Carbon Dioxide Flooding-applJications

Abstract

Abstract This is the second part of a two-part paper and will deal with the reservoir engineering aspects of field applications. First, certain criteria which may be useful in identifying a good candidate reservoir for C02 flooding together with the limitations of the so-called screening procedures are discussed. Second, calculations for CO2 slug size are given, and the following factors dealt with: diffusion, dispersion and solubility phenomena; injection/ production rates and their effect on pressure distribution and maintenance of miscibility in the reservoir; the rate of advance of the displacement front and stability considerations; and oil recovery efficiency. Finally, recommended ways of testing the CO2 flooding process are detailed. Introduction Carbon dioxide flooding is generally regarded as the most promising enhanced oil recovery process(l). At present, there are about forty field applications underway, most having been started in the past few years. The United States leads in the number of field applications. There are no CO2 floods in Canada at the moment. Although the Judy Creed Beaver hill Lake A Pool was proposed and approved for CO2 flooding, the economics appear to have caused cancellation of that project(2). Husky Oil Ltd. is conducting CO2 stimulation/soak in a few wells. The first CO2 flood in Canada may well be in the Joffre Viking Pool and will be undertaken by Vikor Resources Ltd.(3). An earlier paper dealt with the fundamentals and the research considerations of carbon dioxide flooding(4). In this paper, selection of reservoirs suitable for CO2 flooding, determining the CO2 requirements, injection and production strategies, problematic areas in the field and recommendations regarding pilot testing are presented. Selecting a Suitable Reservoir A number of screening criteria have been suggested in order to select a reservoir that is best suited for carbon dioxide flooding(5). Although screening criteria serve some purpose and usefulness in looking at a large number of reservoirs from which one must choose the best candidates for CO2 flooding, the use of such criteria in itself has many pitfalls and does not mean that a successful field application would result. If one has but only a few reservoirs to consider, then some standard reservoir engineering considerations are far superior to any general screening criteria. The first reservoir engineering consideration has to do with whether the displacement is to be miscible or immiscible CO2 flooding. In light and medium-gravity oils one would aim at miscible CO2 displacement and laboratory experiments would have already indicated whether or not the miscibility can be achieved within realistic pressure ranges. Especially old pools should not be subjected to high injection pressures which may cause casing or cement failure, premature breakthrough of the injected fluids due to viscous fingering or coning problems. If the miscibility is not the object, but rather swelling of the oil, reduction of oil viscosity and interfacial effects are the principal phenomena which may lead to increased oil recovery, the reservoir to be selected could have different properties than one which may have been suitable for a miscible displacement process.