Inert Gas Injection Processes

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.