In 2009, ExxonMobil shot one of the largest land seismic surveys ever conducted. An 82-sq-mile (212-sqkm) high-effort compressional (P)-wave survey housed a smaller high-effort 3-D/three-component (3-C) survey, and within that was a smaller static patch.

helicopter

Because of rugged terrain, a fleet of seven helicopters was required to transport people and equipment. (Images courtesy of ExxonMobil)

Overall, the survey had more than 78,000 live channels; 890,000 work hours; 18,000 hours of HSE training; as many as seven helicopters operating at once, completing 9,279 flights; and as many as 300 people on the ground driving vehicles that logged more than one million miles (1.6 million km).

The questions now are, “Did it work?” and “Was it worth the time, cost, and effort?” ExxonMobil geoscientists took delivery of the P-wave data in January 2011. They are hesitant to crow too loudly, saying repeatedly, “The jury’s still out.” But their excitement is palpable.

Piceance Creek

The survey was conducted over part of ExxonMobil’s Piceance Creek asset, a field the company has owned for many years. This asset is a tight gas field that has been treated routinely as a statistical play. The hope was that a better understanding of the subsurface might lead to a more successful drilling program.

TOP: Vibroseis trucks could only operate on the roads in the area.
BOTTOM: A shot hole is drilled using a heliportable rig.

“If we can do something to increase the odds that an individual well will be a better producer, then we can influence the overall statistics of those plays and gain value,” said Dave Johnston, geophysics coordinator for ExxonMobil Production Co. “We put a lot of effort and money into acquiring this survey, but it will add value to the field over a very long period of time.”

Seismic challenges in the field are numerous. Imaging the near surface is difficult, the topography is rugged, a creek valley causes attenuation in the data, and a baking soda-like material called nacholite creates a scattering medium that reduces data quality.

To pull off a survey of this magnitude also was difficult because much of the equipment did not exist. “We initially wanted to do a 44-line rolling spread when we went out to bid,” said John Hefti, geophysical associate for ExxonMobil Exploration Co. and seismic processing lead on the Piceance project. “But nobody could do it.” He added that it took more than a year to secure the equipment before the survey could commence.

Finally, the acquisition window was narrow. Global Geophysical Services crews could not begin until late July and had to be finished by December. Vibroseis trucks were restricted to the roads, so helicopters were required to transport equipment to drill shot holes for dynamite sources. Additionally, the very noisy environment due to traffic and facilities and the delicate ecosystemmade this not a normal, run-of-the-mill seismic survey.

P-wave Results

Brute stacks from the field were not promising. Hefti said one data example showed nothing but noise. “We wondered whether we were actually acquiring any signal at all,” he said.

Working with proprietary ExxonMobil algorithms and those employed by WesternGeco, which did the data processing, Hefti’s team spent nine months removing noise from the data. “It’s like peeling back the layers of an onion and removing small bits of coherent noise in multiple domains so that we’re not clobbering it with a hammer and removing signal,” Hefti said. “We do this to remove the noise and preserve the signal and the amplitudes to perform quantitative attribute analysis and inversion on the gathers and stacks.

“It was a lengthy iterative process, but I think we got a decent product.”

Processors have worked hard to reduce the noise that dominates the brute stack.

So far, the 3-D P-wave prestack time-migrated data is exhibiting better fault definition, higher seismic frequency, better positioning, and a greater level of stratigraphic detail at reservoir level than previous data. “The P-wave data has given us a lot more information than we were originally expecting,” Hefti said. “We’re able to map out a fault network, and we’re seeing areas that correlate to lower pressure and others to more water. It’s given us a lot of insight into what’s going on in the field.”

Added Johnston, “The relationship drawn between the structural framework of this area and the productivity of the wells is a key learning, and it could very well affect how we proceed with the development of the area.”

Converted Waves

Processing of the converted-wave data currently is under way. Hefti said originally a three-interval layer stripping approach was used to analyze azimuthal anisotropy, but input from interpreters has led the team to adopt a nine-layer approach that is more geologically accurate. “Whenwe performed this analysis on the prestack data, we went from something looking random to everything coming into focus very sharply,” he said. “To me, this is incredible, and it reinforces that we’re on the right track in understanding the anisotropy.”

He added that where well information indicates fracture direction, the new seismic data agree with those results.

Naturally the asset team in charge of Piceance is interested in seeing additional results from this survey. But Johnston said that it has a research component as well. “There’s a lot of very interesting stuff in there,” he said. “I think this will not only be of value to the business for many years but will be a value to research as well.” He added that ExxonMobil Upstream Research Co. invested considerable money and people into the project and that effort has been “critical to the team.”

Another test involved a full nine-component 2-D survey, where the sources as well as the receivers include the shear components. This originally was intended to take place during the 3-D survey acquisition, but several complicating issues prevented further acquisition in a delicate environmental area. That test finally took place last year, reacquiring a 2-D line from 2007 that helped design the 2009 3-D survey.

“We’re evaluating the data as we speak,” Hefti said. “That will be another evaluation to see if the future phases should be acquired with shear wave sources.”

Fit For Purpose?

Given the sheer scope and cost of the survey and the host of obstacles posed by the Piceance Creek field, it is possible to question the rationale behind shooting this survey. Apparently some within the company questioned it as well. But Johnston and Hefti stand behind the decision.

“This survey might have been overdesigned; it’s hard to say,” Johnston said. “One of the things we’re investigatingis what level of effort is required in the field. Does interpretation at Piceance really need the high fold and data density that we acquired in this survey? And even if the answer is yes, if you go to other parts of the world, you may not need this level of effort.”

“On the other hand,” Hefti added, “I see the industry as a whole moving toward higher and higher channel counts as the demand to extract more from seismic data increases. The high-end interpretation and analysis methods applied to this dataset require accurate wavefield sampling, and the full-azimuth acquisition method employed at Piceance provides that level of sampling. I’ve never had a dataset where I could look at a suite of full-azimuth stacks at high fold,” he said. “You’re always missing something, always guessing.”

Over all, the survey seems to be paying dividends. “These reservoirs are very complex,” Johnston said. “You have to think of them as an entire system. We’re not trying to find an individual sand; we’re trying to find sand connected to fractures connected to other sands. That system approach is what’s needed to understand how the whole field is going to respond and how we’re going to optimize recovery.

“It’s a phenomenal dataset.”