Thick layers of salt deep in the sea bed in the Gulf of Mexico make locating oil and gas a big challenge. Shell has developed new technology to see under the salt and discover resources that help to meet the world's increasing energy demand. Far below the surface of the sea in freezing temperatures and at pressures too great for divers to bear, advanced technology is helping engineers to explore for oil and gas. "Drilling in deep water is very expensive," says Dirk Smit, Vice President of Exploration Technologies. "So we need first to pinpoint reservoirs with accuracy."
Seismic surveys reveal underground structures and reservoirs in the Gulf of Mexico
Engineers carry out seismic surveys to map out reservoirs below the sea. They use an air gun to shoot sound waves from a boat towing receivers. The waves bounce off the rock below the sea bed and back to the receivers. The speed and direction of the sound waves as they bounce back helps engineers to build up a three-dimensional model of the rock and assess if it might contain oil and gas reservoirs.
But in some locations salt layers distort the sound waves the way static on a telephone line breaks up a conversation, making it hard to build up an accurate picture.
Avoiding obstacles
To overcome the challenge of salt, companies have started to place sound wave receivers on a second boat or on the sea bed. This creates a wide angle between the source of the sound and the receivers. The technique is known as wide azimuth seismic, a mathematical term that describes the precise measurement of the angle in relation to the sea's surface. Sound waves travelling at this angle pass below the salt layer and provide information on rock that was previously invisible.
Until recently the high cost of using a second boat and the computing power needed to process the results restricted the use of wide-angled surveys. But advances in technology over recent years have made the process more economic: now it is more widespread.
Shell used wide-angle seismic surveying in the Gulf of Mexico in 2009-2010. This led to two major discoveries and added more than 200 million barrels of oil to the region's resources.
Looking ahead
Separate boats send and receive signals to pass below thick salt layers in the sea bed
In the future these surveys will be used to overcome the challenges salt poses in the deep water off the coast of Brazil, West Africa and in parts of the Mediterranean.
Shell is also testing with a partner a way to automate the placing of seismic receivers by using self-propelled robots equipped with satellite navigation systems. This would remove the need for operators controlling each robot and more receivers could be used to improve accuracy of the surveys. The new robots would propel themselves under water to pre-set locations to record sound waves and then float to the surface for reassignment.
