CARBON-DIOXIDE IN ENHANCED OIL-RECOVERY

Oil reservoirs are deep underground, with the oil and gas contained in porous rock at high temperatures and pressures. Around 5 - 20%, of the oil can be produced from the field under its own pressure (primary production), but in most fields water is injected to displace the oil. This still leaves at least 50% of the oil behind in the reservoir. Further recovery can be obtained by injecting carbon dioxide that both displaces and dissolves the remaining oil. At least 71 projects worldwide use CO2 flooding and produce a total of over 170 000 barrels of oil a day, worth around $1.3 billion a year. The cost of producing an extra barrel of oil ranges from $5 to $8 and thus is profitable at the present price of nearly $20 a barrel. In the majority of these cases, the carbon dioxide comes from natural underground sources and is piped to the oil field. The potential use of CO2 flooding would be considerably greater, if large quantities of the gas, extracted from power stations, were available at low cost. For every kilogramme of CO2 injected, approximately one to one quarter of a kilogramme of extra oil will be recovered. For most projects about as much carbon dioxide is disposed of in the reservoir as is generated when the oil is burnt. When CO2 is at a sufficiently high pressure to form mixtures with the crude oil that are miscible in laboratory tests, up to 40% of the oil remaining in the field after water flooding can be recovered. Approximately half the water flooded oil fields in the US could be exploited profitably by CO2 injection. Carbon dioxide flooding of the larger North Sea fields is a particularly attractive prospect, because the crude oil is light (composed of low molecular weight hydrocarbons) and the geology of the reservoirs is less heterogeneous than the American fields. A profitable project would be possible if the gas could be provided and piped to the reservoir at a cost of around $3.50 per thousand cubic feet or less.