Secondary recovery of oil

When a large part of the crude oil cannot be recovered simply by allowing the original reservoir pressure to furnish the driving energy, a method for supplying extra energy must be found. Usually, an injected fluid such as water or gas replaces the produced oil and thus maintains the reservoir pressure. When this procedure substantially increases the potential production of oil, it is called secondary recovery. When gas alone is used as the injected fluid, it is usually put into the top of the reservoir, where gas normally collects to form a gas cap. The effective use of this type of secondary recovery is restricted to reservoirs that have a high flow potential so that gravity allows the oil to drain to the lower part of the formation, where it may be produced. In such a situation gas injection can be a very effective recovery method. When gravity segregation of the gas and oil cannot occur, however, or when little vertical relief exists in the formation, only a small increase over the natural pressure decline production is realized.

Another even more widely used secondary recovery method is water flooding. Water, after treatment to remove certain impurities, is injected through some of the wells. In moving through the formation, it pushes oil toward the remaining production wells. The wells to be used for water injection are usually spaced evenly among the producing wells so that the oil has the shortest possible paths to follow. Water is an effective injection fluid and is not dependent upon gravity segregation. A water-flooding project often increases the oil recovery to at least twice that obtained by pressure decline methods. The major exception is oil reservoirs that are connected to jam active subsurface water source and are thus subjected to water flooding without injection.

Although water flooding greatly increases the recovery of oil from a particular reservoir, it leaves one-fourth to one-third of the oil behind, and several other methods have been tried in efforts to achieve even better oil recovery. One such method uses a quantity of fluid, such as alcohol or liquefied carbon dioxide, ahead of the injected water to decrease the quantity of oil bypassed by the water. On occasion the injected water is made more viscous by the addition of jellying agents, which help to increase the reservoir volume invaded by water. Still another method requires that a small amount of liquid hydrocarbon, such as propane, be placed ahead of a gas injection to help push the oil ahead of the gas.

Several recent methods for secondary recovery involve the application of heat to the reservoir to make the oil more flowable. These methods are best suited for reservoirs that contain viscous crude oil. In one method a portion of the oil in the formation is burned by igniting the oil around the well and then forcing air through the formation to push the burning region out into the reservoir. The heat thus generated makes the oil more flowable. In a successful project about 10 times as much oil is recovered as is burned. In another heat method, steam is injected into the reservoir, the heat liberated by the condensation decreases the viscosity of the crude oil. The heated oil is often produced from the same well into which the steam was injected. Several cycles of such injection and production may be used on a single well.