Contrasting storage security in closed and open aquifers: numerical modeling of CO2 sequestration in the Bredasdorp Basin, South Africa

Abstract

Geological carbon storage is a critical pathway for reducing anthropogenic carbon dioxide (CO2) emissions and achieving energy sustainability, particularly in fossil fuel-dependent nations like South Africa. This study presents a numerical simulation of CO2 storage in a saline aquifer within Bredasdorp Basin, offshore South Africa. Using a compositional reservoir simulator, we compared two end-member boundary scenarios, a closed aquifer (no fluid exchange) and an open aquifer (fully permeable lateral boundaries), under identical injection conditions (762,000 m3/day for 20 years, followed by 20 years of post-injection monitoring). Results reveal that boundary conditions fundamentally control storage performance. In the closed aquifer, rapid pressure buildup (a 115% increase to 56,100 kPa) restricted the CO2 plume to the wellbore vicinity, suppressed dissolution, and left 73.9% of the injected CO2 in a mobile supercritical state after 40 years, posing significant geomechanical and containment risks. In contrast, the open aquifer maintained near-hydrostatic pressure (24,900 kPa), enabling extensive lateral plume migration (2-3 times greater lateral extent) and enhancing dissolution; by 2070, only 57.9% of the injected CO2 remained mobile, with 35.7% dissolved in brine. The contrasting density anomalies (∼25 kg/m3 in the closed system versus ∼13 kg/m3 in the open system) imply divergent potentials for long-term convective mixing and mineral trapping. These findings demonstrate that open, hydraulically connected aquifers offer faster progression to secure storage but require larger monitoring footprints, while closed aquifers concentrate the plume but demand rigorous pressure management. This work provides a foundational framework for site-specific carbon storage assessments in South Africa's offshore basins.