By John Kemp
LONDON Dec 5 Hydraulic fracturing to produce
oil and gas has become closely associated in the public mind
with the risk of triggering man-made earthquakes. But the risk
is not high and it is not confined to fracking.
There may be greater danger from geothermal energy
production and pumping carbon dioxide underground as part of
carbon capture and storage projects.
Those are the findings of an authoritative study on the risk
of man-made ("induced") seismicity carried out by the U.S.
National Academy of Sciences at the request of Congress
("Induced Seismicity Potential in Energy Technologies" 2012).
Environmentalists and community groups have seized on
earthquake risk to oppose hydraulic fracturing in the United
States and Western Europe.
Fracking fears appeared to be justified after a British
government enquiry held the technology responsible for a series
of tremors in April and May 2011 at Preese Hall near Blackpool,
the largest of which had a magnitude of 2.3 on the Richter
scale, big enough to be felt by local residents.
Fracking has also been linked to a "swarm" of 50 small
earthquakes in Oklahoma on Jan 18, 2011, ranging in magnitude
from 1.0 to 2.8, which occurred at the same time as the fracking
of a well in the Eola Oil Field.
But these are the only two confirmed cases where induced
seismic activity has been linked to fracking. Far more seismic
events have been traced to conventional oil and gas extraction
(38), secondary oil recovery through waterflooding (27),
geothermal energy (25), waste water injection (11) and
hydroelectric reservoirs (44).
The risk of man-made earthquakes has been known for decades,
and fracking poses far less risk than other technologies because
of the comparatively small volumes of water involved.
The potential for pumping fluids into or out of the earth to
cause seismic events large enough to be felt on the surface has
been known since at least the 1920s.
The most famous example of induced seismic activity occurred
in the 1960s at the Rocky Mountain Arsenal near Denver,
Colorado, where the U.S. Army disposed of hundreds of thousands
of gallons of contaminated water from chemical weapons
production every day by pumping it into a well drilled on site.
Between 1962 and 1967, more than 1,500 earthquakes were
recorded in the area, some as high as magnitude 3 or 4, and
noticed by local residents. "By November 1965, over 700 shocks
had been recorded, and although 75 of these had been felt, no
damage was recorded," according to previous research cited by
the National Academy.
"Research indicated a strong relationship between injection
volumes and earthquake frequency ... Although injection into the
Arsenal well ceased in February 1966, earthquake activity
continued for several more years. The strongest earthquakes
actually occurred after injection into the well was
Induced seismic activity has also been documented at The
Geysers geothermal power project in northern California. The
Geysers generates about 725 megawatts of electricity, enough to
power a city the size of San Francisco, and supplies about 60
percent of the average power demand in the state's northern
Originally, the field produced power from steam flowing up
naturally from 420 wells on the site. But as the underground
pressure depleted, water injection commenced, which coincided
with a significant rise in seismic activity, partly because of
temperature differences between the injected water and hot rock
More than 1,000 seismic events with a magnitude of at least
1.5 are recorded every year, with around 25-30 reaching
magnitude 3.0, and between one and three hitting magnitude 4.0
"Minor damage is occasionally caused by the induced
seismicity ... generally as cracks to windows, or dry walls, or
tile walls or flooring in local communities," according to the
"A system for receiving, reviewing and approving such damage
claims was established six years ago and the homeowners are
reimbursed for their costs to have damage repaired. To date
these reimbursements for home repairs total $81,000 and this
system appears to be resulting in mutually satisfactory
It's not only pumping things into the ground that can cause
earthquakes. Taking things out can also trigger tremors because
of the resulting drop in underground pressure.
Between 1957 and 1983, aggressive gas production from the
Lacq gas field in southwest France caused the pressure to drop
from 660 bars to 160 (1 bar is roughly equal to normal
atmospheric pressure). Some 800 seismic events with magnitudes
ranging up to 4.2 were recorded as the pressure plunged.
Nonetheless, seismic events arising from conventional oil
and gas extraction are relatively rare compared with the large
number of oil and gas fields around the world.
Similarly, there are 151,000 injection wells licensed in the
United States for waterflooding, enhanced oil recovery (EOR) or
waste water disposal and documented instances of induced seismic
activity remain very infrequent.
Fracking poses a low statistical risk. "About 35,000
hydraulically fractured shale gas wells exist in the United
States; only one case of felt seismicity (Oklahoma) has been
described in which hydraulic fracturing for shale gas
development is suspected but not confirmed as the cause,"
according to the induced seismicity report.
"The very low number of felt events relative to the large
number of hydraulically fractured wells for shale gas is likely
due to the short duration of injection of fluids and the limited
fluid volumes used in a small spatial area."
The basic mechanisms that induce seismic events are well
understood: the most important risk factor is pressure changes
linked to the net fluid balance (the total amount of fluid
injected or withdrawn from the subsurface). The probability of
triggering a significant seismic event increases with the volume
of fluid injected (or withdrawn).
In most cases, oil and gas operators seek to maintain
reservoir pressure close to original levels, so the risk of
seismic activity is correspondingly low. The volume of water,
gas or carbon dioxide injected to maintain pressure is roughly
equivalent to the volume of crude, gas and condensate withdrawn
The degree of shaking is closely linked to the magnitude of
the earthquake. Earthquakes with large magnitudes always involve
large parts of the Earth's crust, because the large energies
being released can only be stored large volumes of rock, and
large rupture areas are necessary to produce large fault
To trigger a magnitude 3 earthquake, a fault must rupture
over an area of approximately 15 acres, increasing to 0.5 square
miles for a magnitude 4 event, and 4.2 square miles for
The largest reported induced seismic events have all
involved the injection of huge volumes that caused faults to
rupture over a correspondingly large area.
At Rocky Mountain Arsenal, the U.S. Army injected as much as
148-181,000 gallons (3,500-4,300 barrels) of water a day for
years at a time. Massive injections into poorly selected rock
formations induced 3 earthquakes of magnitude 5.0-5.5, the
largest of which caused estimated damage of $500,000 in 1967.
LOW FRACK RISK
Fracked wells simply do not inject a large enough volume of
fluid over a big enough area to cause seismic events on this
In contrast, water injection at The Geysers has now hit more
than 300 million barrels per year (partly offset by steam
withdrawals). And carbon storage on utility-scale would involve
the injection of truly staggering amounts of supercritical
(gas/fluid) carbon dioxide into saline aquifers or unmineable
Environmentalists have selectively focused on the seismic
risks with fracking while ignoring much greater risks associated
with geothermal and carbon storage, apparently because the
latter are seen as a "clean" form of energy. But it reflects a
poor and partial understanding of the different risks involved
in different types of energy production.
It would probably not be wise to set up a carbon storage
scheme near the San Andreas fault. In most places, however,
fracking for oil and gas will rarely be felt by humans, cause no
more than trivial damage, and pose no risk to life.