A description of binary gas sorption on coal is developed and incorporated into a reservoir simulator for use in predicting primary and enhanced recovery of coalbed methane.
The sorption of methane-nitrogen and methane-carbon dioxide mixtures was measured at 115°F and total pressures of 500, 1,000, and 1,500 psia. A moist Fruit-land coal sample from the San Juan Basin of Colorado was used. The results showed that each gas did not sorb independently, instead both gases were competing for the same sorption sites. An extended Langmuir isotherm provided a reasonable correlation of the data.
A compositional reservoir simulator was modified using the extended Langmuir isotherm as the basis for the equilibrium relationship between the free and sorbed gas. The resulting formulation is functionally equivalent to a compositional representation using K-values expressed as a function of total pressure and the concentration of one component. The modified simulator was successfully tested for the cases of primary recovery of a single sorbing component and the enhanced recovery by nitrogen injection.