<P> While the underground pressure in the oil reservoir is sufficient to force the oil to the surface, all that is necessary is to place a complex arrangement of valves (the Christmas tree) on the well head to connect the well to a pipeline network for storage and processing . Sometimes pumps, such as beam pumps and electrical submersible pumps (ESPs), are used to bring the oil to the surface; these are known as artificial lifting mechanisms . </P> <P> Over the lifetime of a well, the pressure falls . At some point there is insufficient underground pressure to force the oil to the surface . After natural reservoir drive diminishes, secondary recovery methods are applied . These rely on supplying external energy to the reservoir by injecting fluids to increase reservoir pressure, hence increasing or replacing the natural reservoir drive with an artificial drive . Secondary recovery techniques increase the reservoir's pressure by water injection, natural gas reinjection and gas lift, which injects air, carbon dioxide or some other gas into the bottom of an active well, reducing the overall density of fluid in the wellbore . The typical recovery factor from water - flood operations is about 30%, depending on the properties of the oil and the characteristics of the reservoir rock . On average, the recovery factor after primary and secondary oil recovery operations is between 35 and 45% . </P> <P> Enhanced, or Tertiary oil recovery methods, increase the mobility of the oil in order to increase extraction . </P> <P> Thermally enhanced oil recovery methods (TEOR) are tertiary recovery techniques that heat the oil, reducing its viscosity and making it easier to extract . Steam injection is the most common form of TEOR, and it is often done with a cogeneration plant . This type of cogeneration plant uses a gas turbine to generate electricity, and the waste heat is used to produce steam, which is then injected into the reservoir . This form of recovery is used extensively to increase oil extraction in the San Joaquin Valley, which yields a very heavy oil, yet accounts for ten percent of the United States' oil extraction . Fire flooding (In - situ burning) is another form of TEOR, but instead of steam, some of the oil is burned to heat the surrounding oil . </P>

Who showed that oil could be extracted from the ground through a well