Reactivity of glauconitic sandstones in CO₂ experiments

The Palaeogene glauconitic sandstones in the produced Nini West oilfield were successfully tested by pilot injection of ∼4 kT of CO₂ in the Spring 2023 and the application for permanent storage facility is now being evaluated at the Danish Energy Agency. The sandstones are unique by having 25–30 vol% glauconitic clasts, which consist of mixed-layer glauconitic mica/Fe-smectite. The objective of the presented study was to document the reactivity of the glauconitic sandstones with respect to deterioration and carbonation during CO₂ injection. Four flooding experiments were conducted at reservoir conditions whereby supercritical CO₂ (scCO₂) was injected in sandstone plugs saturated with synthetic brine mimicking the formation water. Different flow rates, brine volumes, presence of hydrocarbons, purity of CO₂ and numbers of cycles of brine and scCO₂ were used to evaluate how individual parameters affected the glauconitic sandstones. A range of techniques were used to characterise the sandstone samples before and after experiments, such as Micro-CT, petrographical investigations in optical and scanning electron microscope (SEM), automated quantitative mineralogy (AQM), bulk and clay mineralogy by X-ray diffraction (XRD). The glauconitic clasts show a changed colour in optical microscope and had increased iron content (based on AQM) after the experiments, caused by intraclast precipitation of iron-oxide/hydroxide after partial dissolution of glauconitic clasts and/or corrosion of the test equipment. Dissolution grooves formed in K-feldspar overgrowth during some experiments show that K-feldspar dissolved when exposed to the acidic conditions arising when CO₂ with NO₂ and SO₂ was injected. Intensive dissolution of scattered calcite cement occurred in all experiments flooded with alternating CO₂ and brine. Alteration and dissolution of siderite rhombs intensified in samples without hydrocarbons present. Carbonate dissolution and glauconitic clast alterations resulted in slight decrease in geomechanical strength, although not viewed critical for reservoir integrity.