Partly miscible CO2-water displacement under vertical equilibrium with fines migration in layer-cake reservoirs

Abstract

Recent numerical simulations of geological CO2 storage in deep saline aquifers have demonstrated that vertical equilibrium (VE) models provide a robust and computationally efficient approach for reservoir prediction and upscaling. These studies also underscore the significant impact of fines migration and partial water-CO2 miscibility on the temporal evolution of storage behaviour. Specifically, capillary-driven detachment of clay and silica fines can reduce well injectivity, while water evaporation into the injected CO2 causes near-wellbore drying, salt precipitation, and further injectivity impairment. However, current VE models do not capture these coupled physical and geochemical phenomena. For the first time, this work develops an extended VE model with the following novel features: (i) permeability-reducing migration of natural reservoir fines, (ii) water evaporation into injected CO2, (iii) CO2 dissolution into the displaced brine. For a stratified (layer-cake) aquifer, an exact analytical solution of the developed model has been derived. It provides explicit formulae for sweep efficiency and injectivity decline. The analytical model reveals two distinct displacement fronts – the displacement-dissolution front (advanced front) and the full evaporation front (receded front) – which bound the two-phase flow region. The model downscaling reveals that in formations with decreasing permeability with depth, gas saturation declines monotonically with depth. Alternatively, in reservoirs with increasing permeability with depth, the competition between viscous and gravitational forces can result in non-monotonic vertical saturation profiles. The analysis also shows that fines migration, while detrimental to injectivity, enhances sweep efficiency – highlighting a complex trade-off in the design and assessment of CO2 storage operations.