Recovering from a problem
29/08/2008 Plans to build a gas-fired power station at the Mongstad refinery in western Norway were given the go-ahead in October. This scheme includes the world’s first full-scale carbon dioxide capture plant, due to be fully operational by 2014 with partial government funding. Although no decision has yet been taken, the captured carbon dioxide could help to improve offshore oil recovery.
text: Jan Bygdevoll
Carbon dioxide is a special gas, essential for life on Earth. Plants convert it to oxygen and organic compounds by photosynthesis.
The reverse process occurs when all forms of organic material are burnt, with carbon dioxide and water as the final products after complete combustion of wood, coal, oil or gas.
Pure carbon dioxide is familiar as bubbles in mineral water, beer and other beverages, and as dry ice – itself an indication that this substance differs from other gases.
It does not liquefy at low temperatures under atmospheric pressure, but converts directly from gaseous to solid form. When the pressure rises, however, it becomes liquid quite swiftly.
These properties, and the fact that carbon dioxide displaces air, give it many applications in refrigeration, welding and many other industries.
Injecting water or gas into the reservoir is a conventional technique for optimising oil recovery. Gas injection often yields better results than water, but this advantage must be set against the value of gas sales and the cost of the equipment needed.
Recovery factors are often highest when the combination of pressure, temperature and hydrocarbon properties produces “balanced displacement”.
With characteristics which mean that such displacement often occurs, carbon dioxide can thereby provide an effective medium for driving out more crude.
Many big US oil fields in such locations as west Texas and New Mexico came on stream in the 1930s and 1940s without injection to maintain reservoir pressure.
Water was later injected to improve recovery. This was eventually supplemented by injecting carbon dioxide drawn from virtually pure deposits in Colorado and Utah.
Carried through long pipelines, these supplies are preferred to associated natural gas from the field which can be sold in the market at a good price.
Carbon injection was originally adopted solely to improve recovery, long before emissions of the gas came to be regarded as an environmental problem.
Experience from the USA and elsewhere shows that this technique often yields a greater improvement in oil recovery than other relevant me thods. But higher costs mean that its profitability varies.
Carbon dioxide combined with water boosts corrosion. Some of it also returns to the surface during production, and must be separated out before the associated natural gas can be sold.
Many studies have been carried out to establish whether carbon injection could also be used to boost recovery from Norwegian offshore fields.
As the possible environmental consequences of emitting carbon dioxide to the air have entered the public debate, this question has become increasingly relevant.
A study by the NPD in the spring of 2005 indicated that carbon injection could improve recovery from many oil reservoirs on the Norwegian continental shelf.
Similar investigations have been carried out by such bodies as the Bellona environmental foundation and various oil companies.
While carbon injection would be the best solution on a number of fields, it remains difficult to define projects which meet profitability requirements in relation to risk and uncertainty.
The most important reason for this is the high cost of separating (capturing) carbon dioxide from flue gases in gas-fired power stations and transporting it to the injection point.
Statoil concluded in the summer of 2004 that it would be possible to recover more than 120 million additional barrels of oil from its Gullfaks field in the North Sea.
However, this was conditional on being able to secure at least five million tonnes of carbon dioxide per year at an acceptable price.
Calculations have so far shown that carbon injection for improved recovery first becomes profitable at an expected oil price of USD 30-40 per barrel.
That is significantly above the price normally assumed by the Norwegian government and the oil companies when planning projects with this level of risk.
The players have accordingly called for the environmental and electricity generation sectors to participate in financing and operating any carbon injection schemes.
Statoil and Shell are now working to achieve carbon dioxide injection on the Heidrun and Draugen fields they respectively operate in the Norwegian Sea.
This calls for some 2.5 million tonnes to be captured annually from a planned gas-fired power station at Tjeldbergodden in mid-Norway. However, success again depends on contributions to the overall project economics from other sectors.