Last year, a number of very light earth tremors near gas drilling sites in the UK raised a new line of attack for environmental lobbyists.
These groups, while they’re sometimes shy in saying so outright, are in essence opposed to any and all extraction of fossil fuels, for any reason, by any means, ever. Instead, they either want energy technologies built by companies they favour and who keep failing despite being protected by liberal amounts of government support, or they want humanity to stop using energy for productive purposes at all so we can all regress to some romanticised fiction of bucolic pre-modern idyll.
In particular, they’re opposed to a method for reaching natural gas reserves stored in deep layers of shale, known as hydraulic fracturing. This process involves drilling very deep wells, way past any water aquifers, which are protected by multiple layers of impermeable well sleeving. Once the gas-bearing shale layer is reached, horizontal shafts are drilled and 'fractured' using liquid under pressure. The gas that escapes from the rock can then be brought to the surface and piped to power stations or gas-to-liquids plants. South Africa has a potentially vast reserve under the Karoo, for which oil and gas companies are eager to explore.
A number of recent quakes in the US states of Ohio and Oklahoma, however, have led to headlines such as this: Fracked-off: Gas extraction 'causes quakes'.
On the face of it, such reports sound alarming. But let’s establish some essential context.
The first important observation is that the Ohio quake referenced in the headline above was not caused by hydraulic fracturing as such. It was caused by deep-well injection of the waste water produced in the process. Deep-well injection is a disposal process that has been in common use in many industries for many decades.
This distinction is important because while drilling for shale gas that combines hydraulic fracturing and horizontal drilling is still a matter of ongoing study, the use of wells to dispose of industrial waste water, including hazardous waste, goes back to the 1930s, and has been studied exhaustively by well operators, scientists and government safety regulators.
But before considering the risk associated with injection wells, let’s put the size of the 'earthquakes' in context. The word itself might evoke fears of violent destruction, but are we really talking about major catastrophes here?
The worst of the recent quakes in Ohio measured 4.0 on the moment magnitude scale (the measure of seismic activity that replaced the Richter scale with which most people are familiar). This is hardly terrifying.
A suspected link between deep-well injection and a much larger 5.6 quake in Oklahoma is being investigated. In the UK, the worst quake believed to be linked to gas drilling measured a very pedestrian 2.3 on the scale.
Note that the moment magnitude scale is logarithmic, so six is 10 times worse than five, which is 10 times worse than four on the scale, and so on.
The largest earthquake believed to have been caused by human activity was a quake of 6.3 in India, in 1967, attributed by critics to the building of the Koyna dam. It caused extensive damage, and 180 people lost their lives. The project chief engineer, however, observed that with 45,000 dams in the world, such “reservoir-triggered seismicity” is very rare.
The worst earthquake ever caused by deep-well injection of waste water also occurred in 1967, according to a US Geological Survey report prepared for the US Environmental Protection Agency. It registered 5.5 on the moment magnitude scale, and caused an estimated $500,000 worth of damage at the surface.
Earthquakes triggered by surface events such as blasting, mining, dams, or construction have little in common with tectonic quakes. Triggered quakes are shallow, high-frequency tremors, compared to the potentially far stronger, deeper earthquakes that originate at faults in the earth’s crust. It is theoretically conceivable that a shallow tremor could, in turn, trigger a tectonic quake, but if so, the stresses were already present in the deep rock layers, and it is likely that the tectonic quake was in any case waiting to happen.
How do these triggered earthquakes compare to what we’re familiar with in South Africa? According to a study conducted by the Aon Benfield Natural Hazard Centre, Africa, a collaboration between the re-insurance giant and the University of Pretoria: “The largest mine related event in the history of South Africa occurred on 5 March 2005 in the Klerksdorp (Stilfontein) gold mining district, 200km west of Johannesburg, which reached a magnitude of 5.3. Below ground, substantial damage was observed within the mines, while above ground, the structural damage to property was relatively low.”
The worst-ever earthquakes in South Africa were both tectonic: one on the Milnerton Fault in 1809 and one in Tulbagh in 1969. Both measured 6.3 – much stronger than any suspected tremors related to mining or well drilling. The Tulbagh quake resulted in 12 fatalities, while a 5.2 mining-related quake in Welkom in 1976 caused the deaths of four people when a block of flats collapsed.
Quakes of five or higher occur about once every two years in South Africa. Tremors exceeding four on the moment magnitude scale are relatively common, especially in mining areas such as those up on the Reef. Even less severe tremors, comparable to those that alarmed the UK, are routine.
Back to deep well injection, which caused the tremors incorrectly attributed to fracking. Injection wells aren’t new. They were first used in the 1930s, and grew apace with the growth of 20th century industry. There are 165,000 deep injection wells in the US. Of these, 529 (at last count) are 'Class I' waste wells, which were designed in response to cases of aquifer pollution in the 1970s. These feature very tough sleeve construction and extend several kilometres deep, well below water aquifers. Since 1988, operators of Class I wells must demonstrate that injected waste will not affect the biosphere (including underground water) for more than 10,000 years, and this guarantee includes evaluating seismic risk.
Most hazardous waste in the US (89% of liquid waste disposed of on land) goes down Class I wells.
Although unforeseeable underground anomalies, mechanical failures, negligence on the part of commercial operators, and human error can cause accidents, and on rare occasions do, the risks and costs associated with alternative means of treatment or disposal swing the odds strongly in favour of waste-water injection wells. In fact, the academic literature declares that environmental authorities consider such wells to be “safer than virtually all other waste disposal practices for many chemical industry wastes”.
Anyone who fears that the practice of deep-well injection of waste has been insufficiently studied, or is just a cost-cutting disposal scheme practised by reckless industrial companies, may find a leisurely evening spent perusing said literature instructive. The same goes for the simpler material that ordinary well drillers learn. This stuff is thrilling, I assure you.
Safety regulations are continually adjusted to account for rare cases of hazard, and triggering seismic activity is among these risks. Drillers are instructed to “site them in locations that are free of geological faults and other adverse features”. Wells are constantly monitored for any unexpected activity, including seismicity, and it is routine to suspend operations should any quake risk be discovered during well drilling or waste water disposal.
Disposal of waste water is not the only use for deep-well injection schemes, of course. Storing the US’s strategic petroleum reserve is a less controversial use for the technique. So is carbon sequestration, in which well injection is used to reduce atmospheric carbon dioxide emissions. And hardly anyone ever objects to the use of injection wells for cleaning contaminated or replenishing depleted water aquifers.
Oil and gas drilling also employ a type of injection drilling. In fact, that is exactly what hydraulic fracturing is. So although the cases of quakes and tremors that recently made headlines happen not to be directly linked to hydraulic fracturing itself, there is no reason, in theory, why similar seismicity could not result from gas drilling. There is also no reason, however, to suspect that cases of triggered seismicity should be more severe, or less amenable to safety regulation, than that of any other injection well.
So, what can we conclude from this brief survey of 'fracking earthquakes'?
First, that the quakes reported to date have not been associated with hydraulic fracturing, but with a long-established practice with a proven safety record, which predates horizontal-well hydro-fracked shale gas drilling by over half a century.
Second, that tremors triggered by well-drilling have almost never been severe enough to cause even mild surface damage.
Third, that quakes of similar severity are frequently triggered by other human activities, as well as by natural causes, and that very few of these have ever posed a significant threat to human health and safety.
Fourth, that there is no reason to believe that future triggered quakes would be significantly worse than the rare cases that in the past resulted in much-improved safety procedures and regulations.
Finally, one can only conclude that this is yet another case of environmental exaggeration designed to scare the bejesus out of an innocent public that does not know better, in pursuit of arch-conservative, anti-industrial policies that threaten to impoverish us all.
As the famous scientist Marie Curie once said: “Nothing in life is to be feared, it is only to be understood. Now is the time to understand more, so that we may fear less.” DM
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