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Abstract

This paper comprises a portion of a development study to investigate the applicability of CO₂ for enhanced oil recovery and sequestration in a field in the central UKNS. The study focused on compositional simulations to predict the Minimum Miscibility Pressures, describe prognosed miscibility mechanisms and estimate incremental hydrocarbon recovery from full field simulations.

Depleted gas condensate fields are important targets for CO₂ sequestration/EOR. Once depleted below the dew point, retrograde condensate is deposited in the pore system. CO2 injection in the depleted gas condensate reservoirs may allow enhanced gas recovery by liquid re-vaporization and reservoir re-pressurization or pressure maintenance. It is also noted that an accurate EOS fluid characterization is essential for CO₂ miscible displacement, which is a composition-sensitive process. As such, compositional modelling is preferred.

For the phenomenological stage of the study, 1D slimtube compositional simulation models were used to estimate the Minimum miscibility pressure (MMP) of representative fluids in the Oil and depleted gas condensate legs. Different grid-block sizes were used to consider the effect of numerical dispersion on the MMP. Subsequently, ID finescale block compositional simulation models were used to study and verify the multi-contact miscibility mechanism existent in the depleted condensate legs.

The 3D full field model is highly compartmentalized with nine different panels having separate PVT characteristics.  Each panel has a 12 component Peng-Robinson equation of state which is conditioned to its individual fluid. CO₂ injection for repressurisation is simulated up to and beyond MMP, contact miscibility is attained and gas cycling is implemented for incremental oil recovery. After EOR, production ends and injection is continued for estimation of the maximum amount of CO₂ to be sequestered. Furthermore, typical economic analysis for CO₂ EOR project justification is done. The results enabled correct investment decision making  by showing that CO₂ EOR is technically but not economically justified for this field.