Influence of shrinkage and swelling, of coal on production of coalbed methane and sequestration of carbon dioxide

The potential for enhanced methane production and geologic sequestration of carbon dioxide in unmineable coal seams needs to be evaluated before large-scale sequestration projects are undertaken. Coal is known to competitively adsorb carbon dioxide and methane gases. Laboratory experiments indicate that coal will swell when a gas is adsorbed and shrink when a gas is desorbed. The swelling and shrinkage may change the cleat porosity and permeability of the coal. This paper deals with the influence of shrinkage and swelling of coal on the production of methane from, and injection of carbon dioxide into, a coalbed reservoir. A three-dimensional swelling and shrinkage model was developed. This model is based on constitutive equations that account for coupled fluid pressure-deformation behavior of a porous medium that undergoes swelling and shrinkage. The swelling and shrinkage strains are computed on the basis of the amounts of different gases adsorbed or desorbed. The permeability of the reservoir is modified by the shrinkage or swelling. The paper presents numerical results on the reservoir performance during injection of carbon dioxide in a previous field test that has been reported in the literature. The results indicate that coal swelling and shrinkage can influence the production rates and reservoir pressure distribution in the field.