Abstract

Estimating the EOR potential in producing oil fields is an important input to decision making if large scale CO2-EOR is going to be employed in the North Sea. This paper describes the results from simulations of CO2 injection into conceptual reservoir models representative of water flooded oil fields in the North Sea.

Enhanced oil recovery (EOR) by CO2 injection is an attractive option because it has the potential to increase the oil, gas and condensate recovery of producing fields. In the North Sea a majority of the oil reservoirs have been subject to massive strategic and efficient water flooding resulting in high recoveries for most of the cases. However, more advanced tertiary recovery methods are sought to increase the recovery. Using CO2 as injection fluid has several advantages. Most oil compositions in the North Sea are miscible with CO2 at reservoir conditions something that will enable miscible displacement of the oil targeting residual (capillary trapped) oil after water flooding. The density of CO2 at reservoir conditions is in most cases lesser than the injected water and it may therefore reach other parts of the reservoir and consequently improve the sweep efficiency.

The North Sea shows a great variation in type of oil reservoirs and traps ranging in geological age from the Late Paleozoic to the Cenozoic. To cover the range of different geological classes, traps and recovery strategies a large number of scenarios has been simulated on conceptual sector models. Conceptual live reservoir oil is composed to represent fluid properties of a selection of 55 water flooded oil reservoirs in the North Sea that are considered potential candidates for CO2 injection. The selected reservoirs comprise 30 reservoirs in the UK sector, 20 in the Norwegian sector and 5 reservoirs in the Danish sector. The conceptual fluid model is composed and tuned to reflect the hydrocarbon pore volume (HCPV) weighted average properties of these 55 oil reservoirs.

Representative petrophysical properties and fluid compositions have been used in the models in order to schematically account for heterogeneities and phase behavior of the different reservoir types. Different injection schemes including CO2 injection with and without recirculation of CO2 breakthrough gas and CO2-WAG have been evaluated.

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