Testing CO₂ compatibility of a glauconitic reservoir: Case study from depleted Nini Field, Danish North Sea

A depleted oil field in the Danish North Sea is investigated for possible long-term storage of CO2. The injectivity of this sandstone reservoir, which consists of glauconite clay mineral up to 25 vol.%, to CO2 is monitored during cyclic flooding of CO2 and synthetic formation brine. This 'cyclic CO2 injection configuration' mimics the expected condition during a CO2 storage operation with a reservoir pressure of 200 bara and temperature of 60 °C. In one core flooding test, the initial brine permeability of the sandstone sample is 978 mD. In the first CO2 injection at flow rate of 800 ml/h, the permeability stabilizes at approximately 180 mD. In the second cycle, in the period of CO2 injection after brine injection, CO2 permeability stabilizes at approximately 200 mD. This suggests that the reservoir can sustain a cyclic injection scheme which is a premise for project. Changes in the sandstone minerals are investigated in batch experiments at 180 bara and 65 °C. Analysis of fluids withdrawn after exposing the rock material to CO2 for one month shows that the pH decreases, and the alkalinity and the concentration of dissolved Fe increases. This is consistent with observed dissolution of siderite from petrographic studies.