By John Kemp
LONDON Dec 5 (Reuters) - 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 discontinued."
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 coastal region.
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 formations.
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 or more.
"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 Academy study.
"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 relationships."
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 in production.
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 displacements.
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 magnitude 5.
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 scale.
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 coal seams.
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.
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