Technology Today Series articles provide useful summary information on both classic and emerging concepts in petroleum engineering. Purpose: To provide the general reader with a basic understanding of a significant concept, technique, or development within a specific area of technology. Introduction Gas injection is one of the oldest methods used by engineers to improve oilrecovery, and its use has increased recently. Much of the new expansion(especially in the U.S.) is coming from the nonhydrocarbon gases, nitrogen andCO2. The full impact of CO2 flooding will he felt during the' next severalyears since construction of CO2 pipelines into the west Texas area wascompleted in the 1980's and the larger floods are continuing to respond withgood increases in tertiary oil production. Mechanisms CO2 flooding processes can he classified as immiscible or miscible, eventhough CO2 and crude oils are not actually miscible upon first contact in thereservoir. Recovery mechanisms in immiscible processes involve reduction in oilviscosity, oil swelling, and dissolved-gas drive. While some of the earlyinterest in the 1950's and 1960's involved immiscible processes, recent workhas emphasized miscible processes that recover more reservoir oil. In miscibleprocesses, CO2 is effective in recovering oil for a number of reasons. Ingeneral, CO2 is very soluble in crude oils at reservoir pressures; therefore,it swells the net oil volume and reduces oil viscosity long before miscibilityis achieved by a vaporizing-gas-drive mechanism. As CO2 phase (which now contains many of the intermediate hydrocarboncomponents) can flow together because of the low interfacial tension and therelative increase in the total volumes of the combined CO2 and oil phasescompared with the water phase. phases compared with the water phase. Because ofthese mechanisms, good oil recovery may occur at pressures below those requiredfor the generation of miscibility. However, the generation of miscibilityhetween the oil and CO2 is still considered to he the most important mechanism,and this will occur in most CO2/crude-Oil systems when the pressure is highenough. In general, the high pressures are required to compress the CO2 to adensity at which it becomes a good solvent for the lighter hydrocarbons in thecrude oil. This so-called "minimum miscibility pressure" (MMP) has beenthe target of laboratory investigations in which the pressure required formaximum oil recovery by CO2 is determined in a small-diameter tube (slim tube)packed with sand or glass beads. Correlations have been developed for MMP vs.API gravity and the molecular weight of the C5+ fraction.2-4 Reservoirtemperature also plays a role in MMP.5 Fig. 1 shows the variation of MMP (andthe corresponding CO2 density) with temperature and oil composition.6-8 Becauseof the minimum pressure requirement, depth is an important screening criterion,and CO2 floods are normally carried out in reservoirs that are more than 2,500ft deep.9 Oil composition is also important; a high percentages of intermediatehydrocarbons (especially C5 through C12) is heneficial,8 and surveys show thatthe gravity exceeds 30 degrees API for most active CO2 floods.10 Although themechanism for CO2 flooding appears to he the same as that for hydrocarbonmiscible floods, CO2 floods may give better recoveries even if both systems areabove their required miscibility pressures, especially in tertiary floods.pressures, especially in tertiary floods. CO2 has a much higher solubility inwater than hydrocarbons and has been observed in laboratory experiments todiffuse through the water phase to swell bypassed oil until the oil is mobile.Thus, not only are screening criteria for depth and oil viscosity easier tomeet in CO2 flooding, but the ultimate recovery may he better than withhydrocarbons when above the MMP at similar displacement conditions. Thisconjecture has yet to he proved by comparable experiments or field data, butoil recoveries for some CO2 field projects are exceeding the simulatorpredictions, whereas miscible hydrocarbon projects normally do not. Oil Recovery Potential U.S. Reserve Additions. Potential U.S. Reserve Additions. Ultimateincremental oil recovery from CO2 floods in the U.S. is estimated to be 8 to 15billion bbl,11 but may vary from 5 to 30 billion bbl,6,12 depending on futureoil prices and economic incentives. prices and economic incentives. JPT P. 396