Injectivity analysis and determination of saturation functions during cyclic injection of supercritical CO₂ in Nini West sandstone

Intermittent CO₂ injection, alternating with brine, is a realistic scenario in CO₂ storage operations, often resulting from injection strategies or maintenance activities. This cyclic injection can influence reservoir reactivity and injectivity. To investigate the risk of injectivity impairment due to cyclic injection, a laboratory flooding experiment is conducted on core samples from the Nini-4 well (Frigg sand, Horda Formation, Danish North Sea) under reservoir conditions of 60 °C and 200 bar, simulating supercritical CO₂ (scCO₂) injection and brine backflow. The experiment includes two drainage and one imbibition periods. Produced water is measured during both scCO₂ and brine injection using an acoustic separator, enabling differentiation between water displaced and dissolved in the pore space. A compositional numerical modeling approach using the CMG GEM model is employed for history-matching the experimental data to determine the relative permeability (Kr) and capillary pressure (Pc) curves, besides accounting for water evaporation to predict saturation data. Primarily, the results indicate that injectivity is significantly enhanced over extended periods up to 200 pore volumes and restored to initial brine permeability due to a dominant drying-out effect in both drainage periods, in which differential pressure and saturation data show strong agreement. Secondarily, identical Kr and Pc functions are derived from both drainage periods, differing from the imbibition period, highlighting hysteresis effects. These findings provide insights into near and far wellbore flow behavior under cyclic injection, which are directly relevant for the Nini West site's maturation and certification process, supporting its viability for long-term CO₂ storage.