Adsorption characteristics and thermodynamic property fields of methane and Sichuan Basin shales

Adsorption of methane on three Sichuan Basin shales at 318 K, 338 K, and 358 K has been investigated experimentally. The adsorption equilibrium isotherm data were fitted with Langmuir, DR and DA model. A thermodynamic model on heat of adsorption (Q(st)), considering the non-ideality of gas and the adsorbed phase specific volume, was developed for adsorption fields of methane and shales. The other thermodynamic properties, included specific heat capacity of adsorbed phase (c(p,a)), entropy (s(a)) and enthalpy (h(a)) of adsorbed phase, were also evaluated on basis of the developed heat of adsorption model. The calculated isosteric heat of adsorption shows a temperature independent feature at lower surface loading while a negative temperature dependence at higher surface loading. Specific heat capacity of adsorbed phase (c(pa)) values increases with temperature, pressure and adsorption amount. The change of c(pa) value with temperature are higher at lower pressure and adsorption amount. The entropy and enthalpy of adsorbed phase decreases with the increase of methane adsorption, and the entropy change is larger at lower temperature suggesting lower temperature is conducive to formed ordered arrangement of CH4 molecules.