Résumé -Récupération assistée du pétrole (EOR) et stockage du CO 2 dans des réservoirs pétroliers -L'injection de CO 2 dans des réservoirs pétroliers est une méthode efficace de récupération assistée du pétrole (EOR) et est utilisée par l'industrie pétrolière depuis une quarantaine d'années. La prise en compte des émissions de gaz à effet de serre dans l'atmosphère a mené à étudier ces dernières années le potentiel de cette méthode pour stocker durablement le CO 2 . Si les conditions de réservoirs sont adéquates, elle peut permettre à la fois d'augmenter notablement la récupération d'huile et de stocker définitivement du CO 2 dans les formations géologiques. La plupart des projets passés et actuels d'EOR utilisent du CO 2 peu coûteux et ont un résultat économique appréciable (167-227 sm 3 CO 2 /STB pétrole). Le potentiel de stockage du CO 2 associé à l'EOR est important, à peu près 60 % du CO 2 injecté est retenu dans le réservoir, en ne prenant pas en compte la réinjection. Il est admis qu'il y a peu de défis technologiques majeurs à relever, cependant les contraintes économiques doivent être prises en compte pour les cas de CO 2 cher (comme par exemple celui provenant de la production d'électricité). Dans cet article, un panorama des potentiels de stockage de CO 2 associés à l'EOR est donné. Une étude de cas en mer du Nord est présentée. Abstract -CO 2 EOR and Storage in Oil Reservoirs -CO 2 injection into tertiary oil reservoirs has been widely accepted as an effective technique for enhanced oil recovery (EOR), and has been used by the oil industry for over 40 years. Concerns over greenhouse gas emissions are leading to the investigation and realisation of its potential as a carbon storage method in recent years. With the right reservoir conditions, injection of CO 2 into oil reservoirs can result in incremental oil

Summary The paper describes an air injection improved oil recovery (IOR) process for the recovery of residual oil from light-oil reservoirs. Unlike the air injection technique applied in heavy-oil reservoirs, the main concern for a light-oil reservoir is to remove the oxygen from the injected air by some kind of spontaneous reaction between the oil and oxygen. In-situ combustion in heavy oil reservoirs is a very effective reaction pathway to achieve complete oxygen consumption, as well as to generate heat for enhanced oil recovery. For deep light-oil reservoirs, in-situ combustion is not necessary and may not be readily sustained. More likely, so-called low temperature oxidation (LTO) will prevail. In this study, the potential for LTO reactions to consume oxygen, the reaction rate, and the reaction pathways are investigated. A simplified LTO reaction model has been established based on experimental data obtained from a batch reactor experiment. The model was validated against high-pressure flow displacement experiments in an oxidation tube. A scoping simulation study on a reservoir scale has enabled a sensitivity assessment of the process to be made. The effect of air injection rate, reservoir dip, oil viscosity, formation permeability, numerical grid size, and reservoir temperature on oil recovery and the thermal effect were investigated. Compared with what is normally understood to be conventional air injection (in-situ combustion), the air injection LTO process is flexible in terms of injection rate, stable because of spontaneous reaction (if the reservoir temperature is high enough), and also an economic alternative to hydrocarbon, nitrogen, or carbon dioxide gas. It can also be used as a secondary recovery method in reservoirs that are not suitable for water injection. Introduction Air can be injected into light-oil reservoirs to improve oil recovery. It is freely available and does not suffer from any constraint on supply, as in the case of hydrocarbon, nitrogen, or carbon dioxide gases. The air injection process for light-oil reservoirs is quite different from that of traditional in-situ combustion (ISC), which is applied in heavy-oil reservoirs. In the latter case, a vigorous high-temperature oxidation (HTO) front, or combustion front, needs to be maintained by using a sufficiently high air flux. Also, artificial means of ignition have to be used in most cases to initiate the process. Oil recovery in this case relies on the heat generated by the combustion to reduce the viscosity of the heavy oil, combined with steam and flue gas drives. Air injection in a light-oil reservoir can be viewed as a conventional gas injection process, so long as the oxygen in the injected air is removed efficiently in the oil formation. There have been a number of air injection projects on light-oil reservoirs reported in the literature prior to 1990, which have been reviewed by Yannimaras et al.1 The early application of air injection into deep light-oil reservoirs was dominated by pressure maintenance, repressurization, or gasflooding requirements. In-situ combustion was assumed to occur despite the lack of laboratory data or reliable numerical simulation insight into the process. These processes were developed directly in the field and proved to be successful, both technically and economically, in most cases. Since 1993, there have been an increasing number of detailed reports on air injection projects, both as full-field operations and also laboratory studies.2–7 Examples of current field projects are the Horse Creek reservoir, North Dakota, operated by Total,8,9 and West Hackberry, southwestern Louisiana, developed by Amoco.4,6,10 Other air injection projects are being considered in Indonesia, North Africa, and Argentina. Several important features are anticipated from these projects:Air injection can be used as a secondary recovery method in cases in which water injection is not effective or desirable. It can also be used as a tertiary recovery method to improve oil production after waterflooding.High oil recovery is achieved by gravity-stabilized immiscible displacement. Air/oil ratios (AOR) for air injection projects were reported to be into the range of 5-16 Mscf/bbl.No safety problems associated with oxygen breakthrough into the producers have been reported. It is assumed, therefore, that the oxygen was consumed by vigorous reactions, which spontaneously occurred in the light-oil reservoir. Hydrocarbon backflow into the air injection line and autoignition of the lubricant used in the air compressors are potential safety problems. However, these have been solved by taking special precautions, as in the Amoco and Total projects.7–9Problems associated with air compression and injection, such as compressor failure and corrosion of the injection lines, have also been adequately managed. There is now a large amount of field experience to be shared in the design and operation of new air injection projects. To date, air injection has not been widely accepted as a riskfree technique for either heavy- or light-oil reservoirs. There are a number of reasons for this, including, principally, the high initial cost of compression facilities, the complexity of the reaction phenomena involved in the process, and, finally, the perceived safety problems in operation. The past history of certain heavy-oil ISC projects has also been largely unfavorable. These projects failed mainly because of a lack of understanding of the ISC process, leading to poor design and inappropriate reservoir application. In two recent papers,11,12 these problems have been well addressed. No air injection project in a light oil reservoir has been reported as being unsuccessful. Indeed, a number of projects have been operated for more than 12 years continuously. The chief misconception concerning the two main air injection processes, in-situ combustion and LTO, is because of the lack of understanding of light oil oxidation at reservoir temperatures. This aspect is addressed in this paper. Results of LTO kinetics of light North Sea oils and oil displacement experiments using crushed reservoir core are presented. An LTO reaction model and its application in reservoir simulation are described.

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