Effects of Wetting Conditions and Bottomhole Pressure on CO2 Plume Spread and Saturation in Saline Aquifers

This study develops a fully compositional, geochemically enabled CMG-GEM model, benchmarked to the Illinois Basin-Decatur Project and adapted to a deep onshore Louisiana saline aquifer, to quantify how Bottomhole pressure (BHP) and wettability jointly control long-term CO2 trapping and plume behavior over a 200-year horizon. The novelty of the work lies in (i) explicitly partitioning injected CO2 among supercritical, dissolved, aqueous-ion, residual, and mineral pools through time, and (ii) systematically linking these trapping budgets and plume footprints to realistic operating BHPs and contrasting water-wet versus CO2-wet systems. The framework provides a site-specific but transferable workflow that couples geologic characterization, compositional flow, and geochemistry for storage-security assessment. BHP and wettability are emphasized because they represent the two most influential and practically tunable levers for CO2 storage projects: BHP is a direct outcome of injection strategy and surface facilities, while wettability captures a key subsurface uncertainty that strongly affects capillary trapping and relative permeability. Other parameters (rock and fluid properties, structure) are constrained by site characterization and kept fixed to isolate the impact of these two controls. Results show that lower BHP accelerates the transition from mobile supercritical CO2 to more secure dissolved and residual forms. At 3500 psi, dissolved CO2 overtakes the supercritical inventory roughly five decades after injection ceases (around 2090), whereas at 4000 psi the supercritical phase remains slightly dominant and a full crossover is not reached within the 200-year window. At 4500 psi, supercritical CO2 persists as the largest storage component, leading to the widest plume and greatest long-term migration risk. Wettability further modulates trapping: in the water-wet case, supercritical CO2 still accounts for about 35-40% of stored CO2 after 200 years, while in the CO2-wet scenario it declines to less than 30% and is overtaken by dissolved CO2, which grows to around 40% of the inventory. These findings give industry practical guidance for setting conservative BHP limits and interpreting wettability assumptions in terms of storage efficiency and migration risk in Gulf Coast saline formations. The workflow can support screening of injection strategies, design of pressure-management and monitoring programs, and provide a benchmark for future CCUS optimization and storage-performance studies. Keywords: CO2 storage, wettability, plume migration, saline aquifers, injection pressure, CCUS.