Pore Characterization of Different Clay Minerals and Its Impact on Methane Adsorption Capacity

Clay minerals contain a massive amount of nanopores and play a significant role in gas adsorption in shale. Although the pore structure of clay minerals has been widely studied, the characteristic of pores with diameter < 1 nm remains unclear. To investigate the pore characteristics of different clay minerals, especially for micropores, and to reveal the effect of pore structure on the methane adsorption capacity, the isotherm types, pore size distribution, pore volume, and surface area, as well as the CH4 adsorption capacity of pure clay mineral samples, including kaolinite, montmorillonite, illite, and illite-smectite mixed layer (I/S), were investigated based on low-pressure N-2 and CO2 adsorption and CH4 adsorption isotherm measurements. The results show that the isotherm types of the studied clay minerals based on N-2 adsorption are all type IV, characterized by the presence of hysteresis loops. According to the features of hysteresis loops, it can be inferred that kaolinite mainly has cylindrical pores and slit-shaped pores, while pores in montmorillonite, illite, and I/S are dominated by inkbottle-shaped pores, with a small amount of slit-shaped pores. The studied clay minerals all display pore width peaks around 0.56-0.66 nm and 0.82-0.87 nm. Pores with diameters < 1 nm in kaolinite, illite, and I/S are all interparticle pores. Montmorillonite has microporous interlayer pores in addition to the interparticle pores, leading to its relatively large micropore volume and surface area. The CH4 sorption capacity on different clay minerals is mainly influenced by the surface area, and montmorillonite has the highest CH4 adsorption capacity.