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

Intermittent CO 2 injection, alternating with brine, is a realistic scenario in CO 2 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 CO 2 (scCO 2 ) injection and brine backflow. The experiment includes two drainage and one imbibition periods. Produced water is measured during both scCO 2 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 CO 2 storage. • Injection of scCO 2 up to 200 pore volumes in drainage periods strongly enhances the CO 2 end point permeability • A drying-out effect, which could enhance the CO 2 permeability at the end of both drainage periods, has occurred, showing the repeatability and validity of the phenomenon • No effect of salt precipitation on CO 2 effective permeability has been observed • The effect of hysteresis is captured by relative permeability and capillary pressure functions, comparing the imbibition period with the drainage periods after history-matching the experimental data (both dP and saturations)