Multicomponent Gas Diffusion and Adsorption in Coals for Enhanced Methane Recovery


Abhishek Dutta







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Selective adsorption and transport of gases in coal is important for enhanced natural gas recovery and carbon sequestration in depleted coal seams. Enhanced coalbed methane (ECBM) recovery by injection of CO2 or mixtures of CO2 and N2 is an attractive method to recover additional natural gas resources while at the same time sequestering CO2 in the subsurface. Transport and adsorption of CH4/CO2/N2 within coal is investigated from a simulation perspective.

This thesis presents a numerical model of multicomponent (ternary) gas diffusion and adsorption in coal beds, focusing on CO2, N2-CH4 counter diffusion and simultaneous adsorption associated with CO2 sequestration enhanced coal bed methane (CO2-ECBM) recovery. In this simulation study, we present, for the first time, consistent models to incorporate adsorption along with diffusion of multicomponent (ternary) gas mixtures. The diffusion model was developed based on Fick’s diffusion, integrated with Maxwell- Stefan (MS) diffusion theory. The Maxwell-Stefan (MS) diffusion formulation deals rigorously with the interactions between multicomponent gas molecules. Many of the simulation models assume the linearized theory of diffusion. We avoid this deficiency by making the Fickian diffusivities as functions of MS diffusivities and composition. The extended Langmuir and Ideal Adsorbate Solution (IAS) models are used to model the adsorption phenomenon in ternary gas mixtures. From the literature, it has been found that the extended Langmuir model is inadequate to describe the behavior of ternary CH4/CO2/N2 displacements. A sorption model more sophisticated than the Extended Langmuir model is needed to represent the dynamics of multicomponent systems. We use the IAS model and compare the predicted behavior against the extended Langmuir model. The numerical simulations in one-dimensional (1D) coal-bed are performed using a finite difference discretization method. Spatial discretization is performed using an implicit second-order centered scheme. The numerical scheme is unconditionally stable and guarantees convergence.

The comparative study suggests that the multicomponent sorption equilibria described using IAS and extended Langmuir models are quite different. IAS prediction however is strongly dependent upon the choice of pure gas isotherm equation. The results also show a significant effect of composition on diffusivities. Thus, concentration dependent diffusivities need to be taken into account in modeling the coalbed methane recovery, particularly for a multicomponent domain.

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