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The right mix to get more out
Around two-thirds of the oil in an average field remains underground because it is too costly to extract. If the oil is especially thick, the amount left behind can be much higher. But now advanced technologies are making it economically viable to produce the remaining oil.
When an oil field ages, production declines. Engineers sometimes inject chemicals, gas or steam into the field to boost productivity of these mature fields. This is known as enhanced oil recovery (EOR).
In this approach, a water-based chemical solution flushes the oil out of the field’s reservoir rock. The chemicals can be so-called polymer thickeners or detergent-like surfactants.
Polymers are used to match the thickness of the solution to the thickness of the oil. This helps to prevent the solution from quickly “worming” through the reservoir; a thicker solution sweeps the oil more efficiently out of the field. At Marmul in Oman, polymers are expected to increase the percentage of recovered oil from 15% to more than 25%.
Surfactants clean the reservoir rock of oil, much as a detergent removes an oil stain from clothes. In some cases an alkaline solution rather than a surfactant is injected into a field. The alkali reacts with natural acids in the trapped oil to create the surfactant within the rock. We are testing chemical EOR with a mixture of polymer, alkali and surfactant at sites in Russia and Oman.
Injecting gases that can dissolve in oil – so-called miscible gases – creates a mixture that flows more easily through the reservoir to production wells. In the 1970s Shell was one of the first companies to inject naturally occurring carbon dioxide (CO2) to boost oil recovery from fields in Texas, USA. We built one of the world’s largest CO2 EOR projects there.
Today, we are looking at ways of using man-made CO2 from industrial plants for gas EOR. We also are using natural gas for EOR in a project at Harweel in Oman. The gas there – mostly methane – is also rich in hydrogen sulphide. Using this gas, we hope to recover up to 50% of the oil, compared to 10% using conventional production methods.
STEAM (THERMAL) EOR
Boosting oil recovery with steam injection.
Injecting steam into a reservoir heats the oil, thinning it and making it easier to produce. It also pushes the oil to wells where it can be drawn more easily to the surface. Shell is working on several full-scale steam EOR projects with Petroleum Development Oman (PDO), a company in which Shell holds a 34% stake. For example, at Qarn Alam oil is being drained through the natural fractures in the reservoir. Normally the process would be very slow because the oil is very thick. Injecting steam into the fractures is expected to raise the recovery over the life of the field from 3-5% to 20-35%.
With our partner PDO, we have developed an innovative approach to steam EOR involving the use of sunlight. Mirrors concentrate the warmth of solar radiation to generate the steam. A pilot solar steam plant for this purpose is currently operating in Oman.