Methane Sorption Capacity of Organics and Clays in High-Over Matured Shale-Gas Systems

Abstract

Methane adsorption experiments were conducted on a series of organic-rich shales, isolated kerogens, and pure clay minerals at 60 °C and up to 20 MPa pressure. The maximum adsorption capacities of the two isolated kerogens (type I) (17.45 cm³/g and 12.41 cm³/g) were much higher than those of the clay minerals and shale samples. In the high-over mature stage, the affinity of methane for type I kerogens gradually increased, while the amounts of methane adsorbed decreased with increasing thermal maturity. Among the pure clay minerals, the methane adsorption capacity decreased in the following order: montmorillonite (4.02 cm³/g) > kaolinite (3.48 cm³/g) > illite (3.46 cm³/g) > illite/smectite mixed layer (3.1 cm³/g) > chlorite (0.88 cm³/g); the methane adsorption capacities were controlled by the effective surface areas available for adsorption. These clay minerals with higher Langmuir pressures exhibited weaker affinities for methane than the isolated kerogens. Moreover, the adsorption results of kerogen, shale, and illite at different temperatures (30 °C, 60 °C, and 90 °C) show that the V_L values of kerogen decreased linearly with increasing temperature, while the amount of adsorbed water on clay minerals decreased with increasing temperature, which may have affected the methane adsorption capacity. The results show that the contributions of kerogens to the adsorption capacities of the two bulk shale samples were ∼ 43.08% and 56.58%, and the methane adsorption of clay minerals accounted for ∼ 44.12% and 16.74%.