Remaining oil fractions in a depleted chalk field and the potential effect of CO2 injection

Depleted chalk oil reservoirs in the Danish North Sea have potentially a large CO2 storage potential. However, some of the remaining oil is movable in the CO2-oil phase, while other heavier oil fractions are non-movable or may precipitate in pore spaces which may have an adverse effect on scCO2 injection by clogging pore throats in the reservoir matrix. A thorough understanding of the distribution of the movable and non-movable oil fractions in the depleted oil reservoir is therefore critical. In this study the spatial distribution of remaining oil fractions in a well-constrained part of a depleted chalk reservoir is documented. The oil fraction saturations and oil gravities vary together with reservoir porosity, permeability, and lithological heterogeneities. Laboratory-scale scCO2 flooding experiments under reservoir conditions were conducted to investigate in detail the effect of scCO2 injection on the remaining oil. These results can be used to predict and mitigate potential adverse effects on scCO2 injectivity and help to derisk similar fine-grained carbonate reservoirs elsewhere.