Assessing storage injectivity and rock physics alteration during dry and wet CO₂ injection for the storage prospect of the Gassum Formation in the Stenlille Aquifer, Denmark

Saline aquifers are prime candidates for carbon dioxide (CO₂) storage. However, some projects encounter challenges such as salt precipitation due to water evaporation during CO₂ injection. While salt precipitation alters reservoir porosity and permeability, the level of CO₂ injectivity remains higher compared to brine displacement using water-saturated CO₂. This study examines the effects of water-saturated (wet) and undersaturated (dry) supercritical CO₂ (scCO₂) injection on the properties of brine-saturated Stenlille sandstone reservoir cores from the Gassum Formation. Three high-volume core flooding experiments under reservoir conditions were conducted, analyzing dynamic differential pressure, in situ saturation changes (measured with a high-pressure acoustic separator), and pre- and postexperiment core properties. The results showed distinct differences between dry and wet CO₂ injection, particularly regarding the vaporization and salt precipitation effects on permeability. Dry CO₂ injection reduced water saturation to as low as 4% PV, simulating dry-out near injection wells, and caused absolute permeability changes of 21% on average due to salt precipitation. In contrast, wet CO2 injection, representative of bulk reservoir flow, preserved high water saturations without salt precipitation. Additionally, the final relative permeability to supercritical CO₂ (KrscCO₂) was higher for the dried core (0.62) compared to that of the wet injection experiment (0.40). This suggests that water vaporization during dry CO₂ injection enhances CO₂ effective permeability despite salt precipitation. Comparing wet CO₂ injections in samples with different permeabilities using dimensionless numbers revealed that unstable flow in high-permeability channels leads to higher overall relative permeability and residual water saturation. The comprehensive experimental data from this study provide valuable insights for simulating two-phase scCO2–brine displacement and dissolution processes under reservoir conditions.