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Seabed seismic and James Bond

Thursday, November 12, 2015

John Moses of Seabed Geosolutions explains seabed seismic using the analogy of James Bond trying to understand an object at the bottom of a murky swimming pool.

Imagine James Bond trying to find an object at the bottom of a dark swimming pool.

He has an idea that something is there, and has a powerful torch to see it more clearly.

But he can't work out exactly where it using the torchlight, because the torchlight bends as it goes through the water, and you don't know the water depth or the amount the light is bending.

You can get a better idea by walking all around the swimming pool with your torch, and making calculations.

This analogy starts to indicate why recording seismic data on the seabed gives you a much clearer understanding of what is in the subsurface, said John Moses, regional sales director with Seabed Geosolutions, speaking at the Finding Petroleum forum in Stavanger on June 10, 'Transforming Subsurface Interpretation'.

Seabed seismic recording can pick up more components of seismic data. Shear waves can't travel through water, so recording on the water surface (as with towed streamer) you can only record compressional (P) waves. But on the seabed you can record both shear waves and P-waves. This multi component seismic data is analogous to having a more powerful torch to look in the swimming pool with.

Recording in different directions (azimuths) is expensive when recording with towed streamer, because it means making multiple trips with your source vessels and streamers. But when recording on the seabed, the recording device (node) can pick up seismic coming from many different directions at once. The multiazimuth is equivalent to walking around the swimming pool.

You may need to spend some more money on seabed seismic than towed streamer, equivalent to making more effort to understand the object in the swimming pool before you jump in. But drilling, like James Bond ruining his Tuxedo by jumping in the swimming pool, can only be done once.

There might be a sunbed floating on the swimming pool, which makes it hard to see the object. But recording seismic on the seabed means that the obstructions on the water surface don't matter.

Towed streamer seismic recording is not able to record near any obstructions on the water, such as offshore platforms (which are often built above reservoirs), as well as shallow water, rocks and ice.

Technology

Seabed Geosolutions' technology can do recording at very shallow depths and depths of up to 3000m, with nodes deployed in deepwater using Remote Operated Vehicles (ROVs).

The limit in the number of nodes in one survey is basically linked to the number of nodes you can physically deploy, and surveys are getting larger and larger. A typical offshore vessel in the North Sea can carry 10,000 nodes in containers, he said.

You can record with one node until the battery runs out. Battery capacity in nodes is increasing around 10 per cent a year, he said, and meanwhile the power consumption of the electronics is going down.

Seabed Geosolutions is a joint venture of CGG and Fugro.

Comparing the results

In one example from the Dan Field in the Danish sector of the North Sea, a seismic streamer survey from 2012 was compared with a seabed survey conducted later.

The streamer survey seismic image shows a big gap in image resolution, from the area beneath an offshore platform where the vessel was not able to survey. When ocean bottom node data was recorded, there was no obstruction.

The subsurface has a chalk reservoir with a big fracture going through it. It has 108 wells, half producing and half injecting, with the wells producing and injecting changing all the time.

In another example, data was recorded by Chevron, offshore West Africa in 400m water depth.

The company was using a streamer survey in an oilfield with a massive platform in the centre, and large water currents. 'It's a horror show for a streamer company,' he said.

Seabed Geosolutions flew 3 containers of nodes to the city of Pointe-Noire, and from there they were trucked to the town of Malongo, where they were loaded on the client's ROV vessel. They were placed onto the seabed by ROV. 'It takes 3-4 days to deploy and 3-4 days to pick them up,' he said.

In another example from an Australian field Apache used seabed data to provide a better interpretation of the oil water contact, which was very useful in the drilling program, Mr Moses said.

Drillers could get much more useful information from seismic data than they currently do, he said.

Another issue seabed seismic can help with is when there is gas in the overburden (the rock above the reservoir), which can block the passage of compressional (P) waves, which are the only waves towed streamer seismic can record. But with seabed recording, you can also record shear (S) waves, which can pass through the gas.

This was a particular problem for a field in Malaysia operated by PETRONAS.

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