Influence of capillary pressure and trapping hysteresis on large-scale CO2 migration

Long-term, large-scale CO2 migration in saline aquifers has previously been modelled using a vertical equilibrium formulation as a faster computational alternative to fully three-dimensional models. Previous vertical equilibrium studies have increased complexity from a simple sharp-interface approximation to the inclusion of a capillary fringe which have shown to significantly affect the migrating plume shape and extent in the post-injection period. The focus of this work is the inclusion of a fully implemented hysteresis model for the capillary pressure function within the vertical equilibrium framework. Previous reduced-modelling efforts employed simplified computational implementations for the post-injection problem. The effects of drainage and imbibition cycles in fine-scale capillarity, which in turn affects the vertically integrated functions, are investigated, with the goal of studying the impact on the CO2 migration extent and trapped CO2. The results indicate that higher order hysteresis has little effect on upscaled capillary pressure functions. Their impact on large-scale migration is therefore also limited. First-order imbibition scanning curves have a direct impact on the plume and are important in establishing the characteristics of residual trapping in the aquifer. In addition, the characterisation of the capillary fringe structure affects the plume shape and migration extent, with variation in the capillary fringe model resulting in differences of the same order as those obtained from variability in other model parameters.