The structural versus textural control on the methane sorption capacity of clay minerals

Abstract

In the natural, high-pressure environments of sediments and sedimentary rocks, clay minerals, along with organic matter, contribute to a large portion of the available surface area and microporosity in which methane (CH4) can be adsorbed. However, most of the questions concerning the location of adsorption sites, e.g. interlayer accessibility for CH4, remain unknown. Here, we have separately investigated the textural and structural control on CH4 high-pressure adsorption on pure clay minerals by combining high- and low-pressure gas adsorption techniques, transmission electron microscopy, and thermogravimetric and X-ray diffraction analyses on various cationic forms of montmorillonite, beidellite, and two illites, with different contents of adsorbed water. The results show that CH4 adsorption capacity is controlled by N2-accesible micropore volume. CH4 adsorption sites are located mainly outside of the interlayer galleries unless they are open wide enough (e.g. pillared by organic cations or incompletely dried divalent cations). However, the structure-independent factors like the crystallite planar dimensions and the way that they assemble are the main control on CH4 adsorption capacity. This might be the reason for the inconsistencies in CH4 adsorption reported for clay minerals in the literature. CO2 adsorption measurements, assumed to provide a good proxy for the estimation of CH4 adsorption capacity in natural mudrocks, may overestimate the CH4 adsorption potential in geomaterials containing a significant amount of exchangeable divalent cations due to higher penetrability of CO2 than CH4 in the interlayers of expandable clay minerals.