Experiments on laboratory and field scale have been performed to study physical and chemical interactions in the short term between CO₂ and impurities, brine and reservoir rock.

Focused on the laboratory scale, tests with pure CO₂ and CO₂ containing SO₂ at 150 barg and 45 °C were done. As core findings: the porosity was much lower after the injection of CO₂ with 5%v of SO₂; the effluent pH for the pure CO₂ injection tests was in the range of 7–8 and in the range of 1–2 if impure CO₂ was injected; regarding the ion migration, chemical modifications were detected with the mixture of CO₂ and CO₂, being mainly based on the modification of Mg²⁺ and SO₄²⁻.

The field scale tests were based on the injection of CO₂ and synthetic air in Hontomín Technology Development Plant. The fluid density was calculated using the two pressure transmitters installed at the bottom of the injection well; comparing the base case (pure CO₂) with the case of CO₂ containing 5.1%v of air, the density decreases from 840 kg/m3 to 775 kg/m3 at the storage conditions (i.e. 8% of diminution in the density).

The well head pressure (WHP) needed to inject the mixture of CO₂ and 5.1%v of air increased from 80 bar (corresponding to the case of pure CO₂, to 90 bar. That effect reveals that the injection of N₂ and O₂ in concentrations of 5.1%v of air (79% N2 and 21% O₂) is less efficient than the injection of pure CO₂.