In general, crude oil production from oil reservoirs can include up to three distinct phases: primary, secondary and tertiary (or enhanced) recovery. During the primary recovery of crude oil, the oil is produced because of reservoir energy as the pressure thousands of feet below the surface is high enough to push the oil to the surface. Typically only about 5-20% of a reservoir’s original oil in place is typically produced during primary recovery.
However, with much of the easy-to-produce oil already recovered from domestic oil-fields, producers have attempted several tertiary, or enhanced oil recovery techniques that offer prospects for ultimately producing a total of 60% or more of the original oil in place. Many of EOR technologies are still in their developmental phase with many successful laboratory studies already conducted. Enhanced oil recovery methods can be classified into three major categories: thermal, miscible gas and chemical.
While a Mature Hydrocarbon Province, the U.S. Still Has 400 Billion Barrels of Undeveloped Technically Recoverable Oil Resource. Undeveloped domestic oil resources still in the ground (in-place) total 1,124 billion barrels. Of this large in-place resource, 400 billon barrels is estimated to be technically recoverable. This resource includes undiscovered oil, “stranded” light oil amenable to CO2-EOR technologies, unconventional oil (deep heavy oil and oil sands) and new petroleum concepts (residual oil in reservoir transition zones).  The good thing about CO2-EOR technologies is that CO2 is pumped in ground to displace oil out from the underground. That is not totally eliminating GHG emissions but reduces them by as much as 24 percent. It is like making money out of carbon capture and storage today. 
In view of the world-wide shortage of petroleum and the fact that almost 50% to 60% of the original oil in place is left in the reservoirs at the end of secondary recovery, the importance of enhanced oil recovery methods to produce additional oil can hardly be overstated. U.S leads the world in the EOR technology. As the leader in EOR technology, the U.S. oil industry faces the challenge of further applying this technology towards economically producing the more costly remaining domestic oil resources. While pursuing this remaining domestic oil resource base poses considerable economic risk and technical challenge to producers, developing the technical capability and infrastructure necessary to exploit this resource reduces our dependence on foreign energy sources and helps our domestic energy industry maintain worldwide technical leadership. 
(1) ‘Mechanisms of Surfactant Enhanced oil Recovery in Oil-Wet Fractured Carbonate Reservoirs’. Doctoral thesis presented to the faculty of chemical engineering at University of Houston. By Bhargaw Adibhatla.
(2) ‘Technology’, Chapter 3 of Hard Truths. A report submitted to the secretary of energy by National Petroleum Council.
(3) Undeveloped Domestic Oil Resources Provide Foundation For Increasing U.S. Oil Supply. An analysis by Advanced Resources International, Arlington, VA, for the U.S. Department of Energy’s Office of Fossil Energy.