The third major phase of crude oil recovery. The primary phase is flowing and finally pumping down the reservoir until it is "depleted" or no longer economical to operate. Secondary recovery usually involves re-pressuring or simple water-flooding. The third or tertiary phase employs more sophisticated techniques of altering one or more of the properties of crude oil, e.g., reducing surface tension. This is accomplished by flooding the formation with water mixed with certain chemicals that "free" the oil adhering to the porous rock so it may be taken into solution and pumped to the surface. See Micellar-surfactant Flooding.
