Gas adsorption/diffusion in bidisperse coal particles: Investigation for an effective diffusion coefficient in coalbeds
Conference Paper
Overview
Overview
abstract
Pore structure of coalbeds exhibits multi-scale heterogeneity. It is common practice to characterize the coalbeds using two distinctive porosity systems: a well-defined and uniformly distributed network of natural fractures, and nearly impermeable and non-uniform coal matrix blocks. The blocks consist of microporous solids with large internal surface area and strong affinity for some naturally occurring chemical species such as methane and carbon dioxide. At high coalbed pressures, therefore, these species exist abundantly and/or could be stored in large quantities at a physically adsorbed liquidlike state. Much work has been carried out on adsorption capacity of various coals. Diffusive transport processes within the matrix blocks could be the rate limiting step for adsorption during gas injection and production operations. Identifying these processes and determining their contributions to overall (upscaled) mass transport is a complex and time consuming procedure. The paper presents numerical diffusion models in varying coal particles and investigates transport mechanisms. For this purpose, the coal particle is represented as a microporous solid penetrated by a network of larger interconnected macropores. The solid consists of pores of the order of a few molecular diameters and adsorbs the bulk of the gas. A simple relationship between the apparent and intrinsic (macropore and solid) Fickian diffusion coefficients is shown to exist in the case of single-component (methane) nonlinear Langmuir-type adsorption. Mass transport in the bidisperse coal particle is significantly influenced by the adsorption in microporous solid. The investigation is then extended to study concentration dependence of the microporous solid diffusion for binary (methane-CO2) mixtures. It is found that co-diffusion of the gas molecules enhances significantly, while counter-diffusion diminishes the mass transport in the solid in the presence of competitive sorption dynamics. The isotherm nonlinearity effects and the influence of lateral interactions among the adsorbed molecules in the solid phase are discussed. A sensitivity analysis is given to identify conditions that promote desorbed methane production from and adsorbed CO2 storage in the microporous solid. The work finds application in modeling CBM and ECBM processes.
