Potential CO2 storage sites Stenlille • The reservoir occurs within a domal closure and is overlain by a 300 m thick clay sequence of the Fjerritslev formation. • A 140 m thick upper Triassic Gassum sandstone formation 1500– 1600 m below the surface. • As of 2017, there are 14 injection and production wells as well as 6 observation wells. • Smooth operation of the natural gas storage site since 1989. • The geological setting at Stenlille is assumed to represent a good analog to the Havnsø structure since similar lithologies and burial depths for the Gassum formation are expected. • The average porosity (25%) and permeability (250 mD) in the Gassum sandstone reservoir at Stenlille represent good conditions for geological CO2 storage. • The Stenlille dataset is the most comprehensive onshore dataset in Denmark (cuttings descriptions, core analysis data, wireline logs, well completions reports, well test reports, seismic data, geochemical data, and historical records of injection and production rates of natural gas, as well as bottomhole and wellhead pressures and temperatures). Havnsø • One of the most prospective CO2 storage sites in Denmark. • Proximity of large CO2 emission sources: a biomass-fired power plant and a refinery close to Kalundborg; supplementary CO2 point sources in Copenhagen. • No wells have been drilled and only low-quality seismic data is available (new seismic was acquired in August-October 2022). Reservoir models • CO2-water isothermal model without geochemical reactions • Phase properties are modelled as functions of pressure. Stenlille • The model, covering approximately 7,760 x 3,570 x 137 meters, contains 119,634 active cells with properties, adjusted to represent faults and fractures. • The reservoir consists of 6 zones, separated by impermeable layers. • The model is history matched to natural gas injection/production and pressure data, which makes it a good candidate to model CO2 injection. Havnsø • The reservoir model is based on the Top Gassum map, obtained from seismic interpretation. • A uniform thickness reservoir of approximately 20,000 x 15,000 x 150 m has 4,959,998 active cells. • The distribution of rock properties follows the suggested depositional pattern from Stenlille. In a net-zero CO2 emission economy, the CO2 transportation and storage networks will need to accommodate fluctuations in CO2 feed- flows. This calls for a need to assess different strategies for temporary storage of CO2. The objective of the present work is to evaluate the efficiency of temporary CO2 storage in saline aquifers for the case of two geological sites in Denmark, Stenlille and Havnsø. Specifically, we study the sensitivity of CO2 recovery factor with regards to the parameters of the CO2-water relative permeabilities and capillary pressures. The results of the sensitivity study demonstrate a critical role of rock-fluid interactions and accurate reservoir characterization on the estimates of CO2 recovery. In all considered injection/production scenarios, the efficiency of temporary CO2 storage does not exceed 30%; achieving this recovery factor is only possible if a large amount of CO2 is injected in the reservoir for permanent storage (several hundred thousand tons for the considered cases). The duration of this initial CO2 injection is of the order of years so that the caprock integrity will not be compromised. Sensitivity study on CO2 recovery during temporary storage • Sampling the parameters of the saturation functions within the ranges from the literature. • Quantifying the impact of uncertainty in relative permeability and capillary pressure on the CO2 recovery factor (RF) during temporary storage. • Modelling CO2 injection in a vertical well, located at the top of the Stenlille and Havnsø structures. • Injection for 10 years with a constant bottomhole pressure control, followed by 10 years production with the constant topside pressure control. Stenlille • A large spread in CO2 recovery factors, depending