Mechanisms of Oil Displacement By Carbon Dioxide

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