As mentioned in the previous section, there are in general two types of secondary recovery processes—waterflooding and gasflooding. These will both be discussed in this section. Waterflooding has been the most used process, but gasflooding has proven very useful with reservoirs with a gas cap and where the hydrocarbon formation has a significant dip structure to it.
Waterflooding recovers oil by the water moving through the reservoir as a bank of fluid and “pushing” oil ahead of it. The recovery efficiency of a waterflood is largely a function of the sweep efficiency of the flood and the ratio of the oil and water viscosities. Sweep efficiency, as discussed is a measure of how well the water has contacted the available pore space in the oil-bearing zone. Gross heterogeneities in the rock matrix lead to low sweep efficiencies. Fractures, high-permeability streaks, and faults are examples of gross heterogeneities. Homogeneous rock formations provide the optimum setting for high sweep efficiencies. When injected water is much less viscous than the oil it is meant to displace, the water could begin to finger or channel through the reservoir. As discussed in this fingering or channeling is referred to as viscous fingering and may lead to significant bypassing of residual oil and lower flooding efficiencies. This bypassing of residual oil is an important issue in applying any enhanced oil recovery technique, including waterflooding.
Gas is also used in a secondary recovery process called gasflooding. When gas is the pressure maintenance agent, it is usually injected into a zone of free gas (i.e., a gas cap) to maximize recovery by gravity drainage. The injected gas is usually produced natural gas from the reservoir in question. This, of course, defers the sale of that gas until the secondary operation is completed and the gas can be recovered by depletion. Other gases, such as N2 and CO2, can be injected to maintain reservoir pressure. This allows the natural gas to be sold as it is produced.
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