continental Organic-Rich Shale: A Case Study of the Dongyuemiao Member of Jurassic Ziliujing Formation in the Fuxing Area, Eastern Sichuan Basin

“…The basic principle of the fractal theory is that a self-similar natural system has similar structural characteristics at different scales, which can be quantitatively evaluated using the D value. This D value is an effective and reliable parameter for characterizing the surfaces and internal structural complexity of uneven porous media, such as coal or shale reservoirs. , Several methods and theoretical models, including the FHH model combined with the LTNA data, ,, image analysis, , small-angle X-ray scattering, , and HPMI experiment results, ,, have been used to determine the D value of porous materials. Among these models and methods, the FHH and HPMI methods have been commonly applied to tight sandstone and shale reservoirs because of their wide application range and convenient calculations …”

“…Continental shale is often characterized by frequent changes in the sedimentary environment in the vertical direction and deposits of various shale lithofacies, resulting in complex pore systems and strong heterogeneity in shale reservoirs. − Pore structural characteristics, including pore morphology, pore volume (PV), specific surface area (SSA), and pore size distribution (PSD), are important factors affecting the occurrence of shale oil, fluid migration, and shale reservoir evaluation. − In recent years, various qualitative and quantitative technological methods have been used to elucidate the pore structure characteristics of shale. Qualitative methods, such as field emission scanning electron microscopy (FE-SEM), transmission electron microscopy, and computed tomography, have been widely used to observe the morphology and connectivity characteristics of micro–nanopore networks. − Quantitative methods, including N 2 /CO 2 physisorption, high-pressure mercury intrusion (HPMI), small-angle/ultra-small-angle neutron scattering, and nuclear magnetic resonance (NMR), are often applied to determine the PSD, SSA, PV, and porosity. − Different methods have different applications and limitations, and a combination of different methods is effective for characterizing the full-scale pore systems of shale …”

Hydroxyl Removal in Muscovite by Heating for Low-Fluoride Purification of Natural Flake Graphite

“…The N 2 adsorption/desorption isotherms in Figure 5e show that the shape of the adsorption isotherms of the samples all belong to type IV based on the recommendations by the International Union of Pure and Applied Chemistry (IUPAC), a hysteresis loop is observed between the adsorption and the desorption isotherm branch in the sample, which is given by nonrigid aggregates of plate-like particles. 38,39 When the relative pressure P/P 0 > 0.9, the adsorption isotherm increased sharply, indicating the existence of a microporous structure (>50 nm) 40 in the sample after heat treatment. As shown in Figure 5f, after purification, the number of micropores on the surface of the graphite material increases.…”

Effects of Particle Size Reduction on the Pore Structure and Accessibility in Natural Porous Materials

“…Pore properties of shales are usually evaluated using three characterization techniques: gas adsorption (GA), − mercury intrusion porosimetry (MIP), − and electron microscopy [scanning electron microscopy (SEM), focused ion beam scanning electron microscopy (FIB-SEM), and transmission electron microscopy (TEM)]. , Other methods, such as neutron scattering, , nuclear magnetic resonance, and X-ray micro-computed tomography (micro-CT), , have also been utilized to understand various aspects of the shale pore structure. Often, multiple methods are used in conjunction, leading to a fragmented perspective of the pore structure, which requires careful interpretation within the physical limitations of each method. , For example, the analysis probe can limit the extent to which the pore network can be accessed, i.e., gas molecules in adsorption and pycnometry studies (with kinetic diameters of 0.36, 0.33, and 0.26 nm for N 2 , CO 2 , and He, respectively) or mercury in MIP (limited to 3.5 nm as a result of the maximum applied pressure).…”

“…The gas adsorption method is best suited for studying pore properties in shales; however, the correct application of this method has been a topic of discussion in the research community. For example, one focus for previous research was to identify an “ideal” particle size range most suitable for GA analyses. − ,− Despite all of the efforts, there was no consensus on the ideal particle size range. The GA analyses are particularly sensitive to equilibration time, which is influenced by the pore structure and the particle size. , Shorter intervals in GA studies on shales could lead to skewed results from incomplete equilibration …”