Gas saturation is a critical parameter for characterising CBM and shale gas reservoir. Gas can exist in three states in a coal seam or shale. The major portion of gas stored in coal or shale exists as an adsorbed, liquid-like, molecular monolayer on the walls of the micropore system. The sorptive capacity of coal tends to increase with increasing rank and increasing depth (King et al., 1986). The relationship between gas concentration and pressure is a non-linear function that is generally defined by Langmuir’s equation (Langmuir,1916).    

function   

Where   is the Langmuir volume constant,   is the Langmuir pressure constant and   is the pressure in the cleat system.

The Langmuir constants for a particular coal are determined in the laboratory by grinding a coal sample and measuring the amount of methane held in the coal at various pressure levels.

The actual gas content always lower than the full capacity that coal or shale can adsorb. Gas saturation is ratio of actual gas content to full adsorption capacity.