Gas Injection as an Enhanced Recovery Technique for Gas Condensates. A comparison of three Injection Gases

Gas injection is a recognized enhanced recovery technique for oil reservoirs, but has been given less attention for gas condensate fields. If a gas condensate is produced by natural depletion, the condensate-gas ratio of the produced well stream will steadily decrease after the saturation pressure is reached. The liquid condensed will stay back in the reservoir and will not be produced.

A laboratory study was conducted for a Middle East gas condensate reservoir fluid. Three different injection gases were used- N₂, CO₂ and a lean hydrocarbon gas. A gas revaporization experiment was conducted with each gas on the depleted reservoir fluid. The gas revaporization experiments showed that injection of CO₂ made the liquid dropout in the PVT cell decrease substantially. Already condensed liquid was revaporized and led to an increased liquid content in the released gas. In small concentrations N₂ made the liquid dropout increase, but with a continued injection of N₂ the amount of liquid dropout declined. The hydrocarbon gas made the liquid dropout decrease at all concentrations, but to a lesser extent than CO₂. An analysis of the results showed that injection of CO₂ and a lean hydrocarbon gas may substantially increase the liquid recovery from the actual field, while it is questionable whether N₂ injection will have much impact on the liquid recovery.