Multi-component gas transport and adsorption effects during CO2 injection and enhanced shale gas recovery

• A new mathematical model is introduced based on Maxwell-Stefan formulation.
• Injection and storage of CO2 in shale have the benefit of producing additional CH4.
• Surface diffusion of adsorbed gas molecules is an important mechanism of transport.
• The counter diffusion and competitive adsorption could generate nontrivial effects.

A new mathematical model is introduced based on the Maxwell–Stefan formulation to simulate multi-component (CH4–CO2) transport in resource shale. The approach considers competitive transport and adsorption effects in the organic (kerogen) micropores of the shale during CO2 injection and enhanced CH4 recovery. Following the primary production, injection of CO2 into organic-rich shale initiates co- and counter-diffusive transport and competitive sorption among the molecules. Consequently, the incoming CO2 molecules activate and displace the in-place CH4 molecules. Competitive sorption rates, however, could be controlled by the diffusive mass fluxes during the injection and production operations. Nature of the transport processes should therefore be understood clearly. In this paper, we first show that the widely used single-component Langmuir gas behavior is, in fact, a limiting case of the generalized formulation. The latter, however, includes not only the anticipated binary effects (due to the co-existence of two components with different molecular size and adsorption capacity) but also additional nonlinear effects due to the direction of diffusive mass fluxes and to the lateral interactions of the adsorbed gas molecules in the micropores. Following, we incorporate the multi-component formulation to a shale gas flow model to consider CO2 injection and enhanced shale gas recovery processes in a single horizontal well setup with multiple fractures. The simulation involves primary gas production for ten years followed by three-stages of operations including injection of CO2 for five years, a short soaking period, and finally production for 30 years. Dynamics of the production stages is then investigated with varying initial/boundary conditions. It is shown that the counter diffusion and competitive adsorption in the micropores could generate nontrivial effects at the reservoir-scale such that the predicted CH4 production is significantly enhanced. The investigation is important for our understanding and the design of CO2 injection and enhanced shale gas recovery processes.