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Most
earthquakes in the Barnett Shale region of North Texas occur within a
few miles of one or more injection wells used to dispose of wastes
associated with petroleum production such as hydraulic fracturing
fluids, according to new research from The University of Texas at
Austin. None of the quakes identified in the two-year study were strong
enough to pose a danger to the public.
The
study by Cliff Frohlich, senior research scientist at the university’s
Institute for Geophysics, appears this week in the journal Proceedings of the National Academy of Sciences.
“You
can’t prove that any one earthquake was caused by an injection well,”
says Frohlich. “But it’s obvious that wells are enhancing the
probability that earthquakes will occur.”
Frohlich
analyzed seismic data collected between November 2009 and September
2011 by the EarthScope USArray Program, a National Science
Foundation-funded network of broadband seismometers from the Canadian
border to the Gulf of Mexico. Because of the high density of instruments
(25 in or near the Barnett Shale), Frohlich was able to detect
earthquakes down to magnitude 1.5, far too weak for people to feel at
the surface.
He
found that the most reliably located earthquakes—those that are
accurate to within about 0.9 miles (1.5 km)—occurred in eight groups,
all within 2 miles (3.2 km) of one or more injection wells. Before this
study, the National Earthquake Information Center had only identified
two earthquake groups in the area strongly associated with specific
injection wells. This suggests injection-triggered earthquakes are far
more common than is generally recognized.
The
Barnett Shale is a geological formation in North Texas bearing a large
amount of natural gas that was difficult to recover prior to recent
technological advances such as hydraulic fracturing. The formation lies
beneath Dallas and Fort Worth and extends over several counties, mostly
to the west of those cities. Development of the Barnett Shale and other
unconventional plays—such as the Bakken Shale in North Dakota and the
Marcellus Shale in Pennsylvania, New York and West Virginia—have spurred
dramatic growth in domestic natural gas production.
This
study comes as some policymakers and members of the public are
expressing concern about possible environmental and health effects of
hydraulic fracturing. Most earthquakes identified in the study ranged in
magnitude from 1.5 to 2.5, meaning they posed no danger to the public.
Texas map showing the Barnett Shale (gray) and rectangle indicating region mapped in figure 2. Credit: Cliff Frohlich/U. of Texas at Austin. |
“I didn’t find any higher risks from disposal of hydraulic fracturing fluids than was thought before,” says Frohlich. “My
study found more small quakes, nearly all less than magnitude 3.0, but
just more of the smaller ones than were previously known. The risk is
all from big quakes, which don’t seem to occur here.”
All
the wells nearest to the eight earthquake groups reported high
injection rates (maximum monthly injection rates exceeding 150,000
barrels of water). Yet in many other areas where wells had similarly
high injection rates, there were no earthquakes. Frohlich tried to
address those differences.
“It
might be that an injection can only trigger an earthquake if injected
fluids reach and relieve friction on a nearby fault that is already
ready to slip,” says Frohlich. “That just isn’t the situation in many
places.”
Hydraulic
fracturing is an industrial process in which water and various
chemicals are pumped deep underground in order to fracture rock,
allowing oil or gas to more easily flow to a well. As petroleum is
produced at the surface, most hydraulic fracturing fluids return to the
surface too. Frohlich is careful to point out that he did not evaluate
the possible correlation of earthquakes with the actual hydraulic
fracturing process, but rather the effects of disposing of fracturing
fluids and other wastes in these injection wells.
Support
for this study came from the U.S. Geological Survey and the Jackson
School of Geosciences at The University of Texas at Austin. The author
has no financial ties to the hydraulic fracturing industry. Frohlich has
consulted for the construction industry on seismic risks for projects
including dams, power plants and pipelines. He plans to participate in a
future study relating to hydraulic fracturing in the Barnett Shale by
the university’s Energy Institute.
Source: University of Texas
