Evolution of the Carbon Dioxide Flooding Processes
Authors: Holm Wally L. 
Published: December 2007

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.

Summary.
Carbon dioxide flooding has become one of the major EOR processesin the U.S., with potential recovery of billions of barrels of oil thatotherwise would be left as residual in known U.S. reservoirs. Extensivecommercial-scale CO2 projects are under way, and other new projects andexpansions are planned for the future. This paper briefly reviews the evolutionof the CO2 flooding processes as they are most commonly applied today. Theimportant features of CO2 flooding, which have been defined by extensivelaboratory research and field testing, are summarized. Included are summariesof (1) early developments in the use of CO2, (2) oil displacement mechanisms ofCO2, and (3) other factors involved in the design of CO2 floods-i.e., CO2availability, CO2 injection requirements, mobility control, reservoirconditions, safety, CO2 reinjection, corrosion, and solids precipitation.

Early CO2 Studies
CO2 is one of the most plentiful and useful compounds on, in, and aroundthis planet. So it is not surprising that the idea to use it to recover oilfrom underground reservoirs originated early in EOR history. Investigators inthe laboratories began to look at CO2 flooding in the 1950's. Their mostimportant observations were the high solubility of CO2 in oil at pressuresabove about 700 psi (4.9 MPa) and the resultant dramatic reduction in oilviscosity, making the oil much easier to flow (Fig. 1). Fluid injectionprocesses using continuous CO2 or a single slug of CO2 followed by water orcarbonated water were suggested. As it was noted that CO2 was soluble in water, other publications appeared at that time recommending the injection of onlywater saturated with CO2. Several field tests were conducted with carbonatedwater, but these floods failed to produce significant amounts of extra oil, mainly because not enough CO2 was available at the flood front to saturate theoil and reduce its viscosity. CO2 transfers chromatographically from theinjected water to the oil. However, substantial volumes of carbonated water arerequired to saturate the residual oil, reduce its viscosity, and cause it toflow. Large volumes of CO2-denuded water precede this mobilized oil.
An early field test of the injection of a single CO2 slug followed by waterwas conducted during this early period in the Allegheny field in New York.* Anattempt to make this a miscible flood failed, but, as shown in Fig. 2performance of laboratory model studies. Calculations indicated that viscosityreduction was the major mechanism for EOR. Reduced sweep efficiency was thoughtto account for the lower oil recovery obtained in the field compared to that inthe laboratory.
The well defined phase behavior of CO2 (Fig. 3) shows that in mostapplications as an oil recovery agent, it exists as a supercritical fluid, above its critical temperature (89 deg F (32 deg C)) and pressure (1,070 psi(7.4 MPa)). The increased density of CO2 at these high pressures revealed itssolvency power for various liquids. Recognition of these facts, and notationthat very high oil recoveries were obtained in the laboratories, led to a fieldtrial at high pressures.
The Mead Strawn field test begun in 1964 involved the injection of a largeslug of CO2 (25 % HCPV at reservoir conditions) followed by carbonated water atreservoir conditions of 125 deg. F and as high as 2,000 psi (52 deg C and 13.8MPa). The results of this test showed an incremental oil recovery for the CO2flood of more than 35% above that from conventional waterflooding. Even moreencouraging were the residual oil saturations determined from pressure coresobtained from this sandstone reservoir in the CO2 swept zones as far as 400 ft(122 m) away from the CO2 injection wells. Measured Sor values of less than 5%PV throughout the cored pay zone indicated recovery of residual oil that couldnot be accounted for by oil-viscosity reduction and oil swelling alone. Similarpressure cores recently recovered from the North Crossett flood, a carbonatereservoir, have shown the same ( less 5% PV) oil saturation behind the CO2displacement front.

Displacement Mechanisms
The high oil displacement efficiency of CO2 in the Mead Strawn field testled to further laboratory investigations of the suggested miscible effects ofthe process. Miscible displacement and the resultant very high recovery of oilfrom a reservoir require the injection of a fluid that either will form asingle phase with oil or will generate from the oil a solvent that forms asingle phase with oil, eliminating capillary retention of the oil by water androck.
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