Since the first oil was discovered in 1964 at Fahud, Oman has been heavily developing its oil and gas industry, which today accounts for 80 per cent of export revenues and 40 per cent of gross domestic product. However, Oman oil is highly viscous and only a fraction of the oil contained in its reserves is recoverable using traditional production methods.
Over the past decade, Oman has been working hard to improve the recovery of its oil reserves and has adopted enhanced oil recovery (EOR) techniques on a large scale. As a result the Sultanate has been able to increase oil production to nearly 1 million barrels per day (bpd) from a low of 714300 bpd averaged in 2008. This has also changed the outlook for its oil industry which is now estimated to have at least 40 years of life ahead of it.
What is EOR?
EOR is a term that describes a range of techniques used to increase the amount of crude oil that can be extracted from an oil field. By using EOR 30 to 60 per cent, or more in some cases, of the reservoir's original oil can be extracted compared to 5 to 20 per cent attainable using primary and secondary recovery techniques. It is estimated that the world average oil recovery factor is only around 35-37 per cent, which means that the remaining 63 per cent to 65 per cent is not recoverable at present by using traditional recovery methods.
There are three methods of EOR:
- Thermal recovery/steam injection.
- Chemical injection/polymer injection.
- Gas injection: natural gas, carbon dioxide, nitrogen.
EOR techniques are applied to:
- Decrease the viscosity difference between the oil and water phases.
- Reduce interfacial forces holding the oil in pores within the rock formation.
- Modify the reservoir and oil properties to make the oil more easily released and produced, that is improve sweep efficiency.
Thermal EOR employs high pressure steam or in some cases in-situ combustion. This technique is favoured for the displacement of heavy oil.
Chemical EOR involves injection of interfacial active components, such as surfactants, polymers and alkalis. The mechanism of oil displacement is obtained through the creation of ultra-low surface tension.
Gas injection, the most-widely used approach today, utilises high pressure carbon dioxide, nitrogen or hydrocarbon gas to generate miscibility, thus increasing oil volumes and decreasing oil viscosity.
There are a number of advantages associated with the adoption of EOR techniques; not only can EOR prolong the life of a depleting field by up to 30 years when correctly employed on proven reserves, but it can also save the costs of finding a replacement field. As many fields around the world are starting to reach the end of their productive life, EOR represents the solution to sustain production levels and meet the increasing demand for oil.
The first export of Omani oil took place on 27 July 1967. Initially oil production rose steadily to 341000 barrels per day in 1975 and in 1984 the average daily production reached 400000 barrels a day. However, aware of the low recoverability factor of its fields due to a complex geology, Petroleum Development Oman (PDO) - the largest oilfield operator in the Sultanate - started a series of EOR trials in 1986. The trials proved successful and Oman moved swiftly to implement EOR, boosting production to 714300bpd in 2008 before enhanced oil recovery methods were implemented and to a current level of nearly 900000bpd with EOR techniques. EOR projects commonly yield between 5-15 per cent incremental reserves and PDO expects its EOR projects to contribute around 35 per cent of its total production by 2020. For this reason Oman is seen by many as pushing the limits of EOR technology.
Qarn Alam is the world's first full field EOR project and also the largest of its kind in the world. A sophisticated EOR technique called thermally assisted gas oil gravity drainage (TAGOGD) is employed due to characteristics of the fractured carbonate reservoir where the oil is highly viscous and a very low percentage of recovery is feasible by conventional oil extraction method.
The US$1.2bn facility comprises a water treatment plant producing 24000 tonnes per day of boiler feed water, using reverse osmosis and a steam generation facility producing 18000 tonnes per day of steam, using heat recovery steam generators (HRSG). The application of EOR is adding 60000 bpd, and 1.2MMSCMD of gas to the original output.
Marmul field is located in south Oman. Marmul field is characterised by heavy viscous crude that is difficult to extract by traditional recovery methods. The reservoir has a viscosity of around 90Cp. To improve the reservoir's sweep efficiency, water is viscosified by the addition of polyacrylamide polymers and then injected in the reservoir through polymer injection wells. The polymer flooding at Marmul field will add a further 8000 bpd. The implementation of this technique will lead to an estimated 10 per cent to 15 per cent increase in recovery levels from the Marmul reservoirs.
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Vinod Shah is Managing Director, Mott MacDonald & Company LLC, Sultanate of Oman. www.mottmac.com