High-pressure adsorption of methane on montmorillonite, kaolinite and illite

Methane (CH4) adsorption of Ca2 +-montmorillonite (Mt), kaolinite (Kaol) and illite (Il) at 60 °C and pressures up to 18.0 MPa was investigated, during which the adsorption capacity was evaluated by the Langmuir adsorption model. The influences of adsorbed water and the interlayer distance of the clay minerals on CH4 adsorption were explored by using heated Mt products with different interlayer distances as the adsorbent. Mt, Kaol and Il showed high CH4 adsorption capacities, and their maximum Langmuir adsorption capacities were Mt, 6.01 cm3/g; Kaol, 3.88 cm3/g; and Il, 2.22 cm3/g, respectively. CH4 was adsorbed only on the external surface of Kaol and Il; however, adsorption also occurred in the interlayer space of Mt, which had a larger interlayer distance than the size of a CH4 molecule (0.38 nm). CH4 adsorption in the interlayer space of Mt was supported by the lower CH4 adsorption capacity of heated Mt products (with the interlayer distance < 0.38 nm) than that of Mt at high pressures despite the higher external surface areas of the heated Mt samples. The entrance of CH4 into the interlayer space of Mt occurred at low pressures, and more CH4 molecules entered the interlayer space at high pressures. Moreover, the adsorbed water occupied the adsorption sites of the clay minerals and decreased the CH4 adsorption capacity. These results indicate that clay minerals play a significant role in CH4 adsorption of shale and indicate that the structure and surface properties of clay minerals are the important parameters for estimating the gas storage capacity of shale.