Generalized Simplified Local-Density/Peng−Robinson Model for Adsorption of Pure and Mixed Gases on Coals

Abstract

Simulations of enhanced coalbed methane recovery (CBM) and CO2 sequestration in coalbeds require an adsorption model to predict the amount of adsorbed gas in a reservoir as a function of pressure, temperature, and gas composition. The availability of a coal-structure-based generalized adsorption model would be a valuable tool for use in a reservoir simulator, since it would facilitate predictions of gas adsorption behavior at the conditions encountered in CBM production and CO2 sequestration. Therefore, in this work, we present a coal-structure-based generalized adsorption model capable of accurate a priori predictions of gas adsorption on diverse coals. The model was developed within a rigorous theoretical framework based on a local-density formulation. Specifically, we have utilized the simplified local-density/Peng−Robinson (SLD-PR) model and generalized it in terms of coal characterization information available from ultimate and proximate analyses of the coals. The newly developed generalized model is capable of predicting the adsorption of pure and mixed gases on coals within three times the expected experimental uncertainties, on average. The model was further validated by comparison with an external data set comprised of CO2 adsorption isotherms on 27 diverse coals. Results indicate that, on average, the SLD-PR generalized model provided a priori predictions of the CO2 adsorption data on the 27 coals with an average absolute percent deviation of 12%, based solely on coal characterization information for these coals. As such, the generalized adsorption model appears to be sufficiently robust to be useful in CBM reservoir simulators.