Extension of Vertical Equilibrium Model for Two-Phase Displacement in Layer-Cake Reservoirs: Accounting for Water-CO2 Partial Miscibility and Fines Migration

Numerical simulations of geological CO2 storage in deep saline aquifers have demonstrated that vertical equilibrium (VE) models offer a robust and computationally efficient framework for reservoir optimization and upscaling. These studies emphasize the influence of fines migration and partial miscibility between water and CO2 on the evolving storage behaviour. Specifically, capillary-driven mobilization of clay and silica fines can impair injectivity, while water evaporation into the CO2 phase leads to near-wellbore drying, salt precipitation, and additional injectivity loss. However, conventional VE models do not account for these coupled physical and geochemical mechanisms. This work introduces an extended VE model that incorporates water vaporization into CO2, CO2 dissolution into the displaced brine, and fines migration leading to permeability reduction. For layer-cake aquifers, the model admits an exact analytical solution, providing closed-form expressions for both injectivity decline and sweep efficiency. The analysis identifies two distinct fronts during displacement: a leading displacement-dissolution front and a trailing full-evaporation front. Vertical model downscaling further reveals that gas saturation varies with the depth-dependent permeability profile. In formations where permeability decreases with depth, saturation declines monotonically; in contrast, in systems with increasing permeability, the competition between viscous and gravitational forces can generate non-monotonic saturation profiles. The results also indicate that while fines migration reduces injectivity, it may simultaneously enhance sweep efficiency; highlighting a complex trade-off that must be considered in the design and optimization of CO2 storage operations.