Thermodynamic model for swelling of unconfined coal due to adsorption of mixed gases

Permeability evolution in coal seams during CO₂-Enhanced Coalbed Methane (ECBM) production is strongly influenced by swelling/shrinkage effects related to sorption and desorption of both CO₂ and CH₄. Other gases, such as N2, have also been proposed for injection in ECBM operations. In addition, water vapour will almost always be naturally present. Much work has been done on the sorption/swelling response of coal exposed to pure gases. However, to address in-situ coal-seam conditions realistically, an understanding of the swelling behaviour of coal matrix material due to multiple-species adsorption is needed. Here, we construct new thermodynamic models for swelling of unconfined coal due to adsorption of multiple gas/fluid species, addressing the equilibrium state and focusing on a binary gas mixture. Four models are derived, covering three possible end-member interactions, plus a generalized case. The end-member models considered correspond to adsorption of gases α and β at fully separate sites, at fully shared sites and at partially shared sites (Models 1–3, respectively). We compare our model predictions with literature data on the swelling behaviour of Bowen Basin coal exposed to CH₄/CO₂ mixtures at total pressures up to 15 MPa and at 55 °C. The results show that swelling of the Bowen Basin coal exposed to CO₂/CH₄ mixtures is best explained by Model 3, which in turn implies that adsorption and swelling is in this case determined by both partial pressure and selective adsorption (affinity) effects. Model 3 is easy to parameterize and its applicability to specific coals and gas mixtures is easily evaluated. Though more difficult to parameterize, our generalized swelling model (Model 4) can be applied to any coal rank, and to any binary gas mixture. It therefore offers an important tool for modeling swelling and permeability evolution during ECBM operations.