The Influence of Analytical Particle Size on the Pore System Measured by CO<sub>2</sub>, N<sub>2</sub>, and Ar Adsorption Experiments for Shales
Qing He, Tian Dong, Dongfeng Hu, Sheng He, Rui Yang, Xiaowen Guo
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 (Vmeso, Smeso, Vmacro, and Smacro) gradually increase with decreasing particle size, whereas the micropore volume and surface area (Vmicro and Smicro) have subtle variation. With decreasing particle size, the contribution of Vmeso and Vmacro to the total pore volume (TPV) increases, while the contribution of Vmicro to TPV decreases. Meanwhile, the changing trend of fractal dimensions of the micropore (Dmicro), mesopore (Dmeso), and macropore (Dmacro) 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), Vmeso (ΔVmeso), Smeso (ΔSmeso), Vmacro (ΔVmacro), Smacro (ΔSmacro), Dmeso (dDmeso), and Dmacro (dDmacro) 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.