| Injection of CO2 into deep unminable coal seams is an option for geological storage of CO2 as a way to reduce emissions to the atmosphere from fossil fuel use while enhancing the recovery of CH 4 from coalbeds. In many industrial settings, pure CO2 streams are expensive to obtain and injection of a mixture of CO2 and N 2 would be more cost effective. New analytical solutions are presented for two-phase, multicomponent flow with volume change on mixing in adsorbing systems.; The method of characteristics is used to analyse the simultaneous flow of water and gas containing multiple adsorbing components. Mixtures of N 2, CH4, CO2 and H2O are used to represent the enhanced coalbed methane (ECBM) recovery process. The displacement behaviour of CH4 is demonstrated to be strongly dependent on the relative adsorption strength of gas components. More strongly adsorbing components, such as CO2, preferentially adsorb onto the coal surface, resulting in a shock solution, while weaker adsorbing components, such as N2, displace CH4 by reducing the partial pressure of CH4, resulting in a rarefaction solution. When mixtures of more strongly adsorbing gases and weaker adsorbing gases are injected, the displacement exhibits both shock and rarefaction features.; New composition paths are presented for certain injection gas mixtures consisting of the most volatile component and the least volatile intermediate component. These paths use the increasing saturation branch of the nontie-line path, previously not analysed as an allowable path for continuous variation. Degenerate shocks may be present in the solution structure. This work presents the first instance of degenerate shocks occurring in porous media flow.; Analytical solutions to ECBM recovery processes provide insight into the complex interplay among adsorption, phase behaviour and convection. Improved understanding of the physics of these displacements will aid in developing more efficient and physically accurate techniques for predicting the fate of injected CO2 in the subsurface. |
