Effect of CO2 Impurities on Gas-Injection EOR Processes

Abstract

Abstract Carbon dioxide flooding is a promising enhanced oil recovery method both on technical and, if operating costs are properly controlled, economic grounds. Injecting this greenhouse gas also has environmental merits. Flue gas from power plants is a ready source of CO2; however extracting CO2 for enhanced oil recovery from such a source will increase project costs. Furthermore, to reduce both the net CO2 utilization and the cost of purchasing gas, it is usually necessary to recycle the produced CO2 with as little purification as possible. Therefore, understanding the roles of impurities in fluid phase behaviour and miscibility characteristics is necessary for designing a cost-effective CO2 enhanced oil recovery process. Laboratory studies of the effect of CO2 impurities on phase equilibrium and minimum miscibility pressure (MMP) were conducted on two Saskatchewan light oils covering a range of densities from 29.5°API to 38°API. The results indicate that the MMP for these light oils could increase unfavourably as the N2 and/or CH4 concentration increased in the CO2 stream. The MMP changes as the type and concentration of impurities in the injected CO2 stream change. However, coreflood tests showed that the near-miscible CO2 displacement might employ the same mechanisms as miscible CO2 flooding to mobilize and displace oil; thus, good oil recovery can be achieved in the vicinity of the MMP. While laboratory measurements are essential in the evaluation of a gas injection process, an equation of state (EOS) simulation was demonstrated to be a useful tool in analyzing the phase behaviour of various injection gases, reservoir fluids, and the gas-oil interactions. Introduction Carbon dioxide gas flooding, when properly designed, has proven to be one of the most promising tertiary oil recovery technologies, not only on technical and economic grounds, but also from an environmental point of view. CO2 flooding can significantly increase oil recovery from depleted reservoirs and be eligible for greenhouse gas credits due to CO2 sequestration. A good example of this is the CO2 flood in the Weyburn field in southeastern Saskatchewan, Canada. The construction of a CO2 pipeline to deliver large amounts of CO2 from North Dakota to the Weyburn area greatly improved the feasibility and the economics of this CO2 flooding venture. It was reported that CO2 flooding is not sensitive to reservoir lithology, but it is sensitive to reservoir oil characteristics.[1] In general, reservoirs that have performed well under waterflooding typically will perform well with CO2 flooding. For gas injection projects, two different displacement mechanisms - miscible and immiscible - can occur when CO2 is injected into an oil-containing porous medium. In miscible flooding, which is an ideal displacement mechanism for enhanced oil recovery (EOR), multiple-contact miscibility between the injected gas and oil can be achieved at pressures above the minimum miscibility pressure and below the first contact miscible pressure. If the injected fluid is miscible with the reservoir oil, the interfacial tension at the displacing front is zero, and the capillary forces that trap the oil in the reservoir rocks disappear. Thus, the relative permeability becomes irrelevant resulting in an efficient displacement. The MMP is a very important parameter that is required in a well-designed CO2 flood project. If it is operated well below the MMP, the miscibility between the oil and the injected gas will most likely not be achieved, and the above-mentioned advantages will not be available. In this case, oil recovery will be less than that at pressures near or above the MMP. Therefore, to properly design a CO2 EOR project, it is necessary to have a good knowledge of the thermodynamic properties of the oil-gas mixtures and completely understand the physical and chemical mechanisms involved in oil and gas miscibility.