In this study, a reservoir modeling approach was used to evaluate the miscibility conditions for an Afghan oil reservoir. A model of slim-tube apparatus was built for compositional simulation in CMG-GEM. Minimum miscibility pressure/enrichment (MMP/MME) of crude oil-gas (carbon dioxide, methane and nitrogen) systems were determined by slim-tube simulation. To study the reservoir performance under each injection gas process, we built a quarter of five spot pattern model. The model was assumed to represent a homogeneous section of the reservoir. Simulation results of slim-tube test suggest that, to be miscible with reservoir oil, carbon dioxide requires less addition of enriched gas comparing to nitrogen or methane. But, the reservoir section model showed that a miscible nitrogen injection is better, with higher oil recovery factor than that of miscible carbon dioxide or methane.
Almost sixty percent of original oil in place traps in reservoir as residual oil after primary and secondary oil production (Donaldson et al., 1985). Enhanced oil recovery methods (EOR) target the remaining oil that could not be produced by conventional techniques.
Gas injection is one of the largest implemented enhanced oil recovery techniques to extract oil from a reservoir. In this method, oil is recovered by various means, such as swelling, vaporization and viscosity reduction (Holstein, et al., 2007). A gas injection process could be either miscible or immiscible. Under certain conditions, injected gas and reservoir oil become miscible. Interfacial tension between phases approaches zero in a miscible flood, in such conditions, phase boundary disappear and phases become miscible.
Air, nitrogen, flue gas, carbon dioxide, and hydrocarbon gases have been utilized as injection gases. Gas injection EOR methods are now well understood, based on extensive laboratory and field experiences. The concept of MMP and MME explains the efficiency of oil displacement by injection gas (Taber et al., 1997).
MMP is the pressure at which the injection gas and reservoir oil mix in any proportion. In pressures higher than MMP, the recovery is expected to reach 100% in microscopic scale (Yan et al., 2012).Miscibility could generally be achieved by three methods, including first-contact-miscible (FCM), condensing (or enriched) gas drive and vaporizing (or high-pressure) gas drive (Stalkup et al., 1983).