Carbon Dioxide Miscible Flooding: Past, Present, And Outlook for the Future
Authors: Stalkup F.I. 
Published: December 2007

This paper gives an overview of the carbon dioxide miscible-flooding process, reviews current technology, discusses past and current field process, reviews current technology, discusses past and current field testing, and assesses the state-of-the-art. Carbon dioxide sources are identified and evaluated. Potential oil recovery, producing rates, and future outlook are projected.

Introduction
Use of carbon dioxide to increase oil recovery is not a new idea. As early as 1952, Whorton and Brownscombe received a patent for an oil-recovery method with CO2. Laboratory research was published through the 1950's and 1960's and research continues today. Carbon dioxide has been investigated for miscible displacement, for immiscible displacement, for producing well stimulation, and for carbonated producing well stimulation, and for carbonated waterflooding. There have been a few field tests in the past, and currently some commercial oil recovery exists. past, and currently some commercial oil recovery exists. Since 1973, rising oil prices coupled with declining domestic production have caused an intense interest in enhanced oil-recovery methods. Several recent studies of the potential for enhanced recovery found carbon dioxide miscible flooding could become one of the most important of these methods. Current industry interest in CO2 miscible flooding is high, as evidenced by the level of activity in field testing and CO2 source development. Two studies are in progress to determine the feasibility of building large pipelines to supply carbon dioxide to west Texas oil fields. Although there may be some important future applications for well stimulation and for immiscible displacement, this paper provides an overview of only miscible flooding with carbon dioxide. Current technology, extent of field testing, state-of-the-art, potential carbon dioxide sources, and future outlook are discussed in the following sections.

Review of Current Technology Mechanism for Achieving Miscibility
Carbon dioxide is not directly miscible with most crude oils at attainable reservoir pressures. If CO is added continuously to oil in an equilibrium cell, two or more phases will form, depending on over-all mixture phases will form, depending on over-all mixture composition. Past research shows that with some oils, CO2 partially dissolves in oil as it flows into the reservoir, while partially dissolves in oil as it flows into the reservoir, while extracting or vaporizing hydrocarbons at the same time. 11-13 The displacing gas front becomes enriched with these extracted hydrocarbons and, at sufficiently high pressure, enrichment proceeds to such an extent and so pressure, enrichment proceeds to such an extent and so alters the composition of the gas front that an efficient displacement occurs, which is characteristic of miscible displacement. This mechanism for achieving miscibility is similar to the high-pressure gas mechanism for hydrocarbon miscible displacement with lean natural gas, but CO2 is a much more powerful vaporizer of hydrocarbons than natural gas, extracting hydrocarbons primarily in the gasoline and gas-oil range. Recent research shows that with other oils, continued contact of the oil by CO2 alters the oil composition to the point where miscibility occurs with the CO2. The mechanism here is analogous to the rich-gas drive mechanism for achieving miscibility between oil and a hydrocarbon gas enriched with intermediate hydrocarbons. With either mechanism, the pressure required for miscibility with CO2 is usually significantly lower than the pressure required for miscible displacement with either natural gas, flue gas, or nitrogen.An example of oil recovery by carbon dioxide flooding at various pressures is shown in Fig. 1 for a west Texas reservoir oil. Displacements were made in both a 42-ft consolidated Boise sandstone and in a 20-ft unconsolidated sand pack initially saturated with oil.
JPT
P. 1102