The effect of sample particle size on the determination of pore structure parameters in shales

Abstract

The combination of low-pressure N2 and CO2 adsorption could provide an effective approach for characterizing the pore structure of shales. Although gas adsorption methods generally do not destroy the pore structure during experimental process, sample particle sizes could significantly affect experimental results that can approach or deviate from the real value. Therefore, the determination of pore structure is closely related to the sample particle size. In the current study, 4 fresh core samples of different compositions and total organic carbon (TOC) ranges collected from the Sichuan Basin were analyzed to elucidate the effect of sample particle size on the determination of pore structure parameters. Samples were ground and then sieved into seven groups based on particle size ranges, i.e., <60, 60–80, 80–100, 100–120, 120–140, 140–200 and >200 mesh, for measurements of low-pressure N2 and CO2 adsorption, TOC contents, and X-ray diffraction (XRD) mineralogy.TOC results show a slight enrichment whereas XRD minerals vary irregularly, with sample particle size decreases. Meanwhile, the TOC and mineral contents show insignificant statistical relation with pore structure parameters in all sample particle size ranges. Therefore, variations in organic matter content and mineral composition that result from sieving are unlikely to have a significant influence on the pore structure of shale. Rather, sample particle size may be the most important control on pore structure characteristics in the samples analyzed in this study.The relative standard deviations (RSDs) for Brunauer–Emmett–Teller (BET) N2 surface areas, Dubinin–Radushkevich (D–R) CO2 micropore surface areas and non-local density functional theory (NLDFT) N2 and CO2 nanopore surface areas measurements are <5%, within analytical error. Therefore, in the studied grain size range (60–200 mesh), the sample particle size shows insignificant effects on surface area results. However, samples with smaller particle size have a greater effect on pore volume and pore size, especially for pore size distribution (PSD) of N2 low-pressure adsorption. The RSDs of the Barrett–Joyner–Halenda (BJH) pore volumes and BET pore sizes of all samples in the 140–200 mesh range are obviously greater than the values of other mesh ranges. Moreover, in the dV/dlogw plots of PSD analysis, high N2 peaks and new N2 peaks appeared in the 10–100nm pore-width range, particularly for samples in the >140 mesh range. The 60–140 mesh particle-size range is therefore recommended for N2 low-pressure adsorption. Finally, the sample particle size has insignificant effect on the pore system parameters for grains in the 60–200 mesh range for CO2 low-pressure adsorption. Overall, the results confirm that the 60–140 mesh particle-size range can be used for both N2 and CO2 low-pressure adsorption measurements.