Depleted chalk oilfields may constitute potential CO 2 storage sites, but the remaining oil in the reservoir may pose a risk. This study investigates the effect of scCO 2 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 which 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 scCO 2 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. ESH shows 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 scCO 2 flooding consists of solid bitumen/asphaltenes, in line with low API gravities. The non-movable oil and solid bitumen/asphaltenes 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 scCO 2 may pose a minor risk. Reflected light microscopy shows that lighter oil is commonly concentrated at one side of stylolites associated with solid bitumen/asphaltenes, suggesting oil flow is hindered and only can cross where the stylolites are absent or thinly developed.
Petersen et al. (Thu,) studied this question.