Underground disposal of waste water produced from oil and natural gas wells has been blamed for triggering thousands of small earthquakes in Oklahoma and a number of other U.S. states since 2009.
Heightened seismic activity corresponds closely with the timeframe and location of increased drilling and hydraulic fracturing across the southwest United States, according to the U.S. Geological Survey (“Incorporating induced seismicity in the 2014 United States national seismic hazard model,” 2015).
Most tremors have been barely perceptible to humans, but one at Prague in Oklahoma was recorded at magnitude 5.6, enough to cause severe shaking and damage to buildings.
The quake swarms have sparked a debate about safety and economic opportunity in states and communities that depend heavily on oil and natural gas production for jobs and income.
Waste Water Injection
Most tremors seem to have been caused by re-injection of waste water brought to the surface along with oil and gas back underground into deep rock formations, rather than by the hydraulic fracturing itself.
Water, contaminated with salt, hydrocarbons and even traces of naturally occurring radioactive material picked up from formations underground where oil and gas are found, is actually the largest single output of the oil and gas industry.
U.S. oil and gas wells produced over 57 million barrels per day of waste water in 2007, according to researchers (“Produced water volumes and management practices in the United States,” 2009).
Since then, natural gas production has risen by 30 percent and oil production is up 80 percent, so the amount of produced waste water is almost certainly much higher.
According to researchers, 95 percent of the produced water is disposed of underground by re-injecting it into the oil- and gas-bearing formation to maintain reservoir pressure or into other rock formations.
But it has long been known that the removal or injection of a large volume of fluid into rock formations can trigger earthquakes.
The first and most famous example of man-made earthquakes or “induced seismicity” due to fluid injection was reported at the Rocky Mountain Arsenal in the 1960s and 1970s.
Contaminated liquid waste from a chemical weapons plant injected underground triggered thousands of tremors near Denver, the largest of which measured magnitude 4.8.
Man-made earthquakes have also been linked to the impoundment of large volumes of water for hydroelectric power dams, geothermal energy plants, conventional oil and gas fields, enhanced oil recovery programs and mining.
The magnitude and destructiveness of earthquakes are directly related to the surface area of the rock that ruptures by a sudden slip.
The magnitude of naturally occurring earthquakes follows a well understood distribution. Most are very small, with progressively fewer occurrences of tremors at higher magnitudes.
Below magnitude 2.0, they are unlikely to be felt by humans. Those between magnitudes 3.0 and 5.0 will be felt. Those over magnitude 5.0 are likely to be damaging.
In general, the bigger the volume of fluid injected or removed from a formation, the bigger the maximum potential earthquake, according to the U.S. National Research Council (“Induced seismicity and energy technologies,” 2013).
The Magnitude Scale
Hundreds of quakes are induced by energy production (oil, gas, geothermal, hydro) every year in the United States and probably thousands around the world.
Most are very small at magnitude 2 or lower, with a small number ranging up to magnitudes 3 and 4, which are felt, and very rarely to magnitude 5.
The potential for hydraulic fracturing to cause earthquakes has caused concern among local communities and been seized on by environmental groups and climate campaigners to call for curbs on the practice.
But it is vital to put the risk into perspective. Most of these induced seismic events pose little risk of damage to buildings or humans.
They are better described as tremors or more neutrally as seismic events rather than the more emotive — though common— term earthquake.
The magnitude scale is logarithmic so a magnitude 2.0 or 3.0 seismic event releases a very different amount of energy than a magnitude 5.0 or 6.0 one.
The energy released by a magnitude 3.0 tremor, the sort that might be associated with oil and gas field operations, is roughly 15 million times smaller than the Nepal earthquake on April 25.
Even the worst earthquake in Oklahoma’s current swarm, at Prague, released 2,000 times less energy than the one near Lamjung in Nepal.
While some tremors have been directly traced to the pumping of fracking fluid at higher pressure into undetected fault zones, such as the one at Preese Hall in Britain, most are associated with the disposal of waste water.
Induced seismicity is a side-effect of all oil and gas production rather than the fracking process. Some of the largest recorded seismic events have taken place at conventional fields which have been waterflooded to boost oil recovery.
And induced seismicity is not limited to oil and gas production. Some of the largest earthquakes that may have been triggered by man have been linked to dam projects in India (M6.3) and China (M7.9).
The most frequent induced seismicity in the United States has occurred at the Geysers geothermal power plant in northern California, which triggers 300-400 tremors per year, with one to three of them rated at magnitude 4.0 or higher.
The Geysers has a well-established program to pay for damage to property (such as broken tiles or cracked walls) linked to its operations.
Communities in mining areas and near oil and gas fields have long experienced induced tremors: an average of 15 due to underground works are reported each year in the United Kingdom.
Most induced quakes around the world are limited to between magnitudes 2.0 and 5.0, where they may be felt but are unlikely to do much damage according to researchers at Britain’s Durham University (“What size of earthquakes can be caused by fracking?”, 2013).
Carbon Dioxide Storage
Because the amount of fluid involved in hydraulic fracturing itself is relatively small, just a few million gallons, it is unlikely to generate a large tremor, unless injected into a heavily faulted area. The volumes involved in waste water injection are much larger and pose a greater potential danger.
The risk of activating a large fault system provides a strong case for regulating both fracking and waste water injection and ensuring that operators have an adequate understanding of local geology and that their operations are monitored to detect any seismicity due to undetected faults.
The biggest danger comes from proposals to lock away carbon dioxide underground as part of carbon capture and storage (CCS) schemes.
CCS has been identified as essential if the world is to continue using energy from fossil fuels such as coal and gas while curbing carbon dioxide emissions and limiting the rise in global temperatures to two degrees Celsius.
To have an impact on climate change, however, CCS would have to pump billions of tonnes of supercritical CO2 under intense pressure into deep rock formations. The scale of the injections would pose an earthquake risk far greater than anything currently associated with oil and gas production.
For some climate campaigners and environmental groups, the threat of earthquakes is another reason to ban or severely regulate fracking, and ultimately leave the oil and gas in the ground.
But that response would be neither practical nor proportionate; the risk of earthquakes is associated with plenty of energy technologies that environmentalists like, such as dams, geothermal and CCS.
Unfortunately, the response from some executives linked to the oil and gas industry has been to deny that any link exists and attack the scientific studies, which while not conclusive are strongly suggestive.
A more sensible course would be to accept that there is a strong likelihood of a causal link between oil and gas production and seismic events and work towards sensible and proportionate regulations, recognizing that the quake risks are moderate and that oil and gas production remains essential.
(Editing by Dale Hudson)
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