Hunting for Black Gold

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James Rector is an associate professor of GeoEngineering.

In the classic Clark Gable and Spencer Tracy film Boom Town, the characters determine where to drill for oil by looking at the lay of the land. In reality, a lot of black gold was found that way. Wild catters looked for telltale topographical signs that indicated underground reservoirs. These days though, the tried-and-true technique doesn't work so well. The oil and gas reserves in this country are depleting rapidly, requiring a high-resolution view of the subsurface long before the big drills are brought in. To create these images, UC Berkeley professor James Rector applies technology more commonly seen in the doctor's office.

"To find oil and gas in the United States , we have to look for smaller and more complex reservoirs that may only be a few meters thick," says Rector, a professor of Civil and Environmental Engineering. "That requires much more sophistication in the way we collect our seismic data, process it, and then interpret what we see."

Fifteen years ago, oil companies began using ultrasound systems--similar to the devices physicians use to visualize a fetus in the womb or monitor blood flow through the heart--for three-dimensional seismic imaging. At a possible drilling site, companies use "thumper trucks" outfitted with huge vibrating mechanisms to generate seismic waves that travel deep into the Earth.

"It's much more advanced than medical ultrasound," Rector explains. "The body is essentially a fluid except for the bones. But the earth is a much more complex media that we have to propagate through."

As the wave moves downward, it's reflected differently depending on the kinds of rock it encounters. The reflected waves are then detected and measured by a sensor back on the surface. That data can is finally combined into a 3D image that reveals the boundaries of the rock formations below the surface.

The problem, Rector says, is that these aboveground methods often do not provide the fidelity necessary to find the small reservoirs and understand their structure. His approach is to put the ultrasound system closer to the action, as much as 38,000 feet underground.

This cross-section of the earth was obtained using seismic reflection techniques. The colors represent formation impedance, a production of density and velocity. That impedance, combined with the structure of the area, may indicate where gas or oil may be trapped and recoverable. (courtesy the researcher) [View larger image]

The first step though is for companies to get comfortable with the technique--called borehole seismic surveying--by using it to help calibrate and understand their aboveground 3D images. According to Rector, dropping either the receiver or ultrasound source into a borehole can increase the resolution of the image by a factor of two. One reason for the improvement is that the seismic wave doesn't need to travel all the way down into the Earth and back out before it's detected.

"Eventually, the entire survey could be moved underground," Rector says.

If the source and receiver are properly positioned and tuned, this approach--called a "crosswell survey" because the waves travel from well to well--could improve the resolution of the 3D images by a hundredfold, Rector says. Besides painting a better picture of the subsurface, borehole seismic surveys may also cause less disruption to the area under study.

In March, the US Senate voted to allow drilling in Alaska 's Arctic National Wildlife Refuge (ANWAR). If the ecologically-sensitive area is to be tapped for oil, Rector believes that borehole seismic surveys could reduce damage while the reservoirs are sought out.

"Drilling for borehole seismic surveys in a very limited area deep under the permafrost would have very little environmental impact," he says.

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Currently, Rector and his research group are identifying the ideal depth and placement of the ultrasound and receiver to get the most bang for the survey buck. Another challenge is taking essentially a geometric image of the subsurface and trying to suss out the composition of the material between the layers. Ultimately, Rector says, the aim is to actually detect the oil and gas directly rather than surmising its existence.

"That's what gets me up in the morning," Rector says. "I love the chase."

James W. Rector's home page

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