<P> Occasionally, surfactants (detergents) are injected to alter the surface tension between the water and the oil in the reservoir, mobilizing oil which would otherwise remain in the reservoir as residual oil . </P> <P> Another method to reduce viscosity is carbon dioxide flooding . </P> <P> Tertiary recovery allows another 5% to 15% of the reservoir's oil to be recovered . In some California heavy oil fields, steam injection has doubled or even tripled the oil reserves and ultimate oil recovery . For example, see Midway - Sunset Oil Field, California's largest oilfield . </P> <P> Tertiary recovery begins when secondary oil recovery is not enough to continue adequate extraction, but only when the oil can still be extracted profitably . This depends on the cost of the extraction method and the current price of crude oil . When prices are high, previously unprofitable wells are brought back into use, and when they are low, extraction is curtailed . </P>

How is oil taken out of the ground