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Where Is the Oil?
It is necessary to find the oil before extraction begins. Some oil reservoirs seep up to the surface, but geologic formations deep underground called the "source rocks" are where most are found. The top layer of these source rocks are usually some sort of liquid-impermeable rock, called a cap rock, that has trapped the oil below it in the interconnected recesses of a porous reservoir rock, usually sandstone. Mixed in with the oil are natural gas and connate water.
Geologists usually find the oil deposits by measuring vibrations from an artificially arranged underground explosion. Differences in the time it takes to receive the echoes indicate that the vibrations have traveled through different substances, the right combination of which will tell them that oil may be present. Drilling a narrow hole and testing the composition confirms the presence of oil.
Primary Extraction
Most large oil reserves are naturally under such pressure that simply tapping them will cause the oil to come to the surface on its own at first. This initial pressure is the "reservoir drive," and is the result of a combination of natural physical factors. One such cause is water flowing inward from adjacent aquifers that raises the water level beneath the oil. At the same time, natural gas both above and dissolved within the oil will expand and push the oil downward toward the oil well taps. Finally, the sheer force of gravity on the mass of oil in the reservoir will push down on the oil near the well, also increasing its pressure. This phase of innate pressure will generally allow for recovery of 5 to 15 percent of the total reservoir.
Secondary Extraction
As the natural reservoir drive decreases and pressure no longer pushes oil to the surface in significant quantities, the secondary extraction phase begins, during which artificially induced pressures replace the natural drive. Most often, pumping water downward through a number of other wells into the source rock achieves this. Less often, it proves economical to pump natural gas back down into the reservoir to increase the pressure. Occasional injections of air and/or carbon will also decrease the density of the oil. This phase often manages to extract 30 to 50 percent of the oil remaining in the reservoir.
Tertiary Extraction
As the tapped oil field's lifespan progresses, simply injecting more pressure into the reservoir will no longer yield significant quantities of oil, and advanced methods of altering the oil itself must be employed to make the oil easier to pump.
A few of these methods involve techniques that heat the oil in order to make it flow more easily. Thermally enhanced oil recovery, or TEOR, methods work by injecting hot steam into the reservoir. The use of a cogeneration plant achieves this. A cogeneration plant is a special kind of electricity plant that harnesses its excess heat for productive purposes; in this case, the generation of steam that is then injected into the reservoir.
The second most common technique used in tertiary oil extraction is the process known as "carbon dioxide flooding," in which injected carbon dioxide reacts chemically and physically with both the reservoir rock as well as with the oil in a number of ways that increase oil flow.
Another set of tertiary extraction techniques rely on the introduction of surfactant agents. These chemicals reduce the surface tension of the water molecules in the reservoir and thus allow the oil to flow through the water more easily. The direct introduction of surfactant chemicals like detergent, or the addition of a nutrient-rich solution of microbes that will produce such chemicals and increase oil flow achieves this objective.
This phase of the oil extraction process usually obtains another 5 to 15 percent of the reservoir.