Petroleum is extracted from the earth by drilling. During primary extraction, about 5-15% of the petroleum at that site can be brought to the surface.
What is secondary extraction and why is it necessary?
All new oil wells have a significant amount of pressure pushing the oil up. This is greater than the natural pressure and is called secondary.
Coal and natural gas have a natural pressure exerted on them. This is called primary extraction. Secondary extraction is extra pressure that must be applied to petroleum to bring it to the surface.
As oil is extracted, pressure in the area is reduced and the remaining oil is more difficult to bring to the surface. By injecting fluid like water or brine or a gas like carbon dioxide, pressure can be added that pushes out more of the oil.
Secondary extraction is when a second company must be brought in to finish oil extraction because the primary company went bankrupt.
Secondary extraction occurs after drilling which is known as primary extraction.