JIM STEINBERG / SCIENCE PHOTO LIBRARY JIM STEINBERG / SCIENCE PHOTO LIBRARY
The oil pump, known as a jack pump or a ''nodding donkey'', draws oil out of the ground through a series of non-return valves (not seen here). Newly-drilled oil wells ofen contain oil at a high enough pressure to force the fluid out without needing to be pumped. As the oil is removed, the pressure falls. For a time, the pressure can be maintained by pumping water or air into the subterranean reservoir of oil. Eventually the pressure falls further and a jack pump is required to bring oil out at an economic rate. Even using jack pumps, generally only 30-50 percent of the oil can be extracted.
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