Those of you who follow land seismic surveys know that they are getting really, really big. In the Middle East, technologies such as slip-sweep acquisition enable crews to acquire massive swaths of data in record time.
It is not quite that easy in the Rocky Mountains. But ExxonMobil recently concluded what is arguably the world’s largest 3-D/3-C survey in the Piceance Basin in northwestern Colorado.

Challenges were numerous. Working at altitudes of up to 7,500 ft (2,288 m), crews had to contend with high cultural noise and significant infrastructure, environmental sensitivity, and the need to complete the survey within four months. Additionally, Vibroseis trucks (the preferred source) could only operate on the roads, meaning heliportable dynamite sources were required for the remainder of the survey.

But a good survey with the potential for reservoir characterization was needed. While the reservoir section is thick, the reservoir sands are poorly connected, and performance changes from well to well, in part due to natural fractures. Before shooting the 3-D, a vertical seismic profile study was conducted to examine the overburden and the reservoir. This showed up to 20% shear wave anisotropy in the reservoir sands from aligned and vertical fractures, but no anisotropy in the overburden. The company concluded from this study that a high surface source effort would be required to image the reservoir interval.

In a presentation at the recent Reservoir Characterization Project meeting at the Colorado School of Mines, John Hefti, geophysical associate and lead seismic processor on the Piceance project for ExxonMobil, outlined several business drivers:
• The reduction of reservoir risk through reliable vertical and horizontal prediction of reservoir performance;
• A stable field development plan and a fully leveraged wellbore manufacturing process to enhance project economics;
• The potential to reduce appraisal drilling and improve confidence in reserves estimation; and
• The avoidance of drilling problems related to faults and fractures and the reduction of drilling and completion costs.

Numerous technical drivers also existed, including defining the fault architecture, avoiding high water zones, understanding compartmentalization, delineating sweet spots, characterizing fracture trends, and testing the source effort level for future phases of acquisition.

This was a pretty tall order, made taller by the fact that, in addition to the topographical issues mentioned above, the equipment to conduct the survey did not exist, the source effort was unknown, the preferred acquisition geometry was unavailable, and wide-azimuth acquisition and long offsets were required.

The company designed a survey within a survey within a survey. An 82-sq-mile (212-sq-km) high-effort P-wave survey covered most of the area of interest. Within it was a 42-sq-mile (109-sq-km) high-effort 3-D/3-C survey, and embedded in that was a 28-sq-mile (73-sq-km) high-density 3-D/3-C static patch. In all, the survey comprised nearly 80,000 live channels, 59,200 receivers of which 37,600 were 3-C sensors, and 18,000-ft (5,486-m) offsets.

Begun in late July 2009 and finished the following November, the survey involved more than 890,000 work hours (without a lost-time incident); 18,000 hours of HSE training; as many as seven helicopters operating at once, completing a total of 9,279 flights; and as many as 300 people on the ground driving vehicles that logged a total of more than 1 million miles (1.6 million km). In addition to carrying out this massive survey, some of the workers helped to rescue an injured hiker and transport him to a medical center.

So, was it worth it? Hefti said that processing is still ongoing but that preliminary results show an excellent signal-to-noise ratio and good resolution. “Even with limited processing, the converted waves are demonstrating significant potential for fracture characterization,” he said.

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