A dramatic increase in the number of earthquakes in the U.S. can be attributed to shale gas exploration, claims a new Cornell University study published in the Science journal. What is more, the study found that unlike those caused by fracking, tremors associated with deep-well injections can be ‘destructive’ – causing injury and damage to property.
The study shows that after decades of a steady earthquake rate (average of 21 events/year), activity increased starting in 2001 and peaked at 188 earthquakes in 2011. Human-induced earthquakes are suspected to be partially responsible for the increase.
Microearthquakes (that is, those with magnitudes below 2) are routinely produced as part of the hydraulic fracturing (or “fracking”) process used to stimulate the production of oil, but the process as currently practiced appears to pose a low risk of inducing destructive earthquakes.
More worryingly, deep-well injections can be demonstrated to cause larger earthquakes. For instance, several of the largest earthquakes in the U.S. midcontinent in 2011 and 2012 may have been triggered by nearby disposal wells. The largest of these was a magnitude 5.6 event in central Oklahoma that destroyed 14 homes and injured two people.
Deep disposal is an EPA-approved method of disposing of produced water that is a byproduct of fracking. The process of hydraulic fracturing, or fracking, involves drilling down to the shale layer, drilling a horizontal well through the formation, and than stimulating the rock by pumping in between 2.5 and 5 million gallons water and proppants to fracture the rock and release trapped hydrocarbons.
Some of the water used in fracking (about 5-20%) returns to the surface as the so-called ‘flowback water’. On top of that, ‘produced water’ – water trapped in rock formations released during the production process – also comes to the surface during shale exploration. Both types of water are very saline and contain other contaminants, like salts and radioactive materials present in the formation, and need to be disposed of safely.
Deep well injection is considered to be the best, and the cheapest, method of frac water disposal. The fully loaded cost including amortization of the well is between $0.25 and $1.00 per barrel of wastewater disposed. The process involves injecting the water into the ground to be trapped under layers of impermeable rock that will prevent the wastewater from polluting aquifers.
One of the problems with this method, however, is that if the injection well is in proximity of an existing seismic fault, it may cause earthquakes. Although only a small fraction of the more than 30,000 wastewater disposal wells appears to be problematic — typically those that dispose of very large volumes of water and/or communicate pressure perturbations directly into basement faults – the Cornell study found that earthquakes induced by this method are likely to be larger than those associated with the process of fracturing shale rock and “clearly contribute to the seismic hazard.”
Katie Keranen, professor of geophysics at Cornell University and the person in charge of the study said: “Induced seismicity is one of the primary challenges for expanded shale gas and unconventional hydrocarbon development.”
“Our results provide insight into the process by which the earthquakes are induced and suggest that adherence to standard best practices may substantially reduce the risk of inducing seismicity.”
“The best practices include avoiding wastewater disposal near major faults and the use of appropriate monitoring and mitigation strategies.”
The Cornell study is not the first piece of research pointing to a connection between deep disposal and earthquakes. A report published in May by the Seismological Society of America also linked activities associated with shale gas exploration – such as fracking and deep well injections – to earthquakes, sometimes occurring as far as tens of kilometres from the wellbore.
Limiting the need for fresh water for fracking and ways to safely dispose of wastewater are major problem areas for shale gas industry and a focus of much innovation. One of the more novel solutions came from Kyoto University, Japan. It proposes to dispense with frac water altogether and replace it with CO2. By injecting carbon dioxide into shale bedrock instead of water, the technology will not have to rely on millions of gallons of fresh water, as well as solving the problem of neutralising the toxic produced water. Also, the technology is expected to help combat global warming as it will confine CO2 underground.
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