Petroleum occurs naturally in the earth in porous rock, found anywhere from thousands of meters underground all the way to the earth's surface. The porous layer of rock normally lies between two nonporous layers, so oil does not flow out of its reservoir .In most cases water and/or natural gas occur along with petroleum in the rock. Oil can be extracted from the rock by sinking a pipe into the earth until it penetrates the saturated porous rock. In most cases, oil will then begin to flow of its own accord out of the rock and into the pipe.
The upward flow of oil can be caused by a number of different factors. In some cases, the porous layer of rock is covered by a hollow cap filled with natural gas. Pressure of the gas forces petroleum out of the rock into the pipe. In some cases, gas pressure will be so great as to force oil out of a new well in a fountain-like effect known as a "gusher."
In other cases, the pressure of water also present in the saturated rock pushes oil towards and into the pipe. In still other instances, gases dissolved in petroleum exert pressure on it, forcing it up the pipe.
The ease with which oil flows through a rock layer and into a well depends on a number of factors. In addition to water and gas pressure, viscosity of the oil determines how easily it will move through rock. Even under the most favorable conditions, no more than about 30 percent of the oil in a rock layer can be extracted by any of the natural methods described above.
The term primary recovery is used to describe the natural flow of oil by any of the means described above. By adding a pump to the well—secondary recovery —an additional quantity of oil can be removed. Even after primary and secondary recovery, however, 40–80% of the oil may still remain in a reservoir. All or most of that oil can be recovered by a variety of techniques described as tertiary recovery.
All forms of tertiary recovery involve the injection of some kind of fluid into the oil-bearing stratum. A long pipe is stuck into the ground parallel to the oil-recovery pipe. Into the second pipe is injected a mixture of carbon dioxide in water, steam, or some combination of water and chemicals . In any one of these cases, the injected material diffuses through the oil-bearing rock, pushing the petroleum out and up into the recovery pipe.
Yet another tertiary recovery approach is to set fire to the oil remaining in one part of the stratum. The heat thus generated reduces the viscosity of the unburned oil remaining in the stratum, allowing it to flow more easily into the recovery pipe. Any form of tertiary recovery is relatively expensive and is not used, therefore, until the price of oil justifies this approach. The same can be said for off-shore drilling.
Organic materials washed into the oceans from rivers often settle on the sloping underwater area known as the continental shelf. In this oxygen-free environment , those materials often decay to produce petroleum and natural gas. In recent decades, oil companies have found it profitable to locate and drill for oil in these off-shore reserves.
The technique for drilling from off-shore wells is generally the same as that used on land. The major difference is that before drilling may begin a stable platform on the water's surface for the drilling rig and ancillary equipment must be constructed. The drilling platform must be protected from high winds, waves, and serious storms. In addition, special safeguards must be taken to protect pipes from breaking and releasing oil into the environment.
See also Oil Pollution Act (1990); Oil spills
[David E. Newton ]
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Carey, J. "Hot Science in Cold Lands." National Wildlife 29 (April-May 1991): 4–13.
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Petulla, J. M. American Environmental History. 2nd ed. San Francisco: Boyd & Fraser, 1988.