Carbon dioxide flooding
When a reservoir’s pressure is depleted through primary and secondary production, carbon dioxide flooding can be an ideal tertiary recovery method. It is particularly effective in reservoirs deeper than 2,500 ft., where CO2 will be in a supercritical state, with API oil gravity greater than 22–25° and remaining oil saturation greater than 20%. It should also be noted that carbon dioxide flooding is not affected by the lithology of the reservoir area, but simply by the reservoir porosity and permeability, so that it is viable in both sandstone and carbonate reservoirs. By injecting CO2 into the reservoir, the viscosity of any hydrocarbon will be reduced and hence will be easier to sweep to the production well.
As an oil field matures and production rates decline, there is growing incentive to intervene and attempt to increase oil output utilizing tertiary recovery techniques (also termed improved or enhanced oil recovery). Petroleum engineers assess available options for increasing well productivity, options that include chemical injection, thermal/steam injection, and CO2 injection. Based on data-gathering and computer simulations, the most optimal enhanced oil-recovery technique to maximize well-productivity is determined. To increase the rate of oil production, the pressure within the reservoir must be increased.
In CO2 flooding, the first step is injection of water into the reservoir, which will cause the reservoir pressure to increase. Once the reservoir has sufficient pressure, the next step is to pump the CO2 down through the same injection wells. The CO2 gas is forced into the reservoir to come into contact with the oil. This creates a miscible zone that can be moved more easily to the production well. Normally the CO2 injection is alternated with water injection and the water acts to sweep the oil towards the production zone.
CO2 flooding is the second most common tertiary recovery technique and is used in facilities around the world. In connection with greenhouse gas emissions and global warming, CO2 flooding sequesters CO2 underground and therefore offsets CO2 emissions elsewhere.