3D Static Modeling and CO₂ Static Storage Estimation of the Hydrocarbon-Depleted Charis Reservoir, Bredasdorp Basin, South Africa

An essential greenhouse gas effect mitigation technology is carbon capture, utilization and storage, with carbon dioxide (CO₂) injection into underground geological formations as a core of carbon sequestration. Developing a robust 3D static model of the formation of interest for CO₂ storage is paramount to deduce its facies changes and petrophysical properties. This study investigates a depleted oilfield reservoir within the Bredasdorp Basin, offshore South Africa. It is a sandstone reservoir with effective porosity mean of 13.92% and dominant permeability values of 100–560 mD (1 mD = 9.869233 × 10^–16 m²). The petrophysical properties are facies controlled, as the southwestern area with siltstone and shale facies has reduced porosity and permeability. The volume of shale model shows that the reservoir is composed of clean sands, and water saturation is 10–90%, hence suitable for CO₂ storage based on petrophysical characteristics. Static storage capacity of the reservoir as virgin aquifer and virgin oilfield estimates sequestration of 0.71 Mt (million tons) and 1.62 Mt of CO₂, respectively. Sensitivity studies showed reservoir depletion at bubble point pressure increased storage capacity more than twice the depletion at initial reservoir pressure. Reservoir pressure below bubble point with the presence of gas cap also increased storage capacity markedly.