Repeatability Is Key for 4-D Data

Reservoir Management vs. Monitoring

There’s buzz aplenty that 4-D, or time-lapse, seismic is the next new big thing in the geophysical arena, particularly as it applies to the offshore.

We’re not talking the traditional 4-D method, where the operator commissions additional towed streamer 3-D surveys at specific time intervals to get a handle on what’s happening in the reservoir. Today’s world is becoming increasingly specialized, demanding some very specialized equipment, especially in the deepwater environs.

To meet the anticipated need for more frequent yet cost-effective surveys, contractors and manufacturers have engaged in heavy duty R&D to come up with some amazingly sophisticated, high-tech tools for this challenging environment.

Improvements continue to be made in the realm of permanently placed cable systems, for instance, and significant advances in flexible, self-contained nodal systems have nodal technology poised to become a system-of-choice for many operators.

Valhall’s Example

Of course, this is an industry that essentially has redefined the meaning of slow adaptation; "wait and see" remains the mantra.

The lengthy period between trial status to project status for permanently placed cable technology is a prime example: An experimental permanent seafloor seismic array was first installed at Foinhaven in 1995 by Shell and BP. It wasn’t until last August, however, that the first permanently placed cable system to be financed and purchased by a business unit was finally installed, according to Gary Owens, chairman, president and CEO of OYO Geospace, which manufactured the system.

The locale is the BP-operated Valhall Field in Norway, where the presence of a gas cloud over part of the reservoir distorts the conventional seismic image acquired using towed streamers. Another constraint to streamer technology there is in-place infrastructure.

The initial survey was shot in December 2003, according to Rodney Calvert, geophysical consultant at Shell, which is a partner at Valhall.

"We’ve done a couple of repeat lines on it," Calvert said, "and it is very repeatable, so it looks excellent."

Repeatability has been one of the limiting aspects of using towed streamer data for 4-D programs, various experts say; and, according to Dave Ridyard, business development manager at Input/Output, towed streamer technology has fundamental limits of resolution.

"For starters, how repeatable is it, or how accurately can you put it back in the same place and repeat the noise problem?" he queried. "And you can’t record shear wave with towed streamer, so if you think the primary fluid movement information is going to be in the shear wave information, then you need a seafloor full-wave based system.

"A more subtle limitation I'm seeing is quite a few oil companies are starting to use 4-D not just as a reservoir monitoring tool but as part of an active reservoir management tool," Ridyard added. "Reservoir monitoring tells you what happened in the reservoir, but reservoir management tells you fast enough you can do something about it before the bad thing happens.

"With towed streamer 4-D, by the time I get a boat, shoot, process the survey, take out the differences and get the right answer, I’m just about in time to explain the disaster that just occurred," he said.

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Image Caption

Interval velocities derived from high resolution tomography may be used to determine pore pressure and to predict trends for seismic attributes.
Graphic courtesy of GX Technology Corp.

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There’s buzz aplenty that 4-D, or time-lapse, seismic is the next new big thing in the geophysical arena, particularly as it applies to the offshore.

We’re not talking the traditional 4-D method, where the operator commissions additional towed streamer 3-D surveys at specific time intervals to get a handle on what’s happening in the reservoir. Today’s world is becoming increasingly specialized, demanding some very specialized equipment, especially in the deepwater environs.

To meet the anticipated need for more frequent yet cost-effective surveys, contractors and manufacturers have engaged in heavy duty R&D to come up with some amazingly sophisticated, high-tech tools for this challenging environment.

Improvements continue to be made in the realm of permanently placed cable systems, for instance, and significant advances in flexible, self-contained nodal systems have nodal technology poised to become a system-of-choice for many operators.

Valhall’s Example

Of course, this is an industry that essentially has redefined the meaning of slow adaptation; "wait and see" remains the mantra.

The lengthy period between trial status to project status for permanently placed cable technology is a prime example: An experimental permanent seafloor seismic array was first installed at Foinhaven in 1995 by Shell and BP. It wasn’t until last August, however, that the first permanently placed cable system to be financed and purchased by a business unit was finally installed, according to Gary Owens, chairman, president and CEO of OYO Geospace, which manufactured the system.

The locale is the BP-operated Valhall Field in Norway, where the presence of a gas cloud over part of the reservoir distorts the conventional seismic image acquired using towed streamers. Another constraint to streamer technology there is in-place infrastructure.

The initial survey was shot in December 2003, according to Rodney Calvert, geophysical consultant at Shell, which is a partner at Valhall.

"We’ve done a couple of repeat lines on it," Calvert said, "and it is very repeatable, so it looks excellent."

Repeatability has been one of the limiting aspects of using towed streamer data for 4-D programs, various experts say; and, according to Dave Ridyard, business development manager at Input/Output, towed streamer technology has fundamental limits of resolution.

"For starters, how repeatable is it, or how accurately can you put it back in the same place and repeat the noise problem?" he queried. "And you can’t record shear wave with towed streamer, so if you think the primary fluid movement information is going to be in the shear wave information, then you need a seafloor full-wave based system.

"A more subtle limitation I'm seeing is quite a few oil companies are starting to use 4-D not just as a reservoir monitoring tool but as part of an active reservoir management tool," Ridyard added. "Reservoir monitoring tells you what happened in the reservoir, but reservoir management tells you fast enough you can do something about it before the bad thing happens.

"With towed streamer 4-D, by the time I get a boat, shoot, process the survey, take out the differences and get the right answer, I’m just about in time to explain the disaster that just occurred," he said.

"Pre-emptive intervention is as big a driver toward permanent monitoring systems as the imaging repeatability issue," Ridyard noted.

Issues and Concerns

The payoff has the potential to be substantial.

For instance, the life-of-field seismic reservoir management program at Valhall, where productive life exceeds 20 years, is anticipated to lead to improved drainage and well placement resulting in an additional 60 million barrels of production, according to Olav Barkved, lead geophysicist for the BP Valhall subsurface team.

The learning curve is high.

"We have field tests and related experience (for permanent cable)," Barkved said, but there’s no textbook or reference case.

"There are a number of issues and concerns," he added. "For instance, there is a lot of equipment on the seafloor with a lot of connectors in place, and one concern is, will this last for long enough?

"Another element is how far to push the 4-D response — what type of sensitivity can we really get out of the system," Barkved said.

Results due in March from the initial survey will address this question, among others. Plans include a minimum of three more surveys this year alone, with mobilization already under way for the first.

Given the millions of dollars already expended on new and improved seismic technology for programs such as Valhall, the equipment manufacturers have a lot riding on a success there.

"Even though 4-D has been talked about extensively and encouraged by the E&P companies, in terms of dollars spent 4-D has yet to be a salvation for the geophysical industry," said Steve Mitchell, vice president operations at Fairfield.

Faster, Faster …

But change is in the air, and in a sense Valhall may prove to be an exception in 4-D application rather than the rule — to the advantage of the equipment providers.

Operators are beginning to take a serious look at how to accelerate a drilling program at the start of a field’s life as opposed to the usual approach of applying reservoir evaluation via 4-D late in the game to try to stem the production decline. The goal is to add oil early on when it has more economic impact.

"This also falls in with when people sometimes have a discovery, and they say OK, how big is it?" Fairfield’s Mitchell said. "Top side equipment to develop a 400 million barrel field is far greater than with a 100 million barrel discovery.

"Getting a better seismic survey after a discovery which used relatively inexpensive towed streamer is far less expensive than a $200 million mistake on topside equipment," Mitchell noted. "And it gives a baseline for 4-D."

The secret to good 4-D is to repeat the ray paths in the acquisition process, using the same shot and receiver positions, according to Calvert.

"OBC (ocean bottom cable) is one way to do this," Calvert said, "but to trench OBC in the seabed where it’s in a fixed location is very expensive.

"A potentially cheaper [bottom reference survey] solution is OBS (nodes), where you don’t buy the equipment but rent it for the duration of the survey.

"Expensive cables pay out if there are many, many surveys because you only need a shooting vessel for repeats," Calvert noted. "It looks like six to 10 repeats would be needed before it starts to be cheaper than other methods. If you don’t know how many times you want to survey a field, you may find the answer to your questions after only two surveys. Then you’ve got expensive stuff on the seabed that’s not being used.

"The number of times you will shoot is an important parameter in deciding on cable versus nodal."

Comparisons and Differences

In the cyclical oil and gas industry, where operators can’t afford to ever avert their eyes from the bottom line, unnecessary expense ranks right up there with the plague. Economics aside, however, there are other noteworthy differences between permanently placed cable configurations and the less expensive nodal systems that are drawing attention.

The independent, self-contained nodal units are placed on the seabed via remote operated vehicles (ROV), which enable the equipment to be deployed in high-productive areas safely.

"When you move in with 4-D, you must place it where it was the first time," said Hal Haygood, engineering manager at Fairfield. "ROV technology lets you do this, and you know both the tilt and the direction of the unit within two degrees and placement accuracy within one meter. Repeatability is high"

The absence of any hard link between units circumvents noise issues common to most cable systems, enhancing vector fidelity. This also enables deployment flexibility in areas with subsea infrastructure, where pipeline operators frown at the prospect of cables placed over their flow lines.

Nodal units typically are left in place to collect data, including multi-component, for 10-15 days or so before being brought up to extract data from each unit in a matter of seconds.

"It’s clock crystal technology advances over the last five years that has allowed us to do this, because all the units must be synchronized to take samples at the same time," Haygood said. "We know the exact time each shot was fired."

Orientation uncertainties inherent with most cable and streamer systems are circumvented using nodes because the units are azimuthly symmetrical, meaning the response is the same in all directions. The system can shoot any type survey design, e.g., high-fold wide azimuth for subsalt imaging, whereas cables are incapable of reaching out to accomplish this.

Already proven in shallow water applications, nodal technology for deployment in deep water programs is being readied for a field debut in the coming year, according to Mitchell.

Best of Both Worlds

Differences aside, the consensus is there’s a target market for both cable and nodal technology.

"The big picture is they’re tools, and all tools have a place," Mitchell said. "Sometimes you need a socket, and sometimes you need a wrench."

The market for these tools has the potential to enlarge considerably — it’s a mindset thing.

The big push for 4-D seismic technology has been confined to the geophysical community for the most part, and some folks think that must change.

"The reservoir engineers must take it more on board," Calvert said.

"Half of the reservoir engineers in the world are keen on 4-D, and the other half have resisted," he said. "It has tremendous potential for them because they can see what happened in a field.

"We need a revolution in thinking to really exploit 4-D.

"When you start with a field you can make a model, but you should not believe it and should know it’s a first approximation — fluid flow through the earth is much more complicated than expected," Calvert said. "If you plan from the start for 4-D, you can proceed to develop the field with monitoring in a rational way.

"It’s a sort of myth that you make models, give predictions and all are happy," Calvert said. "Those predictions and models are wrong and why we are getting low recovery rates.

"If we used a phased production approach and are honest about our uncertainty," Calvert said, "then with 4-D we could do a better job."

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