Light Oil Air Injection Process
Figure 2 shows a simple schematic of the light oil air injection (LOAI) process, using a conventional well pattern arrangement, i.e. long-distance displacement. The main displacement mechanism is due to the ‘flue gas’, which is generated in situ by oxidation reactions with crude oil. The displacing front accelerates ahead of the oxidation zone. All of the injected oxygen is consumed in the oxidation zone, thereby creating a safe process. The ‘flue gas’ typically contains 10% to 15% carbon oxides, with the rest being nitrogen. Of course, if higher CO2 concentrations are desirable, this can be achieved by injecting enriched air, up to 50% oxygen or higher concentration. This will also reduce the compression cost.
The EOR process in Figure 2 is horizontal WAG. Depending on the oil reactivity and reservoir properties, oil oxidation can occur slowly, or at very high rates. Either way, the reaction pathway is via low-temperature oxidation (LTO), which can extend all the way from reservoir temperature (about 100°C) (4) up to about 250°C. Combustion only occurs above about 350°C if the fuel is heavy residue or coke. In LOAI, the thermal effect generated by the exothermic oxidation reactions generally will not contribute very significantly to the overall recovery of around 6% to 12% incremental. However, this still represents a very significant recovery on low waterflood residuals. Figure 2 shows that the mobilised oil must displace through the downstream section of the reservoir to reach the production well. In North Sea reservoirs, inter-well distances of 1km are common, so it could take a few years before any oil is seen at the production wells. Even then, the water will bank up first and be produced before the oil. Obviously, some clever interception techniques are required to tap into the oil bank as soon as possible.
Figure 2: Schematic Diagram of Air Injection LTO Process
The main benefits of air injection are that:
- air is available everywhere – a very important factor for offshore locations;
- the LOAI process can be operated without gas breakthrough, so that all the ‘flue gas’ is kept in the reservoir, or continued after gas breakthrough. Separation of the produced gas is then required;
- the CO2 produced is conveniently available ‘at source’, avoiding the high cost of collection and transport. This can save US$0.5 per tonne of CO2/100km; (5)
- the CO2 produced can be injected into adjacent light oil and medium-heavy oil reservoirs; and
- using integrated large-scale IOR, the CO2 produced can be cascaded, reservoir-toreservoir, before being finally sequestrated.
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