Subsalt Remains Deep Challenge

It's an Image Thing

Prices are rising, demand is soaring, supplies are tight — and iffy, depending on the geographic location — and many promising U.S. drilling locales continue to be off-limits.

Not surprisingly, many are looking to the industry's Old Faithful, aka the Gulf of Mexico, as the still-bright hope for increased domestic production. Indeed, it's the only readily accessible region harboring some frontier-type plays — the still-new shallow water deep gas play and the ultra-deep water — despite its lengthy producing history.

The lure of potential new finds in the GOM was underscored in the most recent Central Lease Sale190 held by the MMS, which garnered the highest number of bids of any Central Sale in the last six years. Sixty percent of the bids were on the shelf, apparently reflecting interest in the shallow water deep gas, according to Johnnie Burton, MMS director.

The sale also indicated continuing interest in the deep water, and Burton said the large number of tracts receiving bids in the ultra-deep water was particularly noteworthy.

As of March 2003 there had been 24 deep gas completions, with 17 discoveries, according to Debra Winbush at the MMS, who noted 100 wells were permitted in 2003 and 40 in 2004. The agency reported there have been 11 industry-announced discoveries in water depths greater than 7,000 feet and noted these ultra-deep water discoveries have the potential to open up entirely new geologic frontiers.

But the deep Gulf is salt country, meaning there are obstacles to overcome, and not just in terms of iron.

It's an image thing.

"The imaging challenge pretty much applies to everything," said Chad Harding, team leader seismic imaging group at BHP Billiton. "There are two fronts — acquisition and processing — on which we'll need to advance to improve seismic imaging for subsalt exploration in the Gulf.

Image Caption

Technology such as the "Z pod" seismometer is helping deepwater Gulf exploration. Graphic, photo courtesy of Fairfield Industries

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Prices are rising, demand is soaring, supplies are tight — and iffy, depending on the geographic location — and many promising U.S. drilling locales continue to be off-limits.

Not surprisingly, many are looking to the industry's Old Faithful, aka the Gulf of Mexico, as the still-bright hope for increased domestic production. Indeed, it's the only readily accessible region harboring some frontier-type plays — the still-new shallow water deep gas play and the ultra-deep water — despite its lengthy producing history.

The lure of potential new finds in the GOM was underscored in the most recent Central Lease Sale190 held by the MMS, which garnered the highest number of bids of any Central Sale in the last six years. Sixty percent of the bids were on the shelf, apparently reflecting interest in the shallow water deep gas, according to Johnnie Burton, MMS director.

The sale also indicated continuing interest in the deep water, and Burton said the large number of tracts receiving bids in the ultra-deep water was particularly noteworthy.

As of March 2003 there had been 24 deep gas completions, with 17 discoveries, according to Debra Winbush at the MMS, who noted 100 wells were permitted in 2003 and 40 in 2004. The agency reported there have been 11 industry-announced discoveries in water depths greater than 7,000 feet and noted these ultra-deep water discoveries have the potential to open up entirely new geologic frontiers.

But the deep Gulf is salt country, meaning there are obstacles to overcome, and not just in terms of iron.

It's an image thing.

"The imaging challenge pretty much applies to everything," said Chad Harding, team leader seismic imaging group at BHP Billiton. "There are two fronts — acquisition and processing — on which we'll need to advance to improve seismic imaging for subsalt exploration in the Gulf.

"Probably the most critical thing is in the area of acquisition," he added. "We need to learn how to acquire data sets that better illuminate subsalt targets. We believe — and there's a lot of evidence now — that multi-azimuth acquisition is the way to do that."

Harding noted that this is in contrast to what might be called single or narrow azimuth acquisition, which is what you normally have with conventional marine streamer equipment.

"To image the subsurface salt-related formations in the Gulf, you have to record reflections," said Marty Brandt, contractor acquisition and processing team leader ChevronTexaco Energy Technology Co. "You need those long offset, large apertures to do that.

"The acquisition of long offset information is a challenge," he continued, "along with the subsequent impact it has on processing to deal with anisotropy, tilted anisotropy and depth migration."

Harding said the industry must continue to take advantage of declining computing costs and improve the ability to build seismic velocity models.

"That is a key to better sub-salt images," Harding said. "My view is that velocity model building is a core technical competency for oil companies. Better migration algorithms go hand in hand with velocity model building, and the package of those two things along with better seismic data to start with are what I think will get us where we want to go."

Different Approaches

It all begins with the data acquisition, and Harding noted there are different approaches that afford the capability to acquire the needed wide-azimuth data: ocean bottom sensor (OBS) nodes, ocean bottom cable (OBC) and surface methods using streamers.

Streamers and cables both are widely-used, proven technologies in conventional marine seismic data acquisition, depending on the environment and the survey objective.

On the downside, streamers can become unwieldy around platforms and infrastructure, and placing cables on the seafloor is a high cost undertaking. Cable length and connector integrity are added potential issues of concern in deeper water.

"The ultra-deep is where a lot of new discoveries will be," said Jerry Beaudoin with the BP E&P Technology Group, "and the challenge for cable will be overwhelming because of things like soil hardness, topography, sedimentary features on the surface. Nodes would come into their own there."

Beaudoin, who has co-authored a paper discussing the results of an effort to analyze the relative merits of cables and nodes (also known as autonomous seafloor seismographs) to gather wide azimuth deepwater ocean bottom seismic data, offered his analysis, which was based on three requirements:

  • Wide-azimuth shot coverage is necessary to illuminate a structurally-complex subsurface such as found with irregular salt masses.
  • Dense surface shooting into sparse receivers provides adequate wavefield sampling.
  • Shots within each receiver gather provide full-azimuth coverage with adequate offsets

According to his paper, autonomous ocean bottom nodes are more efficient than OBC technology to acquire deepwater wide-azimuth seismic data for the purpose of properly illuminating complex subsurface structures. The project participants observed that hundreds of nodes can cover a much greater area in a single shooting effort than the most productive OBC crews.

The academic community has a history of using autonomous seismographs for various applications, and ocean bottom sensors have been used successfully to acquire data in the shallow water where they send the data to buoys at the surface.

Deep Waters

Still, the consensus among the E&P folks is that nodal technology is only in its infancy from an equipment and operational perspective, particularly in recording deepwater seismic data.

Companies, however, are scrambling to move nodal applications into the realm of the tried and true.

Fairfield Industries' Deep Z seismic data acquisition system for deep water, for example, is being readied for use in early 2005. Depth-rated to 9,800 feet, the system has advanced to the stage of successfully completing a deepwater data acquisition pilot project in 7,000 feet of water, according to Steve Mitchell, vice president operations at Fairfield.

Each Z pod, or autonomous seismometer, is a self-contained sensor with batteries and a highly accurate clock. Cable-free, they're laid out in a grid in the deep water by an ROV, which later retrieves them to download the data and re-charge the batteries prior to re-deployment.

Even though the pods are placed about 1,300 feet apart, the shot density on the surface is enough to yield a high-fold, wide azimuth survey.

Nodal technology may have an added appeal to engineers and asset managers, Mitchell noted, because it employs ROVs, which they use routinely.

The Z pods are placed using the Sonsub Innovator ROV, which is a large, heavy work-class vehicle.

"They're best suited for this technology," said Wayne Abadie, sales manager at Sonsub, "and limited only by the length of the umbilical. Right now the maximum is 4,000 meters, but we could design for deeper; it's just a matter of getting a larger winch and more cable."

"The Innovator loads one carousel with several nodes and places them in predetermined locations," Abadie said, "and it can continuously deploy maybe several hundred nodes during the course of the survey.

"Also, the vehicle can rapidly change locations on the seafloor if you decide you want a different view of the reservoir," he noted.

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