Uncertainty in static CO₂ storage capacity estimates: Case study from the North Sea, UK

We used a sub-salt Rotliegend Group sandstone saline aquifer in the North Sea as a case study site for Monte-Carlo-based CO₂ geostorage capacity assessment. In the area of interest, this unit is characterized by sparse, low resolution, subsurface data typical of the margins of global petroleum provinces, favored for CO₂ storage. Such data scarcity leads to uncertainty regarding the complex trap geometries and ultimate CO ₂ storage capacity. The Rotliegend reservoir, estimated to have porosity and permeability ranges of 11-27% and 0.2 mD-125 mD, respectively, is sealed by Zechstein salt. The salt, predominantly halite, is a proven hydrocarbon seal in the central and southern North Sea hosting oil and gas columns of >140 m (>450 ft) and >150 m (>500 ft). Utilizing 2D-seismic data, boreholes and analogues, we estimate the pore volume of a 5-km² 4-way dip-closed structure through Monte-Carlo-based capacity simulations. We estimated storage capacity using published methodologies and compared this against a theoretical total storage calculation analogous to the gas in place equation used in the petroleum industry. We found that different methods yield a capacity range of <104 to >109 tonnes CO₂ where sensitivity analysis indicates variability in reservoir properties to be the dominant control. Thus static estimates based upon Monte-Carlo calculations present no advantage over theoretical pore volume estimations. This leaves 3D dynamic modeling of storage capacity populated by 3D seismic data and direct down-hole measurement of reservoir properties to improve confi -dence in capacity estimations as the recommended method.