Study of the Mechanisms of Carbon Dioxide Flooding and Applications to More Efficient EOR Projects

Abstract

SPE Members Abstract The mechanisms of carbon dioxide flooding at pressures below the minimum miscibility pressure (MMP) were studied using a numerical model of a slim tube to determine a means of increasing the efficiency of such floods. Results of these studies indicate that, in multiple contact flooding (MCF), the gas phase at the liquid-gas front approaches a constant composition denoting a bank of solvent approaching conditions of miscibility, but not achieving it because of the quantity of methane, nitrogen, and other light gases that overwhelms it. The ethane plus components (C +.) composed approximately five percent of the reservoir gas phase. This constant compositional gas phase formed early in the flood and persisted throughout phase formed early in the flood and persisted throughout the flood until eventual gas breakthrough. A simulated low-temperature flub of the reservoir gas phase produced a solvent that contained more than 75 phase produced a solvent that contained more than 75 percent ethane and propane. Slugs of this solvent were percent ethane and propane. Slugs of this solvent were used to produce miscible displacements with CO2 gas at pressures 40 percent below the MMP. These findings were pressures 40 percent below the MMP. These findings were confirmed in further studies using fluids from several other reservoirs. Introduction Numerous reservoirs would be suitable candidates for carbon dioxide flooding if their reservoir pressure were not below that required for miscibility. It would be desirable that these types of floods were either multicontact or first contact miscible, since they usually recover nearly 100 percent of the reservoir oil. For these reasons, this study percent of the reservoir oil. For these reasons, this study was undertaken to discover a means of effecting a miscible-like performance for CO2 injected into these under-pressured reservoirs. The minimum pressure needed to attain miscibility, MMP, depends not only upon the temperature and pressure of the reservoir, but also upon the composition of both the reservoir oil and the injected gas. Previous studies nave indicated that the MMP can be lowered by altering the composition of either the reservoir oil or the injected gas. The composition of the hydrocarbons in the reservoir may be changed by injection of light hydrocarbons. These hydrocarbons may be injected as a small slug or alternatively mixed with the injected carbon dioxide gas. A method for adjusting the density of the CO2, while maintaining the required MMP, was reported. Now a method for getting the composition of small slugs of hydrocarbon that will cause a flood to be miscible at pressures below the normal MMP has been found and is pressures below the normal MMP has been found and is the subject of this paper. DESCRIPTION AND APPLICATION OF PROCESS Injection of CO2, into a reservoir will result in either a miscible or an immiscible displacement. Miscible displacements occur whenever the displacing gas is miscible in all proportions with the reservoir fluid under the prevailing reservoir temperature and pressure. prevailing reservoir temperature and pressure. Displacements of this type are called lint-contact miscible (FCM). Carbon dioxide in a FCM flood will recover essentially 100 percent of the reservoir fluid contacted. On the other hand, immiscible displacements are classified by Slobod, et al. as being one of two basic processes:○those in which the equilibrium phases at the gas-oil front are essentially immiscible (IMM) and○those in which the injected gas is sufficiently enriched at the front as to be completely miscible with the reservoir fluid. The second immiscible flooding process is referred to as being multicontact miscible flood (MCM). These MCM floods, like FCM floods, will also displace almost all of-the reservoir hydrocarbons contacted. They are dependent upon reservoir fluid composition, temperature and pressure. More often than not, because the reservoir pressure is below the MMP, immiscible floods are the rule. P. 175