Organic shale micropore and mesopore structure characterization by ultra-low pressure N2 physisorption: Experimental procedure and interpretation model

• Recommend experimental conditions of N2 physisorption for organic shale.
• Identify the improved HK model as the best for characterizing shale micropores.
• First combine minerals and organic matter as adsorbents for the HK model.
• Determine the BJH model to characterize organic shale mesopores.
• Use N2 physisorption to calculate the PSD of shale micropores and mesopores.

Characterizing the micropore and mesopore structure of organic shale is critical for understanding the storage state and flow mechanism of shale gas. In comparison with microscopic observation and radiation detection methods, up to now, N2 physisorption has been widely used for quantitatively analyzing the mesopores in organic shale. Use of this method is rare for micropores. It is well known that the effectiveness of N2 physisorption in pore structure analysis is mostly influenced by both its experimental procedure and the interpretation models. This paper explores the effect of experimental preparation process, including sample size, outgassing temperature, vacuum pump, and equilibrium time threshold, on N2 physisorption. Within, a standard experimental procedure for organic shale sample preparation was recommended. Moreover, a comprehensive validation was performed on the most available interpretation models for characterizing and comparing micropore and mesopore structures. According to the CO2 adsorption results, the Horvath–Kawazoe (HK) model worked best for organic shale micropore analysis, especially when carbon and aluminosilicate oxide ion were used as the adsorbents to represent organic matter and inorganic minerals, respectively. The Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) models worked well for characterizing mesopores in organic shale, but methods based on density functional theory lost their effectiveness for organic shale due to the lack of a suitable kernel.