Earth's 'Safety Mechanisms' Keep Fractures in Check
Ever since the documentary Gasland showed water from a tap catching on fire, people have been increasingly worried about hydraulic fracturing contaminating water supplies with flammable methane.
Now, researchers have found that fracturing, or fracking, at least 2,000 feet below an aquifer will minimize chances of contamination in the United States.
The study was published this week in the journal Marine and Petroleum Geology and is relevant to the Marcellus Shale in Pennsylvania, the Barnett and Eagle Ford in Texas, the Niobrara in Colorado, and the Woodford in Oklahoma.
Most man-made geologic effects have a precedent in nature. Hydraulic fractures can happen naturally, as water-laden rocks buried deep within the ground get pressurized over millions of years. The pressurized liquid causes the surrounding rock to crack and propagate throughout the Earth, until the crack hits the boundary where one type of rock is replaced by another. At this point, the crack ends.
"So the Earth has a number of safety mechanisms which stop natural hydraulic fractures from going on forever," said Richard Davies, co-author of the study and researcher at Durham University in the United Kingdom.
Similarly, during fracking, pressurized water is pumped into the ground to crack shale and release natural gas trapped within.
The worry is that these cracks could travel so far vertically that they connect the fracked rock to water aquifers near the surface. Such a crack, which the researchers have termed a "rogue fracture," would provide a channel for methane to migrate and contaminate the aquifer.
Studying data from five natural gas fields in the United States, Davies and his colleagues found that the largest recorded vertical crack in an oil and gas field is only 2,000 feet long. This does not present a problem since wells are typically fracked below 7,000 feet, while water aquifers are present above 1,000 feet.
The probability of a crack longer than 2,000 feet occurring is less than 1 percent, according to the study.
In fact, the largest known vertical crack anywhere in the world occurred naturally off the shore of Namibia, and is 3,600 feet long, according to the study. And that crack took billions of gallons of water and millenia to propagate.
"To me, that shows the worst the Earth can do. If you've got masses of water available and you've got a long time, the [crack] that forms is 1.1 kilometer [3,600 feet]," said Davies. "When you are hydraulic fracturing in the U.S., you have a limited amount of time -- one to two hours -- and you've got quite limited amounts of fluid."
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