Analytical Modeling of CO2 Sequestration and Enhanced Coalbed Methane Recovery (ECBM)

Injection of CO2 into deep unminable coal seams is an option for geological storage of CO2. In many industrial settings, pure CO2 streams are expensive to obtain and a mixture of CO2 and N2 would be less expensive. New analytical solutions are presented for two-phase, four-component flow with volume change on mixing in adsorbing systems. Analytical solutions have been reported previously for single-phase three-component gas flow (Zhu et al.) and for multicomponent flow of incompressible fluids with adsorption (Johansen and Winther, Dahl et al., Shapiro et al.). <br><br>In this paper, we analyze the simultaneous flow of water and gas containing multiple adsorbing components by the method of characteristics. Mixtures of N2, CH4, CO2 and H2O are used to represent the ECBM-flue gas process. The displacement behavior is demonstrated to be strongly dependent on the relative adsorption strength of the gas components.<br><br>N2 and CO2 recover CH4 through different mechanisms: CO2 preferentially adsorbs onto the coal surface, resulting in a shock solution; while N2 displaces CH4 by reducing the partial pressure of CH4, resulting in a rarefaction solution. When mixtures of N2 and CO2 are injected, the displacement exhibits both shock and rarefaction features. For CO2-rich flue gas, a path that includes a switch between branches of non-tieline paths is observed, a feature not previously reported for gas-liquid displacements. In these solutions, an additional key tieline, at which the switch between non-tieline paths occurs, is required. In the shock along this tieline, the non-tieline eigenvalues of injection and initial segments and the tieline shock velocity are equal.<br><br>Analytical solutions to ECBM processes provide insight into the complex interplay among adsorption, phase behavior 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.<br>