Spotting Oil from the Sky

According to Dan Burns, a research scientist in MIT’s earth resources laboratory, while seismic surveys are currently by far the most common method of imaging oil fields, electromagnetic (EM) imaging is gaining in popularity because it is more reliable. Electromagnetic imaging is a more direct way to detect oil than seismic surveys, since it can measure differences between oil and water, something seismic methods can’t do. “There’s clearly a move more and more toward electromagnetics,” Burns says. “In general, seismic techniques are responding to differences in the rocks themselves, as opposed to fluids, whereas EM methods are much more sensitive to fluids.”

Oil field mapping is growing in importance as oil companies exhaust easily accessible supplies. In addition to revealing untapped pockets of oil, better maps could help engineers place injection wells for steam or carbon dioxide treatments that can help force out stubborn oil.

Because their method reduces costs, eField is also exploring another potential benefit: rapidly scouting for potential oil deposits in new areas or in areas that have already been mapped but with inadequate methods due to high costs. By quickly covering large areas (the Texas survey took in 3,100 miles) and generating maps in weeks instead of months, the new airborne technology can cut costs per “line mile” for large areas to about $100, Johnson says, rather than the hundreds of thousands of dollars per mile he says seismic surveys cost.

Although these quicker surveys lack resolution (higher-resolution images require slower speeds and more passes, thereby increasing the cost), airborne electromagnetic methods, in contrast to stationary ground-based seismic and electromagnetic methods, should be able to reliably identify large deposits of oil quickly and inexpensively, showing companies where to focus their efforts, he says.

Traditional seismic methods, which can provide information about rock structures and detect the difference between rock and gas, are not good at distinguishing water from oil. As a result, geological formations have to be interpreted to determine if oil is actual present – and the interpretations aren’t always right.

One electromagnetic method applies a current to the earth and measures the resistivity of materials in the earth’s crust. It can distinguish different fluids: oil does not conduct electricity well, while the briny water that surrounds it does. Another method measures the charge that accumulates on the surface of hydrocarbon bubbles suspended in water when oil fields are exposed to an electrical current. Johnson estimates that such methods can find oil much more reliably, at rates 2-3 times better than traditional seismic methods.

While these two methods have been used separately in ground-based systems, eField’s system is unique in combining them in an airborne system. In addition, eField surveyors, rather than applying their own electrical current, rely on electrical currents already coursing through the earth’s crust that are caused by lightning and interactions between the earth and solar winds – exchanges similar to those that produce the aurora borealis.

Developing the airborne system has been a challenge, for one thing, because naturally occurring electrical currents are weak. This is overcome in part because large oil fields accumulate relatively large charges from the current, Johnson says.

Still, electromagnetic imaging expert Steven Constable, professor at the University of California at San Diego’s Institute of Geophysics and Planetary Physics, who has not seen eField’s technology, says it is very difficult to image oil fields based on accumulated charge relying on natural electric fields, especially from the air, since the source of the charge is difficult to pin down. “It’s a tricky problem,” he says. “Certainly it will be pretty innovative if they have solved all the issued associated with it.”