The effect of supercritical CO₂ flooding under two different reservoir conditions on the remaining oil in a fine-grained carbonate reservoir, Danish North Sea

Depleted chalk oilfields may constitute potential CO₂ storage sites, but the remaining oil in the reservoir may pose a risk. This study investigates the effect of supercritical CO₂ (scCO₂) injection on the oil composition for two different reservoir conditions of the Halfdan chalk field in the Danish North Sea: non-water-flooded (NWF) state and water-flooded (WF) state. Core plugs were cleaned and used for the preparation of two sets of composite cores that were restored to original NWF and WF oil- and brine-saturation conditions. A typical marine North Sea oil was used for restoration. The cores were flooded by scCO₂ under reservoir temperature and pressure conditions. A total of 21 samples were collected from the core plugs and analysed by extended slow heating (ESH) pyrolysis and reflected light microscopy. The ESH showed that most of the oil in the NWF and WF cores was mobilized and removed, decreasing the original oil saturation considerably. Most of the remaining oil after scCO₂ flooding consists of solid bitumen/asphaltenes, in line with low API gravities. The non-movable oil and solid bitumen/asphaltenes were found to amount to 2.22–3.29%, which is very close to the remaining oil saturation after cleaning. This suggests that, in this case, additional precipitation of heavy, non-movable oil (mainly solid bitumen/asphaltenes) induced by scCO₂ may pose a minor risk. Reflected light microscopy showed that lighter oil is commonly concentrated on one side of the stylolites associated with solid bitumen/asphaltenes, suggesting oil flow is hindered and only can cross where the stylolites are absent or thinly developed.