Caprock integrity: A critical factor for carbon capture and storage in the Vienna Basin

It is commonly accepted that geologic CO₂ sequestration will be needed to meet carbon emission goals and reduce the impact of anthropogenic climate change. While the feasibility of carbon capture and storage (CCS) in saline aquifers and depleted oil and gas fields is proven, the social acceptance for CO₂ injection into geological formations remains low. Most sites under development are located offshore, while onshore storage is generally perceived more critically. The long-term integrity of barrier layers in the storage complex is considered a major risk factor to be acknowledged in approval procedures. This study therefore aims at providing a comprehensive view on the CO₂ seal capacity of mudstones in the Vienna Basin, a potential target area for future onshore CCS in depleted oil and gas fields or saline aquifers. In a first step, the static capillary seal capacity was modelled based on wireline log-derived porosity vs. depth trends. Secondly, all processes potentially causing a CO₂ breakthrough into the seal were identified and their respective contributions to CO₂ leakage from a hypothetical storage complex were quantified. Lastly, a 1D reactive transport model was established to evaluate mineralogy and porosity changes in a seal layer of known composition and formation water chemistry over post-operational time spans of 1000–100,000 years. For both static and dynamic sealing scenarios it is shown that seal capacity in the Vienna Basin is high, and storage risks associated with top seal integrity are likely negligible.