The term secondary recovery technique refers to any method for removing oil from a reservoir after all natural recovery methods have been exhausted. The term has slightly different meanings depending on the stage of recovery at which such methods are used.

The oil trapped in an underground reservoir is typically mixed with water, natural gas, and other gases. When a well is sunk into the reservoir, oil may flow up the well pipe to the earth's surface at a rate determined by the concentration of these other substances. If the gas pressure is high, for example, the oil may be pushed out in a fountain-like gusher.

Flow out of the reservoir continues under the influence of a number of natural factors, such as gravity, pressure of surrounding water, and gas pressure. Later, flow is continued by means of pumping. All such recovery approaches that...