Movable and non-movable hydrocarbon fractions in an oil-depleted sandstone reservoir considered for CO2 storage, Nini West Field, Danish North Sea

Abstract

The Paleocene–Eocene quartz- and glauconite-rich reservoir sandstones of the depleted Nini West oilfield in the Siri Canyon, Danish North Sea, are considered a CO2 storage site. Mobilization of the remaining oil in the reservoir by the injected supercritical CO2 (scCO2) can cause oil displacement and potentially clogging of pore throats by the non-soluble heavy oil fraction. This study focuses on the movable and non-movable oil and solid bitumen/asphaltenes fractions present in the Nini West reservoir, including the effect of scCO2 injection on the residual oil. The residual oil in 14 samples from 13 core plugs from the oil-leg of the Nini-4 well was analyzed, as well as the extracted oil from preserved samples and the Nini stock tank oil (STO). The samples were acquired from preserved, cleaned, restored, and scCO2 flooded core plugs and were analyzed by organic geochemistry, including extended slow heating pyrolysis (ESH®) and organic petrography. The STO is paraffinic with a fairly high asphaltene content, and the preserved samples show that the residual oil occurs in association with glauconite clasts. The extracted oil consists mainly of movable oil fractions and a smaller amount of non-movable oil and solid bitumen/asphaltenes. The organic solvents used in different cleaning methods could remove most of the oil in the samples, but none of them can completely remove the solid bitumen/asphaltenes adhered to the glauconite clasts. The orange-brownish fluorescing solid bitumen/asphaltenes occur in the interior of the glauconite clasts, as coatings on the surface, and in small cracks in the surface, as well as between the laminae of glauconised mica. A sample from a cleaned core that has been restored to original wettability condition while controlling oil and brine saturations, and aged for a month at reservoir conditions, has an oil composition akin to the preserved samples. By scCO2 flooding, the movable oil fractions are entirely removed while the solid bitumen/asphaltenes coatings and remnants in the interior of the glauconite clasts are not mobilized. The presence of solid bitumen/asphaltenes in the organic solvent cleaned samples and in the scCO2 flooded sample suggests that these compounds are highly non-movable. Furthermore, a low proportion of heavy non-movable oil in the scCO2 flooded samples indicates minor asphaltenes precipitation, perhaps due to the high permeability and homogeneity of the sandstones.