The process of oil recovery is essentially the process of getting oil from the places where oil exists in the ground (whether onshore or offshore) and into processing plants for refining so the oil is suitable for industrial and residential purposes. The ways to recover oil through a conventional well bore are known as primary, secondary, and tertiary (enhanced), but some unconventional methods are also becoming popular.
Oil, which is usually called crude oil in its most basic form, is a valuable fuel whose chemical makeup is a mixture of hydrocarbon fuels: kerosene, dissolved natural gas, naphtha, light and heavy heating oils, diesel fuel, tars, benzene, and gasoline. It is formed over millions of years by the action of heat and pressure on organic material buried deep within rock, and typically exists in combination with saltwater, natural gas, and soil. Most of the world's oil comes from huge, seemingly inexhaustible subterranean patches of porous, oil-permeated rock. The oil is confined to a certain location, or "trap," by other layers of impermeable rock (usually types of shale) and/or faults.
Throughout recorded history, people have been using oil (in the form of kerosene) for lighting and other purposes. "Recovery" in those early days of oil use generally meant skimming it off the surfaces of seeps and standing bodies of water, although the Chinese seem to have drilled for fuel in the fourth century A.D. Beginning in about the 1850s, however, whale oil--another major source of fuel--began to become less abundant and people started looking for ways to get more crude oil. A man in Pennsylvania, Edwin Drake (1819-1880), dug what is considered to be the first modern oil well in 1859, and in 1861 the first oil refinery went into business to separate crude oil into all its "fractions," or useful parts.
As oil quickly became a crucial, if not indispensable, product for much of the industrialized world, demand increased annually. The methods people invented for recovering oil from its hidden recesses in the earth becoming more sophisticated as well. At first, oil explorers were content to drill down into the earth where they thought there might be a deposit of oil and then stand by as it exploded out of the ground in a "blowout" or "gusher" as the oil was released from the tremendous pressures that helped create it. Collecting oil forced out by natural pressure caused by water and gas "drives" is known as primary recovery.
As the first pressurized wells began to stop expelling their oil because they were constantly being drawn on, new ways of extracting the remaining oil came about. These artificial oil-recovery methods are known as secondary techniques. One of the tools that became popular for this eventuality was the legendary pumpjack or beam pump, which could raise the oil to the surface artificially. When these pumps stop bringing oil up, sometimes more powerful hydraulic pumps are installed.
The most reliable methods for improving oil extraction were water and gas injection through what came to be known as injection wells. These techniques (still in use today) consist of pumping water or gas in through the injection wells to force the oil up to the surface or into a well from which it can be extracted.
Tertiary, or enhanced, oil recovery (EOR) is perhaps the most common category of modern oil recovery methods. It comes into play after a well's natural drives have stopped working and secondary techniques have recovered as much oil as they can. Even then, there may be as much as 95% of a well's oil remaining.
The main EOR techniques are chemical flooding, water flooding, and thermal recovery. Chemical flooding consists of injecting water that contains surfactants, which lower the water's surface tension and so increase its wetting and spreading capabilities. This causes the remaining oil in the well to mix with the special water, as it normally would not, so that the oil can more easily move through the reservoir's porous rock and toward the well bore. Similarly, injecting alkaline (caustic) solutions into a well causes certain types of crude oil to form surfactants, which have the same effect. Chemical flooding using a micellar-polymer mixture along with surfactants makes the water more viscous (thicker) so it slows down as it passes through rock and picks up more oil on its way to the well bore.
Water flooding is when well operators inject huge volumes of water into "dead" wells near still-producing wells. The water, moving toward the area of lesser pressure, moves some of the oil in the dead wells through the rock and toward the producing wells in the same reservoir. From there, the oil and water are pumped out and separated. This is the most popular and least-expensive EOR technique.
In thermal recovery, heat is the most important agent. Some crude oil is so thick that it will not move toward the well bore on its own. In these cases, well operators sometime use a steam drive, or continuous steam injection, to force the oil out. Not only does the steam reduce the oil's viscosity to make it more moveable, it condenses inside the well to form hot water that transports the oil to the bore. In addition, the heat vaporizes the natural gases in the oil, which provides another drive to make the oil move.
Cyclic steam injection, or "huff and puff," also involves sending hot steam down the well, but then the injection stops and the well sits closed for perhaps 72 hours to "soak." Upon opening the well, the thinned oil and condensed water flow out. Some crews light a fire in a reservoir to create "in situ combustion" or "fire flooding." With sufficient air injected into the well for the fire to burn, the fire starts spreading toward the production wells, pushing the heat-thinned oil ahead of it.
Retorting is the main procedure used for recovering oil from oil shale, a plentiful but difficult-to-process and environmentally-hazardous substance. Oil shale is a sedimentary rock that has large amounts of organic, fossil-based material mixed in with it (kerogen). Retorting is the process of using heat to decompose the kerogen, converting it into a liquid called shale oil. Some experts believe that as the world's conventional sources of oil run out, we will be forced to turn to oil shale to fill our energy needs, should these remain unchanged.
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