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At present, shale nanometer (<100nm) pore structure analysis often uses low-pressure N2 and CO2 adsorption tests to analyze shale nanopore structure characteristics, but these methods are mainly applied to micronoid pore reservoirs or a single pore structure. The traditional analysis of materials is not suitable for shale samples with strong heterogeneity dominated by nanopores.
The traditional Brunauer–Emmett–Teller (BET) method is only suitable for mesoporous and macroporous materials due to its own theoretical limitations. It is not suitable for analysis of materials with micropores, and this limitation is generally neglected in the study of shale pore structure. A few scholars have revised the traditional BET method, but the study of shale nanopore structure is still insufficient. The t-plot method is based on the principle of BET method and is not applicable to shale research. The Barrett–Joyner–Halenda (BJH) method based on Kelvin's thermodynamic equation has a significant underestimation of the calculation of stenotic mesopores. The Dubinin–Radushkevich (D–R) model also has uncertainty regarding the suitability of microporous materials.
The Density functional theory (DFT) and non-local DFT (NLDFT) methods have no limitations on the use of the application process, but current commercial software can only select a single pore morphology model and cannot be based on the strong heterogeneity of shale. A variety of pore morphologies are co-existed to select a variety of pore morphological models, which leads to certain distortion in the calculation of pore structure parameters. The three-dimensional research methods such as nano-CT and FIB-SEM rely heavily on the identification and precision of post-computation simulation and data reconstruction, and the most advanced nano-CT and FIB-SEM instruments are difficult to reach below 2 nm. Nanopore research scale.
Recently, Xiong Yongqiang, a researcher at the Guangzhou Institute of Geochemistry at the Chinese Academy of Sciences, proposed the NLDFT method based on low-temperature N2 and CO2 adsorption isotherms to analyze and characterize shale nanopore structures. The study shows that: (1) According to the latest IUPAC technical report, the N2 isotherm curve of shale is a combination of type I (b), II, and IV (a), not traditionally recognized as type I, type II, or type IV ( Figure 1); (2) NLD and CO2 combined NLDFT method is more suitable for shale nanometers than BET method, modified BET equation, BJH method, t-plot method and DFT method through numerical comparison and reliability statistical analysis. Pore ​​structure surface area and volume characterization (Fig. 2); (3) The NLDFT method based on N2 and CO2 can analyze approximately 0.33–100 nm shale nanopore structure (Fig. 3) with high reliability and accuracy. . The above methods are of great significance for accurate characterization of shale nanopore structures.
Related results were recently published on Microporous and Mesoporous Materials. The study was jointly funded by the Chinese Academy of Sciences' strategic pilot technology project and the China Geological Survey project.
Paper Information: Wei MM, Zhang L., Xiong YQ, Li JH, Peng P., 2016. Nanopore structure characterization for organic-rich shale using the non-local-density functional theory by a combination of N2 and CO2 adsorption. Microporous and Mesoporous Materials 227, 88‒94.
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