Carbon Dioxide Flooding

CARBON DIOXIDE FLOODING

Carbon dioxide (CO2) can sometimes be used to enhance the displacement of oil from a reservoir. Carbon dioxide occurs naturally in some reservoirs, either with natural gas or as a nearly pure compound. It can also be obtained as a by-product from chemical and fertilizer plants, or it can be manufactured or separated from power plant stack gas.

When pressure in a candidate reservoir has been depleted through primary production and possibly waterflooding, it must be restored before CO2 injection can begin. To do this, normally water is pumped into the reservoir through injection wells until pressure reaches a desired level, then CO2 is introduced into the reservoir through these same injection wells.

Even though CO2 is not miscible with oil on first contact, when it is forced into a reservoir a miscible front is generated by a gradual transfer of smaller, lighter hydrocarbon molecules from the oil to the CO2. This miscible front is in essence a bank of enriched gas that consists of CO2 and light hydrocarbons. Under favorable conditions of pressure and temperature, this front will be soluble with the oil, making it easier to move toward production wells.

 This initial CO2 slug is typically followed by alternate water and CO2 injection - the water serving to improve sweep efficiency and to minimize the amount of CO2 required for the flood.

Production will be from an oil bank that forms ahead of the miscible front. As reservoir fluids are produced through production wells, the CO2 reverts to a gaseous state and provides a "gas lift" similar to that of original reservoir natural gas pressure. On the surface, the CO2 can be separated from the produced fluids and may be reinjected, helping to reduce the amount of new CO2 required for the project; thus, the CO2 can be recycled.

 This procedure may be repeated, known as "huff and puff" until oil production drops below a profitable level.