Insight on Coal Swelling Induced by Monolayer and Multilayer Adsorption

Summary Gas adsorption, desorption, and displacement occur throughout the coalbed methane (CBM) and enhanced coalbed methane (ECBM) recovery process, causing the coal pores to deform and affecting injectivity and productivity. The primary objectives of this study include determining the pressure at which the monolayer adsorption transitions to multilayer adsorption and linking the swelling strain to both classes of adsorption. In this study, the simplified local density (SLD) theory was first modified and applied to describe the characteristics of both types of adsorption and to determine the transition pressure. A strain model coupled with the SLD theory was then developed to describe adsorption-induced deformation. Next, the measured methane (CH4) and carbon dioxide (CO2) adsorption isotherms, and strain data on the same coals, were collected for model validation. Results suggest that gas adsorbed on coal surfaces at the very beginning (monolayer adsorption) and the adsorption on other gas molecules continues once the surface has been filled (multilayer adsorption). Results also suggest that swelling strain is proportional to both types of adsorption, with the multilayer case being larger than the monolayer case. This difference may be due to the additional repulsion between the adsorbate and multilayer liquid film.