Fluid-rock interactions during intermittent injection of supercritical CO₂: An investigation for CO₂ storage in the depleted Nini West oil field, Danish North Sea

CO₂ that is planned to be stored in the depleted Nini West oil field will be delivered by ship which results in intermittent injection of supercritical CO₂ (scCO₂). It is important to investigate the effect of this injection scenario on rock specimens in the early phase of the storage program. In this regard, core flooding experiments have been conducted at current reservoir conditions of 200 bar and 60 °C. The state of plug samples pre- and post-scCO₂ injection were compared by coupled scanning with the scanning electron microscope (SEM) and micro-CT scanning to track mineralogical changes at core-scale. The coupled approach shows partial dissolution of garnet, specifically almandine and carbonate minerals such as siderite, rhodochrosite and calcite in the glauconitic sandstone after 5 sets of scCO₂ injection periods. Effluent brine ion content measured after each brine injection operation shows an increase in Fetot, Si+, and Mn2+ in the effluent brine compared with the injection brine, which agrees with dissolution of minerals captured by the SEM and micro-CT scan. ScCO₂ effective permeability at the end of each scCO₂ injection period ranges between 18 and 34 % of the initial brine permeability at 100 % water saturation (Sw). The conclusion is that CO₂ permeability over these periods of injection is not affected by neither the dissolution of minerals, since these minerals are present only in low concentrations, nor the salt precipitation due to scCO₂ injection.