Carbon Dioxide Flooding As An Enhanced Oil Recovery Process

Abstract

Synopsis In earlier work(1–5), the fundamental and the field applications of carbon dioxide flooding were described. The presentation(5) at the Western Regional Meeting of SPE in 1991 focussed on a comparison of CO2 flooding for light and heavy oils, with examples from the field showing that CO2 flooding has recovered significant amounts of incremental oil under miscible as well as immiscible conditions. Carbon dioxide flooding for the enhanced recovery of heavy oils is important for Canada since it provides an alternative to thermal recovery. In this article the author will attempt to illustrate the state-of-the-art on CO2 flooding in general and how CO2 flooding relates to the recovery of heavy oils in particular. Introduction There is a great deal of industry interest in CO2 flooding as evidenced by the large number of field applications which have been initiated in the past decade(6). Additional field tests are certain to be carried out when CO2 supplies of acceptable purity and volumes are found. All major petroleum research centres, institutes, universities and government research institutions have projects relating to CO2 flooding. With two exceptions, the process normally involves the injection of CO2 initially, followed by a period in which CO2 is alternated, as small slugs, with another fluid, and terminates, with the injection of the principal driving or chase fluid which would be either water or gas. The two exceptions are the carbon dioxide stimulation process which is carried out in producing wells and is a stimulation technique very similar to a ""huff-and-puff' scheme. The other exception occurs in reservoirs which have such low pressure that they must initially be pressurized prior to the injection of CO2. Normally, such pressurization is by injection of water. Carbon dioxide flooding may be carried out either as a miscible or as an immiscible oil recovery process and has been studied in the laboratory or practiced in the field in the following modes:carbon dioxide stimulation;continuous CO2 injection;simultaneous injection of CO2 and water;alternating slugs of CO2 and water;injection of CO2 with H2S;injection of CH4 and CO2;CO2 and N2 mixtures;the CO2, SO2, combination;CO2 and LPG mixtures; andLPG followed by CO2. The most common way that CO2 flooding is being applied in approximately 60 depleted light oil reservoirs in the United States has been to initially repressurize the reservoir to the minimum miscibility pressure by injecting water. In a few cases, repressurization was accomplished by injecting CO2 from the start. Of course, this can be costly. For a reservoir depth of, say. 2500 m, one can expect the minimum miscibility pressure to be about 16 MPa and the reservoir temperature to be 90 °C. At these conditions, 350 m3 of CO2 would approximately equal 1 m3 in reservoir volume. Depending on the size of the reservoir and the cumulative past production the CO2 requirements can prove to be prohibitive.