Well-filled toolbox for improved oil recovery

Ekofisk
Ways of improving recovery from Norwegian fields are many and varied – just like the reservoirs involved. These methods include water, gas, water combined with gas, foam plus water and gas, chemicals, bacteria, well management and new technology.

Injecting water or gas will often be decisive for achieving a high oil recovery factor. Conditions specific to particular fields will determine whether water injection, gas injection or a combination of these is the most effective method.

Well number and placing are also important for efficient recovery. A range of different measures have been implemented to improve recovery from fields on the Norwegian continental shelf. One well-known case is water flooding on Ekofisk to displace oil while boosting compaction of the reservoir. This has yielded a big rise in the recovery factor compared with the original estimate.

Advances in drilling provide another important example. Long horizontal wells have become conventional technology, and wells with several branches are used on a number of fields. These developments have contributed significantly to improving recovery on the NCS. They are not only crucial for oil production on Troll, but also benefit a number of other fields.

Natural gas and water-alternating-gas (WAG) injection have also helped to produce high recovery factors, as have better tools for reservoir visualisation and well management. WAG combines the positive effects of injecting water and gas in one and the same reservoir, and can give very low residual oil saturation. The need for gas, which may be in short supply, is also reduced.

Carbon dioxide, nitrogen and air are other possible injection gases. The first of these has been injected for many years in countries with access to reservoirs of pure carbon dioxide – particularly the USA. Injecting carbon dioxide separated from flue gases in thermal power stations or other industry has not been used on the NCS so far to boost recovery. Nitrogen and air injection are not conventional methods, and pose great challenges in terms of reservoir management and safety.

Less traditional methods, such as foam with WAG (FAWAG) and using microbes to form chemicals in the reservoir (microbial improved oil recovery – MIOR), have also been tested on the NCS. FAWAG is being applied in several wells on Snorre, while MIOR has been employed for a considerable time on Norne. These methods have so far had a limited impact on overall NCS oil recovery. Research has also been conducted on adding substances to the injection water which could reduce residual oil saturation, but this approach has still not been adopted in practice.

Most of the research resources for oil and gas recovery has been focused on technology which could be quickly applied. This applies to drilling and well management, and to mapping oil-rich reservoir “pockets” with seismic methods and visualisation so that infill wells can be positioned. That approach has yielded some very good results, helping to maintain the level of oil production on the NCS in recent years.

A longer-term perspective needs to be applied for research into methods to reduce residual oil saturation. As in the rest of the world, Norway’s commitment here has varied with the price of oil. From being a priority in the 1980s, activity in this area was greatly reduced in the early 1990s at both companies and research institutions. A constant research and development effort with a broad range of methods is important in providing options which the industry can use when conditions are right or to solve particular problems. Should the price of oil remain high for a long time, more opportunities for using such advanced methods are also likely to arise.

(This article was first published in Norwegian Continental Shelf)


29.06.2009