The most efficient use of CO2 as an oil-recovery agent is obtained at flooding pressures at which miscible displacement is achieved. The extraction of C5 to C30 hydrocarbons by CO2 at these pressures promotes a displacement efficiency approaching 100 percent. A simple correlation is presented here to determine the optimum displacement pressure for CO2 presented here to determine the optimum displacement pressure for CO2 floods. Introduction The use of carbon dioxide as an oil recovery agent in petroleum reservoirs has been investigated for many petroleum reservoirs has been investigated for many years. Both laboratory and field studies have established that CO2 can be an efficient oil-displacing agent. The various mechanisms by which it can displace oil from porous media have been of particular interest to the petroleum industry. The mechanisms includesolution gas drive,immiscible CO2 drive,hydrocarbon-CO2 miscible drive,hydrocarbon vaporization,direct miscible CO2 drive, andmultiple-contact dynamic miscible drive. Although these mechanisms may be known to some people in the industry, the literature does not clearly distinguish between them and does not point out the differences between CO2 displacement and other types of displacement processes. Nor does the literature show how the miscible type of CO2 displacement occurs with crude oils that have been depleted of gas and LPG. This paper presents experimental data that illustrate the various CO2 displacement mechanisms and shows how they differ from other miscible displacement mechanisms such as LPG, rich gas, and high-pressure gas processes. Properties of CO2 Properties of CO2 Before presenting data on CO2-oil displacement experiments, it will be helpful to review some of the characteristics of carbon dioxide that are effective in removing oil from porous rock. CO2 performs in the following ways:It promotes swelling.It reduces oil viscosity.It increases oil densityIt is highly soluble in waterIt exerts an acidic effect on rockIt increases oil density.It is transported chromatographically through porous rock. We know that the high solubility of CO2 in hydrocarbon oils causes these oils to swell. However, the difference between the solubility of CO2 in gas-saturated reservoir oil and in stock-tank oil, with the subsequent difference in the degree to which the resultant oils swell, has received less attention. Fig. 1 shows the relative oil volume vs pressure characteristics for a typical West Texas crude oil. The reservoir fluid is a mixture of the separator oil and the separator gas. It is apparent that carbon dioxide expands the separator oil to a much larger degree than it does the reservoir fluid. Also, CO2 expands oil to a greater degree than does methane. Because the CO2 does not displace all methane when it contacts a reservoir fluid, less CO2 goes into solution and less swelling of the reservoir fluid occurs. We will see later what effect this difference in solution has on the displacement of reservoir fluids and stock-tank oils by CO2 A large reduction in the viscosity of crude oils occurs as they become saturated with CO2 at increasing pressures (Fig. 2). pressures (Fig. 2). JPT P. 1427
This paper presents additional data related to the correlation between minimum miscibility pressure (MMP) for CO2 flooding and to the composition of the crude oil to be displaced. Yellig and Metcalfe have stated that there is little or no effect of oil composition on the MMP. However, their conclusion was based on experiments with one type of reservoir oil that was varied in C through C6 content and in the amount of C7 + present but not varied in composition of the C7 + fraction. We have found that the Holm-Josendal correlation, which is based on temperature and C5 + molecular weight, predicts the general trend of the MMP's required for CO2 flooding of various crude oils. MMP's were predicted with this correlation and then tested for several crude oils using oil recovery of 80% at CO2 break through and 94% ultimate recovery as the criteria. We now have data showing that miscible-type displacement is also correlatable with the amount of C5 through C3O hydrocarbons present in the crude oil and with the solvency of the CO2 as indicated by its density. Variations from such a correlation are shown to be related to the C5 through C 12 content and to the type of these hydrocarbons. The MMP data were obtained from slim-tube floods with crude oils having gravities between 28 and 44 degrees API (0.88 and 0.80 g/cm3) and C5 + molecular weights between 171 and 267. The crude oils used varied in carbon residue between 1 and 4 wt% and in waxy hydrocarbon content between 1 and 17%. The required MMP for these crude oils at 165 degrees F (74 degrees C) varied between 2,450 and 4,400 psi (16.9 and 30.3 MPa) for an oil recovery of 94% OIP. The MMP was found to be a linear function of the amount of C5 through C30 hydrocarbons present and of the density of the CO2. Introduction Our 1974 paper, "Mechanisms of Oil Displacement by Carbon Dioxide," discussed the various mechanisms by which oil is displaced from reservoir rock using CO2. One conclusion of this study was that multiple-contact, miscible-type displacement of oil occurs through extraction of C5 through C30 hydrocarbons from the reservoir oil by COB when a certain pressure is maintained at a given flood temperature. The mechanism of oil recovery was described as follows. The CO2 vaporizes or extracts hydrocarbons from the reservoir oil until a sufficient quantity of these hydrocarbons exists at the displacement front to cause the oil to be miscibly displaced. At that point, the vaporization or extraction mechanism stops until the miscible front that has been developed breaks down through the dispersion mechanism. When miscibility does not exist, the vaporization or extraction mechanism again occurs to re-establish miscibility. The miscible bank is formed, dispersed, and reformed throughout the displacement path; a small amount of residual oil remains behind all along the displacement path. Also, an optimal flooding pressure at a given temperature for a given oil was defined in that paper as when oil recovery of about 94% OIP was achieved and above which point essentially no additional oil was recovered. This pressure has since been termed the "minimum miscibility pressure" by others. We further determined in our previous study thatthis miscible-type displacement does not depend on the presence of C2 through C4 in the reservoir oil and thatthe presence of methane in the reservoir oil does not change the MMP appreciably. Those findings have been confirmed by Yellig and Metcalfe with the qualification that the CO2 MMP must be greater than or equal to the bubble-point pressure of the reservoir oil. SPEJ P. 87^
Two radioactive tracers have been tested as a means of determining core saturation in multi phase flow studies. Cesium""chloride was tried as a water-phase tracer, but complications in its use in low permeability cores resulted from sorption of cesium by the core or water-wet pads. Iodo'31benzene proved very satisfactory as an oil-phase tracer. The synthesis of iodobenzene from the sodium iodide as received from Oak Ridge is simple and direct. The tracer is insoluble in water and there was no evidence of sorption by any of the core materials used.Use of the method to determine saturation profiles during capillary and dynamic desaturations and relative permeability measurements on oil-water and oil-gas systems is described. Comparisons of the dynamic and capillary methods of relative permeability determination were made using the tracer to check core saturation and saturation distribution. Other experiments are also described in which mobility of the oil phase at various saturations was measured by displacing labeled oil by flowing inactive oil. Similar experiments were made using water labeled with cesium 134.
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