Experimental and numerical investigations of gas sorption on coal, and the subsequent volumetric and permeability changes of the coal were conducted. The goals of the study were to investigate the magnitude of permeability change caused by gas sorption, and develop an algorithm to simulate numerically gas sorption and sorption-induced permeability change. The amount of gas sorption and the subsequent volumetric and permeability change of coal samples as a function of pore pressure and injection gas composition were measured in the laboratory. A constant effective confining pressure (difference between the confining pressure and pore pressure) was maintained in the process of the experiments; therefore, the role of effective stress on permeability was eliminated. Several gases, including pure CO₂, pure N₂, and binary mixtures of CO₂ and N₂ of various compositions were used as the injection gas. The coal sample was first allowed to adsorb an injection gas fully at a particular pressure. The total amount (moles) of adsorption was calculated based on a volumetric method. After adsorption equilibrium was reached, gas samples were taken from the equilibrium gaseous phase and analyzed afterwards. The composition of the gaseous phase prior to and after the adsorption was used to calculate the composition of the adsorbed phase based on material balance. Permeability of the sample was then measured by flowing the injection gas through the core at varying pressure gradient or varying flow rate, and an average permeability was obtained based on Darcy's law for compressible systems. The change of the total volume of the core was monitored and recorded in the whole process of the experiment. Volumetric strain was thereby calculated. Experimental results showed that the greater the pressure the greater the amount of adsorption for all tested gases. At the same pressure, the amount of adsorption was greater for CO₂ than N₂. For the binary mixtures, the greater the fraction of CO ₂ in the injection gas, the greater the amount of total adsorption. Volumetric strain followed the same trend as the amount of adsorption with pressure and injection gas composition. Permeability showed opposite behaviors, decreasing with the increase of pressure and the percentage of CO₂ in the injection gas.

The experimental adsorption, volumetric strain, and permeability data were analyzed to investigate the numerical correlations between gas sorption, sorption-induced volumetric strain and permeability, and pressure and injection gas composition. The relationship between the amount of adsorption and pressure for pure gases (CO₂ and N₂) were readily represented by parametric isotherm models, such as Langmuir and the N-layer BET equations. Modeling efforts of multicomponent adsorption included predicting amount of adsorption and adsorbed phase composition based on the extended Langmuir equations and the ideal adsorbed solution model. Activity coefficients of the components in the adsorbed phase were computed based on the real adsorbed solution model and the ABC excess Gibbs free energy model. Algorithms for modeling the CO ₂/N₂-Coal system were developed, and the constraints and strength of each model were discussed. The experimental volumetric strain was found to be linearly proportional to the total amount of adsorption and independent of the injection gas composition. The permeability reduction could not be readily correlated by the models in the literature unless the change of other coal properties (bulk modulus, axial constrained modulus, etc.) due to gas sorption was incorporated.

The sorption, volumetric strain, and permeability data collected in this study can be used for comparison by other researchers conducting similar studies. The algorithms of sorption modeling and the correlations developed in this study are readily incorporated into the simulation of enhanced coalbed methane recovery and CO₂ sequestration in coalbeds. (Abstract shortened by UMI.)