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 N2 and CO2 adsorption and CH4 adsorption isotherm
measurements. The results show that the isotherm types of the studied
clay minerals based on N2 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.