safer and sounder

Innovations in marine seismic surveys benefit the environment.

Exploring for the oil and gas needed to meet the world’s rising energy demands requires a broad array of scientific expertise and technology. These elements are essential to a sophisticated process for finding and developing energy resources. ExxonMobil is constantly fine-tuning every aspect of that process to improve efficiency, quality of information and environmental soundness.

One such improvement is in the way ExxonMobil explores areas beneath the earth’s surface, searching for formations that may contain oil and gas.

Before the first well is drilled, a team of geoscientists conducts extensive studies, looking for geologic prospects that could yield commercial quantities of hydrocarbons.

When a promising area is identified, the next step is to conduct a seismic survey, a process invented by ExxonMobil in the 1960s. This entails sending sound waves into the earth, then measuring the returning signals. Variations in the signals that are reflected back from geologic layers help scientists visualize the subsurface rock formations.

For example, sophisticated three-dimensional (3-D) seismic imaging generates massive amounts of data that produce multidimensional representations of subsurface formations.

Scientists then construct a detailed virtual “image.” This helps them identify areas where commercial quantities of oil and natural gas may have accumulated. Their analysis guides the decision to drill an exploratory well and where to position it.

On land and sea

Seismic surveys are conducted both on land and at sea. The principles are the same for both operating environments.

However, marine surveys require generating sound waves through significant water depths and then into the earth’s surface. These sound waves are reflected back from subsurface rock layers and recorded on “streamers” that are towed behind a marine seismic vessel at a consistent depth.

“Streamers” are long tubes containing hydrophones that convert pressure differences into electrical signals, and a fluid to maintain buoyancy. A typical streamer is 3 to 4 inches in diameter and covered with a polymer skin about one-tenth of an inch thick. They are typically as long as 9,000 to 30,000 feet.

The seismic vessel may tow one streamer or up to 10 to 12 streamers, depending on the kind of seismic work being done.

ExxonMobil Exploration Company Technical Vice President Rick Weber points out that this raises some unique issues.

“Acquiring seismic data at sea presents its own set of challenges,” says Weber. “The equipment can encounter commercial fishing gear, debris and, of course, ocean currents. In addition, sharks have been known to mistake the streamers for lunch.”

To ensure accurate readings, the streamers must be flexible, buoyant and maintained at a consistent water depth, usually about 15 to 30 feet. Traditionally, this has been achieved by filling the streamers with fluid, usually a form of light diesel fuel. The fluid helps provide the necessary buoyancy and provides coupling for the hydrophones.

Innovations

While fluid streamers have been effective, ExxonMobil has sought to make wider use of other options. The company needed streamers that provided the same buoyancy, flexibility and hydrophone sensitivity as fluid-filled devices, but with increased resistance to tears and other damage that might cause fluid leaks.

ExxonMobil’s marine seismic work is carried out by contractors with world-class experience and global operations. ExxonMobil’s senior management has met with the company’s key contractors to discuss the wider use of two alternatives to fluid-filled streamers.

One is a “solid streamer.” It’s filled with high-density polyurethane foam and a small quantity of light diesel around the hydrophones.

The other is filled with gel. The gel consists of a nonhazardous, petroleum-based synthetic urethane polymer.

The company’s contractors share ExxonMobil’s goal to minimize the environmental impact of seismic surveys by using solid and gel streamers. Many had already been making the conversion.

For example, the French company CGGVeritas, which has the industry’s largest fleet of 20 seismic vessels, has converted 12 vessels to solid streamers. They plan to convert approximately 90 percent of their entire fleet by 2010.

Norway-based PGS plans to have 17 vessels in its fleet by 2009, and all will be equipped with solid and gel streamers.

WesternGeco, the surface seismic company of Schlumberger, has acquired two large 3-D projects for ExxonMobil this year in New Zealand and in the Canadian Beaufort Sea using solid streamers.

“The cooperation we’ve received from our contractors has been essential and gratifying,” says Rick Weber. “And it’s paying off. Today about 45 percent of our seismic fleet uses solid or gel streamers. We expect that to rise to 70 percent in 2009. This shows what we can accomplish when all the parties work together to achieve an important goal with tangible environmental benefits.”