PETROLEUM
usually sufficient for a while after reservoirs are first tapped. In some reservoirs, such as
in the Middle East, the natural pressure is sufficient over a long time. The natural
pressure in most reservoirs, however, eventually dissipates. Then the oil must be
extracted using "artificial lift" means. Over time, these "primary" methods become less
effective and "secondary" production methods may be used. A common secondary
method is "waterflood" or injection of water into the reservoir to increase pressure and
force the oil to the drilled shaft or "wellbore." Eventually "tertiary" or "enhanced" oil
recovery methods may be used to increase the oil's flow characteristics by injecting
steam, carbon dioxide and other gases or chemicals into the reservoir. In the United
States, primary production methods account for less than 40 percent of the oil produced
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