Experimental Investigation of the Effect of Major Impurities in the CO2 Stream on Carbon Storage in Chalk Reservoirs

Abstract Depleted oil and gas reservoirs in the Danish sector of the North Sea are predominantly composed of chalk, characterized by high porosity, low permeability, and reactivity. While storing CO2 in these reservoirs presents challenges, due to the reactivity of calcium carbonate and the low permeability, mitigating the risks could unlock substantial CO2 storage capacity. This study specifically investigates the impact of major reactive impurities such as H2S, NO2, and SO2 in the injected CO2 stream on calcite dissolution and its implications for rock integrity. These impurities can be present in captured CO2 and an important factor in carbon storage feasibility studies is the maximum tolerable levels of impurities for storage safety. Dynamic injection experiments were carried out on reservoir material from a Danish North Sea mature oil field. The experiments were carried out in a core flood injection set-up designed to mimic reservoir conditions. These experiments were designed with alternating Gas mixture/Water injection scenarios into separate core plugs, with the effluent brine samples analyzed by ion chromatography to assess rock dissolution. Porosity and permeability measurements were conducted before and after the experiments. Results revealed that calcite dissolution notably increased when SO2 was present in the CO2 stream, with H2S enhancing dissolution to a lesser extent, and NO2 exhibiting the least effect. Comparison with a base case of pure CO2 exposure showed calcite dissolution but no significant changes in porosity and permeability. The dynamic experiments demonstrated that the measured calcite dissolution occurs regardless of injection rate and takes place primarily at the gas-water interface. The experiments consider the worst-case scenario locally in the reservoir, with a relatively high concentration of reactive impurities in the CO2 allowing for a comparison between the three impurities H2S, NO2, and SO2, and the core scale and in experiments lasting several days. This research contributes novelty to the study of CO2 storage in chalk reservoirs, which remains relatively pioneering due to safety concerns. The exploration of impurity effects under dynamic conditions, as presented in this study, represents a notable innovation, addressing a gap in the literature dominated by batch experiments and thermodynamic modeling.