Influence of Sample Partical Size on Gas Adsorption Experiments——A Case Study of Longmaxi Formation and Wufeng Formation Shales in Changning Region of Sichuan Province

Although the low-pressure gas adsorption method has been widely used to characterize the porosity of shale porous minerals, there is no unified standard to regulate the granularity of samples so far.Taking the outcrop shale samples of Longmaxi Formation and Wufeng Formation in Changning region of Sichuan province as an example, this paper aims to explore the influence of sample particle size on gas adsorption experiments and try to propose a particle size range suitable for low-pressure N₂ and CO₂ adsorption measurements. In this paper, the basic petrological characteristics and pore characteristics of shale samples were obtained by TOC, R_O, XRD tests and scanning electron microscopy. And low-pressure CO₂ and N₂ adsorption experiments were carried out on samples with different particle sizes. The results show that the change of particle size has different effects on CO₂ and N₂ adsorption experiments. In the case of extremely fine crushing (200 mesh), part of the native pore structure is destroyed, the adsorption is significantly weakened, the total pore volume is reduced, the specific surface area is reduced, and the experimental results have large errors. The pore exposure degree of 140 ～ 180 mesh samples is the most sufficient, which is recommended as the optimal particle size range for CO₂ and N₂ adsorption experiments. High organic matter content is conducive to the development of pores. Compared with the samples of Wufeng Formation, the samples of Longmaxi Formation have higher TOC content, so the gas adsorption amount is higher, and the specific surface area and pore volume are larger. The above results are of great significance for improving the sample processing standards of gas adsorption experiments and improving the accuracy of quantitative characterization of shale pores.