Diffusion Equilibrium Times for Langmuir Isotherm Adsorption in Shale and Coalbed Methane Systems

The ‘quasi-steady state’ (QSS) assumption is a commonly applied simplification to numerical dual porosity models. However, the assumption only becomes valid when concentration changes within the fractures are slow in relation to the time required for diffusive equilibrium to occur within the matrix (Barker, 1991). This thesis seeks to explore diffusion equilibrium times (DET) for gas diffusion with Langmuir isotherm adsorption by numerical solution and sensitivity analysis. A simple, heuristic function is derived, permitting rapid estimation of DET for individual cases. Four short case studies exploring application to shale gas and coalbed methane systems are presented.