How Oil Is Produced
The naturally occurring pressure in the underground reservoir is an
important determinant of whether the reservoir is economically viable or not.
The pressure varies with the characteristics of the trap, the reservoir rock and
the production history. Most oil, initially, is produced by "natural lift"
production methods: the pressure underground is high enough to force the oil to
the surface. Reservoirs in the
Over time, these "primary" production methods become ineffective, and
continued production requires the use of additional "secondary" production
methods. One common method uses water to displace oil, using a method
called “waterflood,” which forces the oil to the drilled shaft or "wellbore."
Finally, producers may need to turn to "tertiary" or "enhanced" oil
recovery methods. These techniques are often centered on increasing the
oil's flow characteristics through the use of steam, carbon dioxide and other
gases or chemicals. In the United States, primary production methods
account for less than 40 percent of the oil produced on a daily basis, secondary
methods account for about half, and tertiary recovery the remaining 10 percent.
Both the varying reservoir characteristics and the physical
characteristics of the crude oil are important components of the cost of
producing oil. These costs can range from as little as $2 per barrel
in the Middle East to more than $15 per barrel in some fields in the