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 spontaneous LTO reactions with the residual oil in the reservoir. The product of the LTO reactions is a "flue gas", which displace 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.
Gas injection into light oil reservoirs is a proven IOR technique. The IOR potential for gas injection in the UKCS has been estimated at 1.4 BSTB.' 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.
Yarmimaras 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 Hackbeny Field at Louisiana (Amoco3), in Medicine Pole Hills Unit at North Dakota and Buffalo, South Dakota (Koch4), most recently, in the Horse Creek field at North Dakota (Total5), and for 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 1) used accelerating rate calorimeter (ARC) to screen light reservoir oils for continuous exothermicity. In general, for light oils about 20 percent were considered 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 the light oil reservoirs with sufficiently high reactivity to react with (and consume) oxygen in the injected air.