Compositional characterisation of Nini dead oil and remaining oil in preserved, cleaned, restored, and CO₂-flooded core plugs from the Nini-4 well, Nini West reservoir. Project Greensand Phase 2, WP 3.3

The depleted Nini West oilfield (Siri Canyon, Danish North Sea) is considered a potential storage site for the safe sequestration of CO₂ in the Paleogene quartz- and glauconite-rich reservoir sandstones. The current study focuses on injectivity risks associated with reactions between injected CO₂ and the unproduced, remaining oil present in the depleted reservoir of the field. The main process investigated is the scenario in which the injected supercritical (sc) CO₂ dissolves certain fractions of the remaining oil, while other fractions, such as solid bitumen/asphaltenes, may be immobile and may have an adverse effect on reservoir capacity and CO₂ injectivity by clogging critical pore throats of the pore network.

The current study investigates the composition of the Nini stock tank oil (STO), preserved core extracts, and the remaining oil in core plugs from the reservoir sandstone. A total of 13 core plugs from the Nini-4 well oil leg were analysed, including three ‘fresh’ preserved plugs, three preserved plugs stored in brine, five plugs cleaned by three different methods, one restored plug, and one restored scCO 2-flooded plug. The hydrocarbon composition of the STO and the core extracts was characterised by gas chromatography and organic petrography. The oil in the core plugs was analysed by organic petrography and extended slow heating pyrolysis (ESH®) providing a detailed insight into the occurrence and distribution of the oil and solid bitumen/asphaltene fractions in the various plugs.

Minor oil-based mud (OBM) contamination in the diesel range was recorded in the STO and core extracts indicating that it will not interfere with the heavy oil fraction >nC 20 which may pose problems with clogging. The STO contains only a minor amount of asphaltenes. The remaining oil in the preserved plugs consists mainly of mobile oil and a lesser amount of immobile oil and solid bitumen/asphaltenes. All three cleaning methods remove most oil fractions, but none of them, including the hot Soxhlet extraction method, can remove all the solid bitumen/asphaltenes associated with glauconite. These solid bitumen/asphaltenes occur in the interior of glauconite clasts, as coatings on the surface, and in small cracks in the surface, as well as between the laminae of glauconised mica. The remaining solid bitumen/asphaltenes are ‘carried’ over to the restored (STO saturated and aged) plugs used for scCO₂-flooding experiments. A hot Soxhlet extraction cleaned plug was restored by saturation with synthetic formation brine, drainage to initial water saturation, and at the end saturation with STO and ageing for one month. The oil occurs as surface coatings and in the interior of glauconite clasts, between the laminae of glauconised mica, and as a hydrocarbon ooze. The restored oil composition is, overall, comparable to the composition of the oil in the preserved plugs.

Supercritical CO₂-flooding of a restored plug shows that the mobile oil fractions are removed while the solid bitumen/asphaltenes coatings and remnants in the interior of the glauconite clasts were not mobilised. This suggests that the solid bitumen/asphaltenes are non-movable, and that the glauconite clasts associated with solid bitumen/asphaltenes can be considered to have oil-wet surfaces. The solid bitumen/asphaltenes may have a protective effect thus mitigating glauconite swelling, disintegration, and fines generation. Glauconite clasts not associated with solid bitumen/asphaltenes are likely water-wet and potentially prone to both swelling and disintegration into fines. Negligible amounts of immobile oil are left behind in the scCO₂-flooded core plug, and the saturation of solid bitumen/asphaltenes is very similar to those of the cleaned core plugs which together with the inferior immobile oil fraction indicate no or very low risk for precipitation of solid bitumen/asphaltenes.

Measured He-porosity is slightly underestimated due to the presence of non-movable solid bitumen/asphaltenes in the cleaned plugs. A method to convert the results of vol.%-ESH to %-saturation has been derived, and the equation required to correct the He-porosimeter porosity measurements by a correction factor has been developed based on the results of the ESH from cleaned core plugs. The porosity underestimation ranges from 0.432–1.138 PU. The calculated total oil saturation from ESH data of a restored plug provides almost the exact measured saturation value, and the determined non-movable oil saturations of preserved and restored core plugs are within the range of measured residual oil saturations. This indicates that the ESH method might be used to provide a fast and low-cost estimate of the residual oil saturation after water flooding. This assumption should be verified by ESH analysis of water flooded samples.