A 4-D confidence builder

OBS Passes Repeatability Test

OBS node deployment by ROV in the Gulf of Mexico’s Deimos Field. Photo courtesy of BP (Atlantis)
OBS node deployment by ROV in the Gulf of Mexico’s Deimos Field. Photo courtesy of BP (Atlantis)

An ocean bottom seismic (OBS) node 3-D survey at the Shell-operated Deimos Field in the Gulf of Mexico included an unusual twist.

Geophysical contractor Fairfield Industries and Shell implemented a 2-D node repeatability study simultaneously with the 3-D survey that took place in 1,000 meters of water.

The late 2007 repeatability study proved to be a giant step forward in documenting the ability to acquire time lapse, or 4-D, seismic data, via nodes on the seafloor. Repeatability, i.e., accurate repetition of each successive survey over time, has long been the challenge to successful 4-D application.

During the productive life of a field, changes in hydrocarbon saturation and pressure occur in the producing reservoir(s), impacting how the reservoir behaves. Time-lapse technology can help significantly to identify exactly where the changes occur, enabling reservoir monitoring and, subsequently, improved reservoir management.

Time-lapse surveys have proved to be a valuable tool for reservoir management in the North Sea. One high profile area is the BP-operated Valhall Field in Norway where BP installed a permanently placed ocean bottom cable system (OBC) in 2003.

Will It Work?

A downside to installing – particularly trenching – an OBC system for 4-D purposes is the considerable up-front cost, especially given that the need for testing may diminish as the reservoir becomes more predictable.

Towed streamers are another option for 4-D that has proven quite successful in a number of instances over the years. Some industry folks, however, question just how accurately streamers can go back and record over the same exact place. Infrastructure poses yet another obstacle to application of this technique, resulting in holes over some of the survey area.

In contrast, autonomous self-contained OBS nodes can be placed most anywhere on the seafloor, no matter how dense the congestion.

Nodes are deployed via remotely operated vessels (ROVs), ensuring positional accuracy and repeatability, according to David Hays, vice president of the technology group at Fairfield.

“For us, at the start of the Deimos survey, there were two purposes,” noted Frans Smit, senior operations geophysicist at Shell E&P Co. “First was to do the repeatability test, and second was to actually see Fairfield’s deployment and retrieval methods in operation.

“Deploying 20 nodes ahead of the survey, retrieving them and downloading the data from each with 100 percent success gave us a lot of confidence,” said Smit, who helped design the repeatability program at Deimos along with Fairfield.

Hays noted no one has ever conducted a full-scale time-lapse seismic experiment with nodes.

“We didn’t this time either,” he said, “but we wanted to get insight into how nodes would measure up compared with other methods by doing a limited study where we just acquired one swath within a 3-D survey.

“Before we started production on the 3-D survey at Deimos using our Z3000 system, we laid out 16 nodes on a single 2-D line in the normal positions they would occupy in the 3-D grid,” Hays said. “We then shot a swath of seven dual source sail lines into that one receiver line of nodes during what we refer to as Day 1.”

Hays distinguished sail lines from shot lines, noting the boat has two gun sources on it, so there are two separate tracks of shots that are acquired in one sailing, or one pass of the boat. In the swath there are seven sail lines that produce 14 actual shot point tracks.

In addition to the initial 16 nodes deployed, there were four extra nodes laid out side-by-side – or co-located – with four of the regular grid locations, Hays noted.

“The purpose was to give a side-by-side look if you had identical shots and replaced a node almost exactly on top, just how repeatable the data would be,” he said. “We did this as kind of a sideline experiment.”

Staying True to Life

After acquiring the swath of data into the one line of 16 nodes, then those original 16 nodes were recovered via ROV, and the data were downloaded and set aside.

“On Day 6, it was time to begin the production 3-D survey,” Hays said. “But before getting started on that, we went down to the same line and redeployed 16 like nodes with the attempt of getting back to the same location, knowing we wouldn’t precisely get to it but to the same nominal grid location.”

Smit noted the instruction for this redeployment was not to search for the imprint of nodes on the seafloor from the first layout but to place them where they thought they should be.

“I later looked at the diving video,” Smit said, “and it showed that the ROV operators did not get out of their way searching for the imprint of the previous deployment. They instead redeployed the nodes where they determined they should be, as will be the case in a ‘real-life’ 4-D survey.”

This second set of 16 nodes was subjected to a second acquisition via the seven sail lines, and the node set stayed on the seafloor for the 60-day duration of the Deimos acquisition program.

On Day 45 the crew was back over this same location and acquired another set of seven sail lines – constituting the third experiment – as part of the production 3-D.

Following completion of the 3-D survey on Day 60, one last shooting occurred before the 16 nodes were recovered.

Repeat Performances

In a perfect world, the data acquired from a repeatability exercise such as Deimos could be processed to form a seismic image to hand off to the interpreter. All would be identical, and nodes could be declared absolutely repeatable.

Forget perfect world.

“They’re not decidedly identical and are different for various reasons,” Hays said. “But you can measure those differences, and there’s a statistic that’s calculated to come up with a hard number to quantify that difference.”

It’s known as normalized root mean square (NRMS).

Simply speaking, when the NRMS number is low the data are very repeatable, according to Hays, who noted, “If it’s zero, you have identical sets of traces from the experiments.”

Hays cited differences encountered at Deimos.

The acquisition on Day 6 (the monitor survey) is a good repetition of Day 1 (the baseline survey) with an NRMS statistic of 10 percent (ignoring edge effects).
The acquisition on Day 6 (the monitor survey) is a good repetition of Day 1 (the baseline survey) with an NRMS statistic of 10 percent (ignoring edge effects).

“Between Day 1 and Day 6 (data), we had rather typical statistics of 10 percent NRMS, which is good,” he said. “In the North Sea, streamer typically is 20-50 percent and OBC typically is 15-25 percent.

“The positive news of the story is in the deepwater environment, seismic data acquired by nodes is very repeatable, more so than streamer and OBC,” Hays said. “They’re fairly easy to put back close to where they were originally – in this case it was about five meters.

“You could never control an eight-kilometer-long streamer and varying currents and put every trace back with that precision,” Hays pointed out.

Convinced

When doing the comparison of Day 1 and Day 60, the NRMS registered a still-respectable 20 percent – but also prompted the question, “what’s different this time from the first time?”

“In that 60 days, the conditions in the ocean changed, affecting the temperature and salinity of the water,” Hays noted.

“One of the important things we learned is that to get a repeatable survey, then processing has to comprehend the wave speed differences in the water layer itself,” he said. “Water is more dynamic than the crust of the earth, so things can change.”

“It’s a complicating factor,” he said, “but when comprehended properly that repeatability statistic goes down.

“The hypothesis was that nodes will be a good tool for time lapse, and the conclusion is that’s right, based on this experiment,” Hays said.

Smit noted three pertinent aspects of the repeatability program:

  • The survey showed the importance of subsea positioning – being able to get to the same location with new nodes.
  • The ability to repeat source positions, especially in the presence of loop currents like in the Gulf.
  • The shearwave noise on the vertical geophone is very location dependent and non-repeatable, so it’s quite critical that this noise be removed, and the Fairfield processing showed this can be done.

“This test did a lot to convince us at Shell that OBS node technology is suitable for time-lapse seismic,” Smit noted.

“It likely could be a part of future projects,” he added.

Hays noted that while time-lapse seismic is common in the North Sea, it’s not common in the Gulf of Mexico. He emphasized, however, that its value is being recognized, and it’s positioned to become a key component of many programs.

“It will catch on, especially in these high dollar fields where the wells are so expensive,” he said. “They really want to exploit these fields with maximum efficiency.

“You can replace a lot of guessing with data if you acquire the seismic signal more than once,” Hays noted, cautioning that “not all fields have a strong 4-D signal, and those are hard to do.

“For the ones where producing the reservoir does change the seismic response,” Hays said, “time-lapse is highly useful.”

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Emphasis: Seismic