American Oil and Gas Reporter - February 2015 - 144

to the Northstar 1.
McGarr points out earthquakes continued to occur for six months after the
well operator allowed the injected water
to flow back into the wellbore.
A swarm of earthquakes outside
Trinidad, in southern Colorado, starting
in 2001 and culminating in a magnitude
5.3 earthquake in 2011 sparked concerns
the triggering mechanism was fluid injection
arising from coalbed methane production
activities, but the Colorado Geological
Survey asserts the evidence is inconclusive.
The area also was hit by earthquakes in
1966, 1973 and 1992, it points out. According to seismic research, data on the
earthquakes was consistent with the source
being a previously unmapped northeastsouthwest trending normal fault dipping
steeply to the southeast, CGS says.
The survey says it has joined with its
Oklahoma counterpart in criticizing the
U.S. Geological Survey's 2012 conclusion
that the increase in earthquake activity
across the Mid-Continent, including Colorado, was "almost certainly man-made.
The survey directors for both states have
independently characterized the conclusion
as premature."
Minimizing Seismic Events
One of the major issues for disposal
well operators is injecting fluids close to
basement formations where unmapped
faults could be stressed critically, Friberg
says. In cases such as the Youngstown
well, state regulators asked operators to
plug their wells above the formation lying
directly above basement rock. He adds
other well operations have reduced the injection volumes, which has been shown to
reduce pore pressures and seismic activity.
McGarr points out that only an extremely small fraction of injection wells
have been linked to earthquake responses.
He adds that disposal well operators can
employ local seismic networks capable
of detecting low-magnitude earthquakes,
which would allow them to determine
early on if a disposal well was likely to
trigger larger events after a period of injection. He says once the fluids in the
ground percolate away from the injection
well, it becomes progressively more difficult to control seismic situations.
Majer estimates six monitoring stations
would be the minimum needed to determine the location of seismic events around
an injection facility.
The U.S. Environmental Protection
Agency has regulatory authority over
Class II injection wells, McGarr points
out, although he notes the regulations do
not target seismic activity.
"The agency's regulations are almost
all involved with whether you have well

casing failures or other pollution issues,"
McGarr says. "But EPA now is addressing
induced earthquake questions.
"The other factor is federal authority
can apply everywhere, but EPA has, in
most cases, delegated this authority to
state agencies," he continues. "State regulations are required to be at least as rigorous as federal regulations."
That means each state determines how
much seismic data to collect and how to
handle it, he says, adding that some have
been more diligent than others about
maintaining and releasing data that might
be useful to those studying induced seismic
events. Several states-such as Ohio and
Colorado-are very cooperative in sharing
data on injection operations, he says.
Geologists continue to struggle with
determining whether a seismic event is
natural or man-made, McGarr admits.

"The short answer is we really do not
know," he says. "If we find there is a
good correlation in space and time between
an injection operation and an earthquake,
then we are fairly certain that it is induced
by the injection activity. Often, that is
not the case."
The analysis gets more difficult because
there is nothing in the seismograph data
to indicate whether an earthquake was
natural or induced, because basically it
is caused by the same applied stresses
that nature provides, McGarr asserts.
"The injection activity is increasing
the pore pressure and weakening the
rock," he maintains. "That injected fluid
is lubricating pre-existing faults, but the
basic stresses causing the earthquake are
those that are supplied by nature: the
long-term deformation of the earth's crust
that goes on everywhere."
Ì

Geothermal Operators Manage
Seismic Risks By 'Traffic Lights'
BERKELEY, CA.-As the oil and gas
industry deals with public concerns about
seismic events linked to wastewater disposal wells, geothermal operators offer
a range of resources that could help mitigate public fears and the resulting regulatory overreach.
"Geothermal operators get local stakeholder buy-in by educating the local populace that earthquakes could result from
geothermal power plant operations," says
seismologist Paul Friberg, owner of Instrumental Software Technologies Inc.
"This has gone a long way toward making
the felt earthquakes caused by this industry
to be viewed more as a nuisance than a
hazard. The classical case is the Geysers
Geothermal Field, north of California's
Napa Valley, where the operator has done
a great job in education about earthquakes
induced by his operations."
Ernest Majer, a staff scientist and division deputy director of the earth sciences
division at Lawrence Berkeley National
Laboratory, says many California geothermal facilities set thresholds for the
types of risks that may arise, employing
a traffic light system to guide response
mechanisms, while other companies take
a more detailed approach through realtime monitoring of seismic events.
Art McGarr, a geophysicist at the
Earthquake Science Center at the U.S.
Geological Survey's Earthquakes Hazard
Program, notes that traditional geothermal
fields, in which meteoric water percolates
readily to depths where the rock is hot
enough to produce steam, have long since

144 THE AMERICAN OIL & GAS REPORTER

been discovered. Expanding the geothermal industry requires taking advantage
of rock that is hot, but of such low permeability that water cannot percolate
through it so as to be heated. Instead, he
says it is necessary to create a network
of fissures throughout the hot rock mass
to allow cold water from above to flow
through and acquire heat-a process termed
enhanced geothermal systems (EGS).
"The idea is to inject a large volume
of cold water under high pressure into a
hot crystalline rock mass so as to produce
a distributed network of narrow fissures
that serve as effective systems to extract
heat from the rock," he says. "Injection
usually takes place over a matter of days.
Because of the short injection time, earthquakes occur almost immediately and at
quite a high rate. Presumably, the water
does not have a chance to migrate very
far from the injection point."
Although small earthquakes are unavoidable in developing an EGS, McGarr
points out it is important to avoid inducing
earthquakes large enough to be felt at
the surface, for several reasons. A large
earthquake fault would compromise the
effectiveness of the EGS, and a large
event might give rise to public concerns.
For these reasons, an operator may use
a traffic light system, based on earthquake
magnitude thresholds to guide his operations. McGarr says, explaining that occurrence of events with specified magnitudes
might require reducing injection rates or
even stopping injection altogether so as to
avoid earthquakes of greater consequence.

American Oil and Gas Reporter - February 2015

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