Complex process couplings in systems pushed far-from-equilibrium: Applications to CO₂ sequestration in carboniferous formations

We explore important couplings between the infiltration of CO₂ into fractured coals and into shale caprocks to examine changes in the storage and transport properties that may be helpful in ensuring long term sequestration. These effects include the potential sorption of gas into organic matter in both the coals and shales with the effect of long term fixation and also the swelling-induced reduction in permeability in shales that may create a self-sealing capability in caprocks. We show how changes in permeability in both coals and shales are indexed to sorption-induced swelling, even for conditions of zero mechanical restraint. A model of a discretely-fractured medium is used to identify modes and magnitudes of permeability change, as a function of gas pressure and effective stress and related gas content. This model is used to explain observations of non-monotonic changes in permeability of both coal and shales infiltrated with CO₂.