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.

A new air injection technique, low temperature oxidation (LTO) process, is described. Improved oil recovery from deep, light oil reservoirs is achieved by removing the oxygen in the injected air by LTO reactions with the residual oil in the reservoir. The product of the LTO reactions is a "flue gas," which displaces the oil. Preliminary results of LTO reaction kinetics and oil recovery have been obtained using four North Sea light oils. The paper also contains some discussion of the safety issues related to air injection offshore. Introduction Gas injection into light oil reservoirs is a proven improved oil recovery IOR technique. The IOR potential for gas injection in the United Kingdom Continental Shelf (UKCS) has been estimated at 1.4 bSTB(1). However, the application of gas injection is limited by gas availability and cost, particularly for many mature fields, with the prospect of abandonment unless economic methods can be developed to extend the field life. Therefore, there is now growing interest in air injection because of its availability. Air injection has been widely used in the past for production of viscous heavy oils, where the heat generated by in situ combustion is a necessary part of the recovery process. Air injection can also be used for the recovery of light oils, but in this case, heat generation is not necessary for the displacement. Some form of oxidation is only required in order to remove the oxygen from the air and prevent it from reaching the production wells. Yannimaras et al.(2) have discussed the benefits of air injection for IOR from deep, light oil reservoirs, wherein the principle objective was to generate flue gas (85﹪ N2, 15﹪ CO2) by in situ combustion. There are a number of ongoing successful air injection field projects, notably in the West Hackberry Field, Louisiana [Amoco(3)]; in Medicine Pole Hills Unit, North Dakota; Buffalo, South Dakota [Koch(4)]; most recently, in the Horse Creek Field, North Dakota [Total(5)]; and Total's proposed LTO pilot test in the H Field in Indonesia(6). In the latter case, core flooding studies were undertaken to investigate the effect of various parameters on oxygen uptake by the oil. Previous field projects and simulation studies have considered that high temperature oxidation (HTO, or in situ combustion) is needed to remove the oxygen and enhance oil recovery. Christopher(7) [see also Yannimaras et al.(8)] used an accelerating rate calorimeter (ARC) to screen light reservoir oils for continuous exothermicity. For light oils they found that about 20﹪ were good candidates for propagating full in situ combustion. This suggests that perhaps a majority of light oils will sustain only low temperature oxidation (LTO). Thus, when the primary objective is only to generate nitrogen and carbon dioxide in situ, then a less intensive oxidation process, without combustion, is sufficient. The focus is therefore on a spontaneous LTO process, which can be applied in all light oil reservoirs with sufficiently high reactivity to react with (and consume) oxygen in the injected air.

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TX 75083-3836, U.S.A., fax 01-972-952-9435.Abstract THAI -'Toe-to-Heel Air Injection' is a radically new process for the recovery and in-situ upgrading of heavy/medium crude oil and bitumen. It is an integrated-horizontal wells process, which creates its own natural sealing mechanism (in the horizontal producer well), enabling stable combustion front propagation along the horizontal producer well to be achieved. CAPRI is an extension to the basic THAI (thermal) process, which involves the emplacement ('gravel-packing') of a standard refinery HDS catalyst, around the horizontal producer well.A series 3-D combustion cell experiments were performed to physically simulate the THAI-CAPRI process, using Lloydminster heavy crude oil (11.9 o API). In separate THAI and CAPRI experiments, high temperature combustion was sustained (500-550 o C), achieving stable combustion front propagation. Very high oil recovery was achieved, exceeding 79 % OOIP for THAI and CAPRI. The first-stage of downhole conversion, or in-situ upgrading, is accomplished by thermal cracking (THAI), and thermally/catalytically, in the second stage (CAPRI). Thermal upgrading of the produced oil by THAI averaged 18.3 o API -an incremental conversion gain of 6.4 API points, with a maximum of 22 o API. Higher upgrading was achieved by CAPRI using a regenerated CoMo HDS catalyst. The produced oil averaged 23 o API, essentially a light oil product. The oil viscosity produced by THAI or CAPRI was as low as 20 to 30 mPas.

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