HOUSTON — For decades, the giants of the oil industry were confounded by salt.
While oil companies for years had shot sound waves into the deep to help create images of undersea geology, salt located far under the floor of the Gulf of Mexico was unpredictable. It muffled reflections, or bounced them away from survey vessels, leaving geophysicists in the dark.
But that was before a series of recent seismic imaging breakthroughs involving supercomputers and the largest moving objects in the ocean. The advancements helped oil companies peek under salt layers located miles below the Gulf and have spurred a number of discoveries and billions of dollars in new investment in offshore oil production.
“It’s like a medical imaging experiment that we’re all familiar with when they take an ultrasound and show us an image of the baby, only it’s done on a planetary scale,” said Craig Beasley, chief geophysicist for Schlumberger’s WesternGeco subsidiary.
Using the new and evolving methods, oil companies have found ways to shoot sound miles below the surface and capture more echoes of those sound waves than they ever have.
Teams of mathematicians, geophysicists and software engineers use large computer systems to translate those echoes into three-dimensional images of reserves more than six miles below ocean, rock layers and salt, a feat that was little more than a dream a decade ago.
The innovations have helped oil companies tap into some of the largest offshore oil reserves ever discovered and are a big reason why four towering oil platforms were under construction this summer at a single yard in Ingleside near Corpus Christi. The platforms are the culmination of multibillion dollar projects to extract oil from reservoirs that would have been challenging to target prior to seismic imaging advances.
“If the seismic had stayed the same as it was in 1999 and 2000…today we would be effectively drilling in the dark,” said John Etgen, BP’s distinguished adviser for seismic imaging.
Beasley said salt previously created havoc in seismic images.
“The easiest way to think about it is the swimming pool where you put your pole in the swimming pool, and it looks like there’s a kink in the pole,” he said.
The shape of the pole hasn’t changed, of course, but its image is distorted. Salt in underground layers as thick as 18,000 feet can have the same effect, sending sound waves back at different speeds and angles and leaving gaps in data and distorting images.
Since salt reflects sound in a variety of directions, geophysicists had to find a way to capture more echoes. Covering the ocean surface with sensors isn’t practical, so experts at BP decided to experiment with additional sources of sound, Etgen said.
Most seismic surveys involve boats towing arrays of cables up to five miles long and half a mile wide, perhaps creating the largest moving bodies in the sea. The cables are equipped with hydrophones, which measure sound in water.
To generate sound waves, oil companies use air guns to create explosive sounds under water in intervals as long as 15 seconds.
Environmentalists say the noise can endanger sea life.
The air guns can produce underwater noise of more than 130 decibels, 10 miles from the source, said Michael Jasny, director of the Natural Resource Defense Council’s marine mammal protection project. That’s as loud as a jackhammer and almost as loud as the sound of a jet engine, which measures 140 decibels.
A settlement earlier this year among environmentalists, regulators and a leading industry group puts some limits on air gun use.
When they are used, each blast sends sound waves deep below the ocean and seafloor. Their echoes return and hit the hydrophones.
BP in 2004 pioneered the wide azimuth towed streamer method, a new approach to seismic that would change the industry. The process captures more sound waves with the same set of cables by using multiple boats firing air guns, Etgen said.
Each boat sends soundwaves into the ground, delivering reflections to the surface from multiple angles. Powerful computers use mathematical processes called algorithms to combine the echo images, creating a sharper view of what lies beneath the salt, Etgen said.
The effect is similar to sports broadcasts that use multiple cameras to show the action from different angles. If one angle produces a distorted or incomplete image of the underground rocks, measurements of other reflection angles help fill in the missing parts.
“The more of that thing that I can catch and record, the better chance I have of making an image,” Etgen said. The wide azimuth survey produced 16 times as much data as conventional seismic surveys, he said.
The success prompted other experiments.
Beasley said WesternGeco championed an approach involving just one boat with an air gun and streamers attached. The guns fired as the vessel moved in a giant circle, with a radius of up to 5 miles. Then it moved over slightly and made another, overlapping circle. The method allowed a single array to encircle the reflections bouncing up from the subsurface, helping a survey vessel capture more sound waves from each air gun blast, Beasley said.
WesternGeco now is advancing seismic imaging that involves multiple sounds being fired at the same time, cutting out the waiting period needed between air gun shots, Beasley said.
On the ocean floor
Another revolutionary approach places sensor nodes on the ocean floor rather than towing them on the surface, said Roger Keyte, director of marketing and strategy for Sugar Land-based Fairfield Nodal.
It produces the yellow, disk-shaped nodes in its Sugar Land facility, then ships them by the thousands to locations where they’re dropped off boats, sink to the ocean floor and listen for sound.
They can produce the most detailed images possible because they don’t encounter ambient noise from moving through the water and can be placed in position to capture sonic reflections, he said.
The technique, called on-bottom seismic, allows oil explorers to examine plays beneath existing structures like platforms and pipes, which isn’t possible for unwieldy vessels towing strings of hydrophones. BP first used the method in a commercial survey at its Atlantis field in 2005, Etgen said.
Although they produce more data than towed hydrophones, on-bottom seismic surveys can cost as much as four times more, Fairfield Nodal’s Keyte said.
But companies have found on-bottom seismic critical to expanding their drilling efforts.
“I view it as a ground-breaking kind of technology that’s allowing us to see the subsurface like we’ve never seen it before,” said John Hollowell, Shell’s executive vice president for deep water in the Americas, in a July interview. “And the success of these projects in many respects is your ability to see the subsurface better than anybody else.”
Deep water discoveries
Several of the largest oil fields ever discovered were not produced until many years later, after advances that helped oil companies better plan for the deep obstacles they might face.
BP’s Thunder Horse field, the second largest field discovered in the Gulf with an estimated 1.1 billion barrels of oil equivalent in recoverable reserves, was discovered in 1999, but production didn’t begin until 2008, according to research from energy consulting firm Wood Mackenzie.
Shell’s Mars-Ursa field, which was discovered in 1989, is the largest in the Gulf of Mexico, with 1.3 billion barrels of oil equivalent in recoverable reserves, according to Wood Mackenzie. Production didn’t begin at the Mars field until 1996.
Reserve estimates for both fields, and others, have increased as seismic imaging advancements have helped companies better understand underground rock layers. And a new effort to produce oil from the Mars field is currently underway, following advancements in seismic imaging that helped Shell drill new wells there.
Advancements since then have led to a rush of huge new Gulf discoveries, some that have lifted the profile of smaller oil companies, such as Cobalt International Energy. Cobalt explicitly targets reserves located below salt layers, focusing on its use of advanced seismic imaging to help it find oil, the company says.
Cobalt made one of the largest gulf discoveries in the last decade when it found the North Platte field last year. The field holds 500 million barrels of oil equivalent in recoverable reserves, according to Wood Mackenzie.
Beasley said that further seismic advances are likely to help oil companies find more oil that they couldn’t see before.
“I’m sure there’s a limit somewhere,” he said. “We haven’t reached it yet.”