Gasflooding

Gasflooding was introduced in where the injection of an immiscible gas was discussed in retrograde gas reservoirs. Gas is frequently injected in these types of reservoirs to maintain the pressure at a level above the point at which liquid will begin to condense in the reservoir.^10,11 This is done because of the value of the liquid and the potential to produce the liquid on the surface. Reservoir gas is also pushed to the producing wells by the injected gas, similar to oil being pushed by a waterflood, as discussed in the previous section.

A second type of gasflooding is that shown in Fig. 11.3. A dry gas is injected into the gas cap of a saturated oil reservoir. This is done to maintain reservoir pressure and also for the gas cap to push down on the oil-bearing formation. Thus oil is pushed to the producing wells. Obviously, the producing wells should be perforated in the liquid zone so that the production of oil will be maximized.

Figure 11.3 Schematic of a typical gasflooding project in an undersaturated oil reservoir.

Steeply dipping reservoirs may yield high sweep efficiencies and high oil recoveries. A concern in gasflooding in more horizontal structures is the viscosity ratio of the gas to oil. Since a gas is typically much less viscous than oil, viscous fingering of the gas phase through the oil phase may occur, resulting in poor sweep efficiencies and low oil recoveries. Often in horizontal reservoirs, to help with the poor sweep efficiencies, water is injected after an amount of gas injection. The water is followed by more gas. This process is referred to as the water alternating gas injection process, or WAG. Christensen et al. have shown the effectiveness of this process in several applications.¹²

N₂ and CO₂ have been used in gasflooding projects. With the increased desire to sequester CO₂, the injection of CO₂ has developed into a viable option as a secondary recovery process.^13–15