Miscible CO2 gas injection has nowadays become one of the most important enhanced oil recovery (EOR) processes. For optimal displacement efficiency, CO2 flooding should be conducted at pressures greater than a minimum miscibility pressure (MMP), which is a function of temperature and composition of injection gas. The MMP has been defined as the minimum pressure at which the injected gas and the oil become miscible with each other. Approaching miscibility, the interfacial tension (IFT) between phases decreases resulting in decreased residual oil saturation, hence, higher oil recovery efficiency.
This paper presents two new experimental approaches to determine the MMP for the miscible CO2 gas injection. The first approach consists of measuring the interfacial tension (IFT) between the injected gas and the oil at reservoir temperature and varying pressures using a prototype Vapour Liquid Equilibrium-Interfacial Tension (VLE-IT) equipment. The miscibility of the injected gas and the oil is evaluated on the basis of the vanishing IFT between the two phases.
The second experimental approach involves the MMP determination by displacement test using a prototype Gas Extraction & Miscibility Analyser (GEMA)*. The invention incorporates a capillary-sized unpacked column or a micro-slim tube (MST) as opposed to the conventional packed slim-tube widely reported in literature. The MMP analysis time reduces up to one tenth of the conventional slim tube or sand-pack flood.
It is demonstrated in this study that the approaches are reliable and fast for determining CO2 MMPs. The IFT method yields the quantitative information and the IFT MMP obtained is verified by the micro-slim tube displacement tests. The MMP obtained also falls within the range of values predicted by published correlations.