This
wall collapsed in Timpson, Texas, following a 4.8 magnitude earthquake in 2012.
A new satellite study confirms the temblor was induced by injection of large
volumes of wastewater from oil and gas activities.
The largest
recorded earthquake in East Texas was triggered by the high-volume injections
of wastewater from oil and gas activities deep underground, according to a
study co-authored by Stanford geophysicist William Ellsworth.
A 4.8
magnitude earthquake
that rattled residents and damaged buildings in East Texas in 2012 was
triggered by the injection of large volumes of wastewater from oil and gas
activities into rocks deep beneath the surface, a new satellite study by
Stanford scientists and others confirms.
Geophysicists
have long suspected that the 2012 temblor, which was the largest ever recorded
in the region and followed by several strong aftershocks, was a manmade, or
"induced," quake resulting from the activities of nearby wastewater
injection wells, but definitive proof had been lacking – until now.
"Our
research is the first to provide an answer to the questions of why some
wastewater injection causes earthquakes, where it starts and why it
stops," said study co-author William Ellsworth, a geophysics professor at
Stanford's School of Earth, Energy & Environmental Sciences.
In a new
study, published in this week's issue of the journal Science, Ellsworth
and his co-authors used a remote sensing technique called Interferometric
Synthetic Aperture Radar, or InSAR, to measure ground deformations near the
wells in East Texas where the quake occurred. InSAR satellites use radar to
detect tiny, centimeter-scale changes in the shape of Earth's surface.
"Our
study reports on the first observations of surface uplift associated with
wastewater injection," Ellsworth said. "The detection of uplift when
combined with well-injection records provides a new way to study wastewater
injection."
The team
focused on four high-volume wells used for disposing wastewater, located near
the town of Timpson, Texas, where the 2012 quake was centered. The four wells
began operations between 2005 and 2007 and at their peak injected about 200
million gallons of wastewater per year underground.
Brackish
water naturally coexists with oil and gas within the Earth. After extracting
this slurry using hydraulic fracturing or other techniques, drilling companies
separate the "produced water" from the oil and gas and then reinject
it into Earth at disposal wells. Approximately 180,000 of these disposal wells
are currently in operation in the United States, primarily in Texas,
California, Oklahoma and Kansas. "You can think of the wastewater as ancient
ocean water," Ellsworth said. "It's too salty and too contaminated
with other chemicals to treat economically, so the only viable solution at
present is to put it back underground."
But where
that wastewater is injected can make a huge difference. Injecting wastewater at
a depth of over 1 mile, two of the wastewater disposal wells the scientists
examined lie directly above where the earthquake occurred. The other two wells
injected similar volumes of wastewater, but at shallower depths, just over a half
mile below the surface.
The InSAR
measurements revealed that wastewater injection at the shallow wells resulted
in detectable ground uplift up to 5 miles (8 kilometers) away but only a modest
rise in pore pressure, which is the pressure of fluids within the fractures and
cavities of rocks, at the depth at which earthquakes happen 2 or more miles
below the surface.
Increasing
pore pressure within a geologic fault can cause the two sides of the fault to
slip and release seismic energy as an earthquake.
This did not
happen at the shallow well sites in East Texas because a thick layer of nearly
impermeable rock beneath the injection sites of the shallow wells prevented the
pore pressure from migrating downward towards the crystalline basement, a deep
and faulted rock layer where earthquakes originate. Instead, the injected
wastewater caused a subtle uplift of several centimeters over a broad area
around the injection wells.
The
situation was different at the deep-well injection sites. There, the
combination of stiffer rock and the impermeable "blocking formation"
above allowed the rising pore pressure to migrate downward and build up until
it triggered earthquakes in 2012 along an ancient fault line. The quakes ended
in late 2013, when pressures began to decline after wastewater injections were
scaled back considerably.
The new
findings highlight the importance of understanding the local geology for
wastewater injection operations, the scientists say. "The recent upturn in
seismicity in Oklahoma and Kansas commonly happens where injection occurs close
to the crystalline basement, so we're getting lots of earthquakes in those
places," Ellsworth said. "Injecting at shallower depth above a
blocking formation would reduce the ability of the pore pressures to migrate to
the basement and activate the faults."
The research
also demonstrates the potential of remote sensing for understanding how and
where pressure changes move through Earth. "Moving forward, we need to
predict where pressures will increase in order to reduce the potential for
inducing earthquakes," Ellsworth said. "Our research uncovers new
possibilities for operating wells in ways that reduce earthquake hazard."
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