4-C Projects Harvest Gulf Data

Emerging Technology Matured in North Sea

Storms were raging over New Orleans and much of southern Louisiana one day in late June, but at that same moment the skies were clear and the waters were smooth about 30 miles out in the Gulf of Mexico.

On land, people were dodging lightning bolts.

On the water, people were hoping that lightning would strike -- not as an electronic discharge, but as a bolt of good fortune to their efforts.

No, not just hoping. They were aggressively seeking subsurface data, using an emerging seismic technology that could be the next big step in the industry's march toward finding new sources of energy.

And under the water? A gigantic, thick, heavy orange cable was being lowered and coupled to the seafloor, carrying an arsenal of 75-pound receiving stations that would be the new windows for geologists and geophysicists to view subsurface treasures.

Hour after hour, mile after mile, this was the scene: Under ideal conditions, gliding smoothly past dozens of offshore oil structures that dotted the horizon, three boats carrying scores of geoscience crew members worked a massive seismic survey designed to find and help unlock new oil frontiers in a very old area.

One boat laid the line, one boat shot the energy-seeking source guns, one boat recorded the story. Constant activity. Constant precision.

The source ship Snapper -- a one-time Norwegian ice-buster -- carried twin air gun source arrays, and a new blast rumbled through the water every 14 seconds.

And with each vibration, another piece of the subsurface was revealed. Each blast signaled new data for the crews and computers aboard the Dolphin and Manta recording and tracking vessels.

For most of the workers, most of the time, their day revolved solely around their individual duties: You, lay the lines. Position the boat. You, fire the guns. Get the data. Check the quality. Retrieve the cables. Check the receivers. Turn the boats, find the new track and do it again.

One worker, one duty. A team-effort in action.

In those moments, the grand-scale -- and perhaps grand significance -- of the operation was a distant thought.

But not always.

"It's a big operation," one worker said between blasts from the source guns, "and so far things have gone good. No, things have gone real good."

Lots of Interests

Welcome to West Delta, site of a two-month seismic operation that carried the flag for both the cutting edge of seismic technology and the future of Gulf of Mexico shelf development.

The operation was a speculative 4-component (4-C) seismic survey shoot using ocean bottom cables by crews of Schlumberger Geco-Prakla, and the project's results and ramifications could have impact far beyond the interests of it and its partner Seitel.

The size of the operation was news enough.

The West Delta shoot, which started in early June and ended in late July, covered 530 square kilometers of choice offshore real estate just southwest of New Orleans.

To put that size in perspective, according to Michael Spradley, Schlumberger's point man on the project, "people consider that a 'huge' operation of 4-C seismic usually is about 50 square kilometers. Before this year, the largest 4-C seismic project ever was 130 square kilometers, acquired by Schlumberger Geco-Prakla in the North Sea."

The West Delta crews had previously completed a true 3-D/4-C seismic operation over West Cameron, about a couple of hundred miles west-southwest of West Delta in the Gulf of Mexico, and that gig was the record setter in terms of size (just under 1,000 square kilometers) for this technology.

West Delta has its own claim to fame, however, namely in that it is the world's second largest conventional 4-C OBC/extracted grid survey ever shot.

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Image Caption

These boats, the Manta and Dolphin, represented two-thirds of the West Delta seismic operation, and their crews alternated duties of line laying, line retrieval and data acquisition. By the end of the day they would be miles apart. Photos by Bob Compton Captured Light Photography

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Storms were raging over New Orleans and much of southern Louisiana one day in late June, but at that same moment the skies were clear and the waters were smooth about 30 miles out in the Gulf of Mexico.

On land, people were dodging lightning bolts.

On the water, people were hoping that lightning would strike -- not as an electronic discharge, but as a bolt of good fortune to their efforts.

No, not just hoping. They were aggressively seeking subsurface data, using an emerging seismic technology that could be the next big step in the industry's march toward finding new sources of energy.

And under the water? A gigantic, thick, heavy orange cable was being lowered and coupled to the seafloor, carrying an arsenal of 75-pound receiving stations that would be the new windows for geologists and geophysicists to view subsurface treasures.

Hour after hour, mile after mile, this was the scene: Under ideal conditions, gliding smoothly past dozens of offshore oil structures that dotted the horizon, three boats carrying scores of geoscience crew members worked a massive seismic survey designed to find and help unlock new oil frontiers in a very old area.

One boat laid the line, one boat shot the energy-seeking source guns, one boat recorded the story. Constant activity. Constant precision.

The source ship Snapper -- a one-time Norwegian ice-buster -- carried twin air gun source arrays, and a new blast rumbled through the water every 14 seconds.

And with each vibration, another piece of the subsurface was revealed. Each blast signaled new data for the crews and computers aboard the Dolphin and Manta recording and tracking vessels.

For most of the workers, most of the time, their day revolved solely around their individual duties: You, lay the lines. Position the boat. You, fire the guns. Get the data. Check the quality. Retrieve the cables. Check the receivers. Turn the boats, find the new track and do it again.

One worker, one duty. A team-effort in action.

In those moments, the grand-scale -- and perhaps grand significance -- of the operation was a distant thought.

But not always.

"It's a big operation," one worker said between blasts from the source guns, "and so far things have gone good. No, things have gone real good."

Lots of Interests

Welcome to West Delta, site of a two-month seismic operation that carried the flag for both the cutting edge of seismic technology and the future of Gulf of Mexico shelf development.

The operation was a speculative 4-component (4-C) seismic survey shoot using ocean bottom cables by crews of Schlumberger Geco-Prakla, and the project's results and ramifications could have impact far beyond the interests of it and its partner Seitel.

The size of the operation was news enough.

The West Delta shoot, which started in early June and ended in late July, covered 530 square kilometers of choice offshore real estate just southwest of New Orleans.

To put that size in perspective, according to Michael Spradley, Schlumberger's point man on the project, "people consider that a 'huge' operation of 4-C seismic usually is about 50 square kilometers. Before this year, the largest 4-C seismic project ever was 130 square kilometers, acquired by Schlumberger Geco-Prakla in the North Sea."

The West Delta crews had previously completed a true 3-D/4-C seismic operation over West Cameron, about a couple of hundred miles west-southwest of West Delta in the Gulf of Mexico, and that gig was the record setter in terms of size (just under 1,000 square kilometers) for this technology.

West Delta has its own claim to fame, however, namely in that it is the world's second largest conventional 4-C OBC/extracted grid survey ever shot.

So that, alone, is cause for pause.

But also, this operation could one day be seen as one of the important steps leading to a new era of offshore development in proven and/or congested areas, and it could prove to be a showcase for an emerging technology whose time has come.

Real success, of course, will be claimed only when the data is finished and studied -- a project that targets December for its completion date.

But the project is important for other reasons, too -- and to a lot of other people.

West Delta's 154 blocks are almost all leased to a variety of companies -- both large and small -- that may be very interested in what the survey's new technology reveals.

Among those with West Delta holdings are Chevron, Agip, BP Amoco, Oxy, Exxon, Amerada Hess, Texaco, Mesa, Vastar, Chiefton, Burlington and Sonat.

"Prospects (in West Delta) are getting harder to find," said Richard Fossier, with Schlumberger Reservoir Evaluation, adding that West Delta had an existing P-wave data set (Pressure-wave) that was older and was acquired on a looser grid.

"This operation is designed for imaging objectives for production groups," he added. "They're (companies) hoping this is going to be the last data set they need."

For example, "There are a number of opportunities that we could put a rig on right now, or we could spend the money and get a new data set," said Greta Lydecker, manager of Chevron's Harvest Asset Team now at work on West Delta 117.

"We're working hard to identify additional wells," she continued. "We're just waiting for the data to help us fine-tune our locations and further reduce risks.

"We're open to new technology," she said. "We need the new tools to help us better define the geology and possibilities of the projects."

Team geologist Abby Hymel agreed.

She said Chevron is interested in the new 3-D survey over the area because it is seeking ways to extend the life of the field.

Hymel is currently working Chevron's West Delta Block 117 Field, on which hydrocarbons were first discovered in 1963. She described it as a complexly graben-faulted anticlinal structure.

New seismic data, she said, may lead to:

  • Reduced dry hole risk by better imaging the faults in a structurally complex field.

  • The identification of new drilling opportunities due to possible new interpretation.

  • The identification of possible bypassed hydrocarbons by changes in amplitudes from previous 3-D surveys.

"We anticipate that the new survey will better image the faulting," she said, "due to the improvements in acquisition and processing technology since our previous 3-D surveys were shot."

Finally, An Emerging Technology

For many, 4-C seismic may be a technology that has been heard of but not understood.

For starters, it's simply a "production enhancement technology," experts say, which makes it even more valuable in light of today's industry realities.

In fact, it's probably been talked about for more years than it's actually been used.

Four-C development dates back to the mid-1980s, and it became a big industry buzzword by 1996 -- remember those heady days before the crash? -- when there was money to spend on R&D and when there was a "tremendous ramp-up" for the technology, according to Jack Caldwell, manager of reservoir geophysics for TomoSeis in Houston.

Caldwell was with Schlumberger in those days, and was a key player in the technology's development.

"But then came the crash (1997), and high tech projects were the first to go when the activity slowed down," he said.

But now activity is starting to return, both to new frontier areas and, as in the case of West Delta, to older areas that may have a few surprises left.

Caldwell believes in the technology's value -- passionately so -- and is happy to see it getting a chance once again to shine.

"(It) is an emerging technology that could be used very commonly over the next five years if the cost of oil stays over $21 a barrel."

While Schlumberger is a big player in 4-C technology development, it's not the only player in the game. Western and PGS -- "the other big guys," Caldwell said -- have been active, with Veritas and Fairfield also becoming or on their way to becoming players.

Most 4-C work has to date been done on the North Sea -- not for geographic purposes, Caldwell continued, but because the Norwegian government has put pressure on its oil industry to improve recovery rates.

In such an environment, new technology is embraced -- in fact, anything that will help return rates there is embraced. So 4-C was utilized in the North Sea early in its development life.

Of the 145 4-C projects in the world (Geco-Prakla has shot the majority of them), more than half are found in the North Sea.

And because of success there, 4-C has moved on to other regions, most notably the Far East, Middle East, West Coast of Africa, Alaska's Cook Inlet and, now, the Gulf of Mexico.

Most of the industry's work has been speculative, Caldwell continued, and it is intended for offshore operations. When it works, he said, "we can get much more information on the properties of the reservoir" than with conventional seismic.

Schlumberger's Spradley is succinct: It (4-C) is best at "removing the blind spots" that often occur in acquiring seismic data.

That made it valuable for the West Cameron shoot, and officials now hope it will prove the same for West Delta.

4-C Advantages

Here's how 4-C works.

In conventional 2-C operations, Spradley said, you have one hydrophone and one geophone per receiver station. In 4-C operations, you have one hydrophone and three geophones per station -- one that is vertical, one that is in-line and one that is cross-line.

"That allows you to gather omni-directional seismic data," he said.

In the West Delta operation, the receivers were placed about 165 feet apart. Each 4-C unit weighs 75 pounds.

"That assures excellent coupling to the earth," Spradley said, "to make sure we get the best possible data."

The two extra horizontal geophones are important because, while the hydrophone and vertical geophone measure P (pr primary) sound waves, they measure the Shear volocity of each molecule in the solid earth (S, or shear, waves).

The Schlumberger proprietary OBC system is called the Nessie 4-C multiwave array, which debuted for the company in the North Sea in 1996, according to Jason Robinson, the company's business development manager.

"Until the 1970s, seismic exploration was limited to P-waves," Robinson said, which "do a good job of imaging the subsurface in most locations.

"However, just making an image of subsurface geology often is not easy with P-waves, and we are sometimes tricked by the images created by P-waves," he said. "This is where S-waves can often help to clarify the image.

"This is the principal application now here in the Gulf of Mexico -- to use the S-waves to image through gas clouds so we can see what geologic structure is down there."

The advantages of OBC 4-C seismic, he said, include several technical distinctions, including:

  • Lower ambient noise level (no streamer noise).
  • Potential for greater bandwidth (no receiver ghost).
  • P-wave and mode-converted S-wave data can be acquired.
  • Operations are less sensitive to weather, because with cables on the seafloor "we are not limited by weather noise that often shuts down operations of a streamer operation."
  • Infill shooting unnecessary.
  • Permanent emplacement is possible.
  • Operation in and around obstructions is easier -- a plus in an established offshore region like West Delta.

"The main problem (in areas like West Delta) is obtaining continuous seismic data in areas with many surface obstructions," Robinson said. "This, however, is the ideal application of OBC, because we can deviate the cable around the obstructions, yet still record data beneath (it)."

'Consistently Efficient'

Schlumberger's West Delta operation involved three ships -- an important facet in helping to make the shoot an economical project.

Having the second recording ship meant non-stop shooting for the source boat. One boat was always ahead of it laying cables, and one boat was always behind, recording and picking up the cables.

Then the two vessels switched roles, back and forth, all day long, in north-south swaths across West Delta.

"Having two ships in continuous operation like this allows for continual shooting," Spradley said. "By having cloned recording vessels we are constantly ahead of the shooting vessel.

"This is why we have been consistently efficient."

Computers and global positioning technology kept all vessels on the right track, creating an automatic pilot for steering that could be overridden by the human pilot if and when obstacles -- such as fishing boats -- got in the way.

For the West Delta shoot there were 84 receiver lines, with the four-component receiving units spaced every 50 meters apart. Located on the lines close to each of those units were other receiving units that were used to help check location along the sea floor.

Being on the sea floor is what made this operation special.

"In order to record shear waves we have to put the sensor in contact with the sea floor," Schlumberger's Robinson said. "S-waves do not propagate in fluids."

And, because the sensors are on the floor, "we are isolated from swell and wave noise that affects the top 10-20 meters of the water column, depending on the weather, location and season.

"When we started putting sensors on the floor we had to find a way of making it effective and efficient," he continued, "so in the case of Nessie 4-C we designed an array of multiple sensors to attenuate noise -- and with the GOM crew we used a heavier single sensor package to ensure good coupling to the sea floor."

If all went well, Schlumberger will deliver a fully stacked, pre-stack migrated 3-D coverage over about 20 West Delta blocks by December.

And a new chapter of production may begin in the history that is the Gulf of Mexico.

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