807 sensors used

OBS Data Answer Field Questions

Deep sweep: Ocean bottom seismic nodes were deployed in the Gulf’s Deimos Field. Photos courtesy of Shell E&P
Deep sweep: Ocean bottom seismic nodes were deployed in the Gulf’s Deimos Field. Photos courtesy of Shell E&P

You don’t have to dig too deeply to know that optimistic stories about the world of E&P are, to be kind, becoming hard to find.

Still – surprisingly? – oil field activity actually is chugging along as usual for many folks, even in today’s complex and unusual business environment.

For example, the large scale, deepwater Gulf of Mexico programs that require many years to segue from concept to production and development are not based on commodity prices at any given time, meaning they tend to keep on going whatever the market environment.

This deepwater action continues to rock ‘n’ roll in large part because the sophisticated, savvy operators who venture into these typically high risk, pricey plays are always on the prowl seeking new technology to bring improved efficiency and cost-savings to each and every project.

One high-tech application garnering increased attention is the use of ocean bottom seismic (OBS) nodes to acquire seismic data.

The deepwater Gulf is rife with subsurface salt sheets that severely distort seismic signals resulting in an inferior image of the deep, complex reservoirs.

A now-proven solution to this problem, particularly for development projects, is all-azimuth illumination using OBS node technology to acquire full, or true wide azimuth seismic data via recording in all directions, according to Mike Spradley, acquisitions marketing manager at Fairfield Industries.

This is a stark contrast to conventional streamers that routinely record narrow azimuth data with a single illumination direction or else re-shoot in several directions to acquire additional azimuth data – an expensive undertaking.

Pass the Salt

The Shell-operated Deimos Field in 1,000 meters of water in the Gulf’s Mississippi Canyon area is the site of a recently completed OBS node technology application using Fairfield’s Z3000 node system designed to work in water depths down to 3,000 meters.

The Deimos OBS 3-D program took place in 2007 and covered about 134 square-kilometers. It was only the second deepwater node survey worldwide, according to Frans Smit, senior operations geophysicist at Shell E&P Co.

The initial deepwater node program – using the same Z3000 system – wrapped up successfully in 2006 at BP’s Atlantis Field in the Gulf. BP is Shell’s non-operating partner at Deimos.

Deimos sits beneath a salt overhang that had prevented adequate imaging of the site. As a result, when the Shell folks began developing the field about two years ago, they quickly recognized they needed a whole different kind of seismic data because there were myriad uncertainties about the volumes, where to put the wells, what field development system to select, etc.

“We felt we needed more certainty to determine a final development plan,” Smit said.

“We did some testing with long offset streamer data before the node survey, and it was established we needed very long offsets,” he said. “It would be very expensive to get these offsets (using streamers), and we had infrastructure in the area that would make it difficult to acquire a survey like that.

“When you want wide azimuth (WAZ) over a small area, it’s very hard to make it economically efficient,” Smit added. “We were automatically drawn to nodes because that tends to be more economical for a limited area.”

Starting With Seismic Modeling

The node program at Deimos kicked off with a seismic modeling exercise to determine the optimal placement of the nodes on the ocean floor during acquisition. The modeling process utilized a wave equation modeling (WEM) method that handles multiples capably.

The modeled data reaffirmed that very long offsets acquired at a range of azimuth angles would be critical to illuminate the target, Smit said, noting these long offsets effectively undershoot the salt body.

There were a number of node deployment scenarios modeled to ensure maximum illumination of the sub-salt target area. Smit said the modeling gave them considerable confidence the program would be successful.

The plan was to cover the target area in a single patch using all of the available nodes, which numbered 900, according to Reagan Woodard, Fairfield’s operations supervisor for the Deimos project. He said 807 nodes ultimately were used.

Prior to the actual survey, a select number of nodes were deployed on the seafloor, acquiring a limited amount of data before being retrieved, according to Woodard. This served a two-fold purpose:

  • To determine the optimal gain setting for each type of sensor.
  • To verify the operational readiness of the nodes and the deployment/retrieval methodology.
The Work Begins

Field operations at Deimos were implemented via a remotely operated vessel (ROV) and a dual source shooting vessel.

Each Z3000 system autonomous node is an independent self-contained unit with a battery and highly accurate clock. Deployment on the ocean bottom using an ROV ensures positional accuracy as well as repeatability.

Smit noted a major advance during the survey entailed the efficiency gained via use of a nodebasket – a tool newly developed by Fairfield for the operation. Once operational, the nodebasket reduced the strain on the ROV significantly by limiting the number of ROV trips required.

The acquisition logistics at Deimos were governed in large part by the 60-day battery life of the nodes.

“All of the relevant data had to be recorded before the nodes were retrieved to recharge the batteries and download the recorded data,” Woodard said. “During the survey, the speed with which the nodes were deployed and retrieved proved critical to the rate of progress.”

As valid testimony to the operational flexibility of ocean bottom node acquisition in highly congested and otherwise challenging environments, all nodes were deployed and retrieved as planned, according to Spradley.

Adding to the infrastructure challenge at Deimos, there were strong loop currents in the area. This would have made streamer operations even dicier than usual, particularly long streamers, Woodard noted. Repeatability, which is a fundamental feature of node surveys (see related story, page 18), would have been impossible using streamers.

The pre-processing stage of the Deimos program focused on delivering four principal products:

  • Accurate source and node positions.
  • Accurate timing of the recorded data.
  • Removal of any S-wave leakage on the vertical geophone.
  • Splitting the data in up-and-down-going wavefields.

A global process was used to deal with the post survey positioning calculations as well as the uncertainties in the timebreak of the recordings. Regarding shearwave leakage, a method was used where removal of the leakage was closely linked to the wavefield separation.

The pre-processed data were taken through mirror migration, initially using the same WEM algorithm utilized for the modeling, according to Smit. Using this approach – which is especially effective in deepwater environments – the downgoing energy is used for imaging.

Big Advantages

A comparison of the resulting OBS node data with existing narrow azimuth data revealed significant improvements on the nodal data in terms of S/N, multiple content and structural definition, especially under the salt overhang.

“One of the big advantages for us on the interpretation and processing side was the ability to test a lot of alternative velocity models very quickly,” said AAPG member Mark McRae, senior staff geophysicist on the Deimos Field team at Shell. “That’s a function of the way OBS is acquired.

“When you process the data you do a reciprocal operation on it so you turn the nodes into shots in the processing,” McRae said. “We had 807 nodes, so when we process and migrate we only have to do 807 shot migrations, whereas with a conventional WAZ you do a whole lot more shot migrations.

“The very fast migration turnaround times were very beneficial,” McRae added. “As an interpreter, I need to test many models.”

More specific to the interpretation, McRae said their well-to-seismic ties are substantially improved with OBS nodes. He noted this is relative to the two different directions of narrow azimuth streamer data they have over the area.

“A key point there is that in tying seismic to wells, we’re now for the first time in the Deimos Field confident we can see the hydrocarbon fluid effects,” McRae said. “We can see an amplitude effect that is correlatable to where the hydrocarbons are in the reservoir.

“This gives us a lot more confidence in future development of the field,” he added.

When queried about the business value of the Deimos node program, McRae noted they’re extremely pleased with the investment they made and highly confident they’ll do well relative to the investment.

McRae acknowledged BP – fresh off its Atlantis Field Z3000 node survey – for working closely on the Deimos project and aiding their ability to acquire good data.

The program clearly attracted a lot of attention.

“We get questions about node surveys almost every week,” Smit said. “There’s a lot of technical interest.

“But these sort of surveys also are very expensive,” he cautioned, “and each asset must consider their commercial aspects.”

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Emphasis: Seismic