Carbon Dioxide Flooding-fundamentals

Abstract

Introduction This is the first of two papers dealing with the fundamentals and application of carbon dioxide flooding. First, the basic mechanism involved is discussed. Then a laboratory experimental program as well as predictive techniques to determine the minimum miscibility pressure, swelling of oil, viscosity reduc tion, PVT properties, solution gas drive effect and increased injectivity are presented. Finally, laboratory tests directed toward the determination of residual oil saturation and the anticipated incremental oil recovery are discussed. Introduction Since 1950, the petroleum industry has been carrying out considerable research into the use of Carbon dioxide to increase the recovery of oil from underground hydrocarbon reservoirs(22). In the last few years, about a dozen new field applications have been initiated and more could come if economically priced CO2 supplies are found. Most petroleum research centers have projects relating to CO2 flooding, indicating the current interest in CO2 flooding and promising further innovations in the process. This paper deals with the fundamentals and the research aspects of carbon dioxide flooding. The basic mechanisms which may lead to an increased oil recovery, the data required to evaluate a potential application in the field, certain basic properties of CO2 and carbonated fluids, and the all-important considerations of residual oil and incremental oil recovery are described and discussed. A second paper covers the reservoir engineering aspects of field applications, such as selecting a reservoir suitable for CO2 flooding, determining the CO2 requirements, injection/production strategies and certain recommendations regarding pilot testing the process in the field. Basic Mechanisms Carbon dioxide flooding may be applied in the field as a secondary or tertiary process and would normally involve the injection of CO2 and some other fluids, either sequentially or in an alternating fashion. Generally, in reservoirs in which the displacement is horizontal or nearly so, the CO2 flooding process would involve alternating injection of CO2 and water to attempt to control the mobility of the fluids, whereas in vertical floods the various fluids would be injected sequentially. For example, in vertical downward displacement, the CO2 may be followed by a lighter gas to maximize the advantage of gravity segregation and minimize -viscous and gravity" fingering; in vertical upward displacement, CO2 would be followed by water, again taking advantage of the gravity segregation. Regardless of how CO2 flooding is applied in the field, the following factors may contribute to increasing the oil recovery:reduction of crude oil viscosity;swelling of crude oil;miscibility effects;increase of injectivity;internal solution gas drive. The effects of the foregoing on oil recovery have been discussed in the literature(12,22). Therefore, only the new and hitherto unrecognized aspects will be discussed here. With regard to swelling of crude oil, the additional oil recovery has generally been considered to be due to an increase in the oil formation volume factor so that the residual oil is smaller in volume at surface separator conditions.