The use of inert gases, in particular CO2 and N2, as injected fluids in miscible processes, has become extremely popular. The representation of the process with CO2 or N2 on the ternary diagram is exactly the same as the high-pressure vaporizing process, with the exception that either CO2 or N2 becomes a component and methane is lumped with the intermediates. Typically the one-phase region is largest for CO2, with N2 and dry gas having about the same one-phase size. The larger the one-phase region, the more readily miscibility will be achieved. Miscibility pressures are lower for CO2, usually in the neighborhood of 1200 psia to 1500 psia, whereas N2 and dry gas yield much higher miscibility pressures (i.e., 3000 psia or more).
The capacity of CO2 to vaporize hydrocarbons is much greater than that of natural gas. It has been shown that CO2 vaporizes hydrocarbons primarily in the gasoline and gas-oil range. This capacity of CO2 to extract hydrocarbons is the primary reason for the use of CO2 as an oil recovery agent. It is also the reason CO2 requires lower miscibility pressures than natural gas. The presence of other diluent gases such as N2, methane, or flue gas with the CO2 will raise the miscibility pressure. The multiple-contact mechanism works nearly the same with a diluent gas added to the CO2 as it does for pure CO2. Frequently, an application of the CO2 process in the field will tolerate higher miscibility pressures than what pure CO2 would require. If this is the case, the operator can dilute the CO2 with other available gas, raising the miscibility pressure but also reducing the CO2 requirements. Due to the recent consideration to sequester CO2 because of its contributions to greenhouse gases, companies may find it very desirable to use CO2 as a flooding agent.
The pressure at which miscibility is achieved is best determined by conducting a series of displacement experiments in a long, slim tube. A plot of oil recovery versus flooding pressure is made, and the minimum miscibility pressure is determined from the plot.
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