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The oil companies might still have been relying on seismic surveys alone had a key person in Statoil not had some money left in the research budget as Christmas 1998 approached.
The Statoil man and some of his colleagues had been in the USA and had seen some earlier experiments with electromagnetism (EM) in and between closely spaced boreholes, and as a result he put the following question to Harald Westerdahl and Fan-Nian Kong at NGI: Can electromagnetism from the surface be used to determine whether a reservoir contains oil or water?
"This story teaches us how important it is to have funds available for research. Most pilot studies do not yield the desired results and are shelved, but this time we hit the jackpot," says John H. Løvholt, who has been involved in the administrative side of activities since the start.
The most common means of exploring for oil today involves the use of seismic surveys. Strong sound waves are sent down into the subsurface, and the "echo" of the sound waves which is reflected back is recorded and used to create an image of conditions in the depths. The method can be compared to using a camera with a flash, which emits a light, and then using the reflection to create an image of the subject. |
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Seismic surveys can be used to find reservoirs, but not to distinguish between oil and water reservoirs. In the past, oil companies therefore had to drill wells into reservoirs to determine whether they contained water or oil. A typical exploration well can easily cost NOK 150-200m, and it is not unusual to have to drill ten wells to find oil in one of them.
The two researchers were hunting in the dark. "It was very difficult to record the electromagnetic waves that had penetrated both the seabed and the oil reservoir and become very attenuated, while the electromagnetic wave transmitter continued to transmit strong signals. How do you hear a mouse whispering while you have a lion roaring in your ear? That was the problem we thought we were facing," explains Løvholt. |
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Earlier attempts had shown that EM waves that were sent down into the seabed did not return to be recorded by the receiver sensors. But one day they discovered that turning the antennae that sent and received EM waves in relation to one another caused a dramatic change in the response. And this was the breakthrough. |
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New analyses and modelling results suddenly showed that EM waves would propagate horizontally along the length of the reservoir, and that it should be possible to pick them up again at the other end with a strength that was great enough to distinguish the signals from the reservoir from those that had taken other routes to the receivers.
"This is about polarised EM waves, i.e. waves that oscillate in different directions. You can compare it to a salmon fisherman who doesn¿t see anything under the water, but then he puts on polaroid sunglasses and sees the fish just below the surface of the water. We put 'sunglasses' on our receiver antennae, and suddenly we saw the reservoir," relates Westerdahl, who heads geophysical work at NGI today.
The new EM technology is now used in combination with seismics and provides a much more reliable estimate of whether the subsurface structure contains hydrocarbons. This means a sharp reduction in exploration drilling costs, which is important in view of the fact that a typical EM survey can cost about NOK 10-11m. |
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Seabed logging and oil exploration with the aid of EM take place by means of ships towing a transmitter just above the seabed. The powerful electromagnetic waves that are transmitted are propagated through the seawater, on down into the seabed, down into the reservoir, through the reservoir, up through the seabed again, and into a network of receiver antennae which are deployed on the seabed. |
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Electromagnetic waves propagate differently through oil/gas and other media, and the return signals can therefore be used to create an image of the subsurface and any reservoirs, which increases the probability of making discoveries.
"We are busy developing equipment. In other words, both the senders, and in particular the receivers, can be even better. NGI is also working intensively on the software that is used to analyse the returning signals and create images of the reservoirs. Moreover, we have a project that involves installing either the transmitters or the receivers, or perhaps both, down in an oil well. This brings us nearer the reservoir, and we can 'snap' even sharper pictures", explains Westerdahl. |
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Today some 15 NGI employees are busily engaged in developing EM technology, both in their own projects and on assignment for companies such as the world-leading company EMGS (ElectroMagnetic Geoservices ASA) and StatoilHydro.
We also believe that EM can be used to direct exploration drilling for oil more accurately. Sometimes oil companies drill a well and 'miss' the reservoir, perhaps by just a few metres. Then we can lower EM equipment down the well and find out which side of the well the reservoir lies on, and how big it is. We are working on a large number of 'spinoffs' of this type," concludes Westerdahl.
NGI has been an active trouble-shooter for the petroleum industry since the very start of Norwegian oil and gas activities, and has a separate Oil and Gas Division. We carry out analyses and numerical modelling, subsea instrumentation and subsea technology and geophysical reservoir mapping. We work in the fields of petroleum geomechanics, geohazards, concept development and engineering, model testing in the field and the laboratory and site and laboratory investigations. |