Stability of two-phase flow with interfacial flux in porous media: CO₂ mineralization

The primary objective of carbon capture, utilization, and storage (CCUS) applications in various natural and engineered porous materials is to achieve a stable and planar CO₂ displacement front, thereby suppressing viscous fingering. A stable front can ensure uniform and exhaustive CO₂ mineralization throughout a reactive medium (i.e., mineral carbonation). Drawing inspiration from experimental observations of CO₂ flooding into cores of portland cement-based materials, we examine the stability of such systems. Under these conditions, the injected CO₂ continuously dissolves into the resident water phase, which becomes chemically disequilibrated with the solid minerals and leads to mineral carbonation on the wetted surfaces. Focusing on the early injection time allows us to reduce the complex multiphysical problem to a simple two-phase flow scenario of immiscible displacement with a CO₂ interfacial flux sink. The formulated equations are investigated using numerical simulations and linear stability analysis, which results in a closed-form criterion, and provide fundamental insights into system stability. Overall, the results show that several effects combine to stabilize the system, including the sink effect, which acts to eliminate instability; the reduction in flow velocity along the flow path, which limits flow focusing; and the relative increase in stabilizing capillary forces. Therefore, if the system is stable at early stages, it will likely remain stable later on. Finally, this research demonstrates the use of theory to simplify complex problems and shows that even when flow is inherently coupled, the state of systems can often be determined from fluid stability alone.