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Sensing the wheat from the chaff

13/12/2002 A new electromagnetic seabed logging technique could complement traditional seismic surveys based on sound waves in identifying whether a subsurface structure contains water or hydrocabons.

By Torbjørn Goa

This articles was published in [The Norwegian Petroleum Diary No.4/2002]
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Seismic data may be a necessary and natural oil industry tool, but they have their deficiencies and involve some degree of uncertainty. Given that an exploration well can cost NOK 100-300 million, players clearly have a financial motive for seeking to improve the accuracy of placing such probes.

Sensors
Statoil?s recently developed seabed logging method uses subsea sensors to pick up reflected electromagnetic waves. These can specifically identify what a structure contains.

In addition, this technique is able to provide valuable information on the level of saturation of possible hydrocarbons in a reservoir rock.

Traditional seismic surveys give us flashing green lights and the hope of discoveries even when such saturation is low,? explains chief geophysicist Terje Dahl at Statoil.

Seabed logging can tell you whether the hydrocarbons occupy five or 80 per cent of the available reservoir capacity. ?But we can?t open the champagne yet, because this method is still at the research stage and has a number of shortcomings which must be improved through intensive development.?

Statoil has already applied the technique ahead of the 17th licensing round on the Norwegian continental shelf in 2002, and off Angola.

So far, the method works best in 500 metres of water or deeper, and the low-frequency electromagnetic waves react only to large formations and reservoirs,? Dr Dahl comments.

These are obvious limitations, since few really large fields remains to be discovered on the NCS.?

The next step is to continue developing and improving the technology for use in shallower water depths, he says. 

At present, it?s confusing to try and interpret information gathered in shallow water. So we?re hoping to develop methods and computer programmes which help to sort wanted from unwanted data.?

Bats
Seeing with the aid of sound waves is a well-known concept. Bats and submarines do it - and people.

Traditional seismic builds on the same principle.  Sub-surface structures are mapped by emitting low-frequency sound waves at up to 200 Hertz. This is usually done offshore with the aid of air guns placed five to 10 metres beneath the sea surface.

Shots are fired at set intervals by releasing compressed air from chambers in the guns, which are located in an array behind a survey ship. This vessel also tows cables up to a kilometre long containing hydrophones - sensors which pick up the sound waves reflected back from below ground.

Seismic techniques are used today to identify sub-surface rock types, and in some cases they can indicate whether an undrilled structure contains hydrocarbons or water.

Dr Dahl stresses that seabed logging will not make this traditional approach superfluous.

These two methods complement each other, and can combine to reduce uncertainty. Seabed logging otherwise only provides a rough picture of a structure, while a seismic survey gives more detailed and specific information about its quality and size.?

Value
According to Dr Dahl, both four-dimensional (4D) and seabed (4C) seismic will make a growing contribution to value creation on the NCS.

Conventional 2D seismic describes thin sections of the sub-surface. The 3D method can map a structure from many angles, giving a better spatial picture or more specific understanding of its form. 

A 4D seismic survey extends the value of 3D data by monitoring a producing reservoir over a long period. That allows you to see how the oil and gas are moving through the structure.?

Major financial benefits are conferred by the 4D solution because it indicates the optimum location for new production wells. It can also be used to monitor carbon dioxide injection into sub-surface formations.

The 4C technique involves installing geophones - acoustic sensors - on the seabed, rather than towing hydrophones on cables behind a ship at the surface. 

In future, we`ll see an integration of seabed logging with 4C and 4D seismic methods, Dr Dahl predicts.

Its interesting to note that seismic surveys now look like playing a part throughout a field?s producing life, rather than being confined to the exploration phase.

That`ll allow us to produce from ?intelligent? fields with more continuous observation and an easier flow of information.?

Updated: 04/09/2009