By John Kemp
DUBAI, April 17 (Reuters) - Recent government reports have concluded hydraulic fracturing and other fluid injection activities associated with oil and gas production were responsible for a series of small tremors in northern England last year, and they have also been blamed for a six-fold increase in the number of tremors in the midcontinent of the United States since 2001.
But the uptick in seismic activity is unlikely to stop the spread of fracking. The tremors are small and no worse than those traditionally associated with mining for coal, salt and other minerals. They pose little or no threat to structures or humans.
In the United States, President Barack Obama’s decision to issue an executive order on April 13 “Supporting Safe and Responsible Development of Unconventional Domestic Natural Gas Resources” is a clear sign policymakers are prepared to override concerns about the risk linked to small earth tremors in order to boost domestic energy production.
In Britain, the government has launched a six-week consultation. But it too is clearly preparing to endorse the practice, albeit with heightened safeguards.
The chief scientific adviser to the Department of Energy and Climate Change (DECC) launched the review by suggesting it contained “a set of robust measures to make sure future seismic risks are minimised, not just as this location but at any other potential sites across the United Kingdom.”
Hydraulic fracturing at the Preese Hall well near Blackpool in northern England “induced” a number of earthquakes in April and May 2011, the largest of which had a local magnitude of 2.3, according to the findings of an official government review published this week (“Preese Hall shale gas fracturing: review and recommendations for induced seismic mitigation” April 2012).
It was not the pressure pumping itself which caused the tremors but the accidental injection of fluids into an adjacent fault. The largest quakes occurred approximately ten hours after the start of the injection, when the well was shut in under high pressure.
The potential for man-made activities to trigger small earthquakes is well known. Underground mining, deep artificial water reservoirs, oil and gas extraction, geothermal power generation and waste disposal have all resulted in cases of induced seismicity, according to the report’s authors, who included scientists from the British Geological Survey and Keele University.
In 2001, pressure maintenance injections at the North Sea’s Ekofisk oil field induced an earthquake with a surprisingly large magnitude of 4.1. But most tremors are much smaller and are barely perceptible or measured only with sensitive instruments.
Half of the seismic activity experienced in the United Kingdom in the last century was caused by coal-mining, according to a government briefing note. Despite the widespread shut down of most coal mines, traditional mining areas still experience occasional tremors linked to mine flooding and the restoration of water levels (“Induced seismicity in the UK and its relevance to hydraulic stimulation for exploration for shale gas” April 2012).
Post-mining tremors range from those only detectable very close to the source with sensitive instruments up to a maximum of magnitude 3.2 for one quake in Scotland’s Midlothian region. But most tremors are below 3.0.
In the United States, researchers for the U.S. Geological Survey (USGS) have found a remarkable increase in the number of earthquakes occurring each year of magnitude 3.0 or greater, concentrated along the border of Colorado and New Mexico, and more recently in Arkansas and Oklahoma.
Between 1970 and 2000, the entire midcontinent region, of which these areas form a part, averaged 21 +/- 8 events at magnitude 3.0 or greater. From 2001 to 2008, this increased slightly to 29 +/- 3.5 events at 3.0 or greater. But then in 2009, 2010 and 2011, the number surged to 50, 87 and 134 events respectively.
In Oklahoma, the rate of magnitude 3.0 or higher events has surged from 1.2 per year in the previous half century to over 25 per year since 2009. This number excludes one strong earthquake of 5.6 in November 2011 and its aftershocks.
“The modest increase that began in 2001 is due to increased seismicity in the coal bed methane field of the Raton Basin along the Colorado-New Mexico border,” according to USGS. “The acceleration in activity that began in 2009 appears to involve a combination of source regions of oil and gas production, including the Guy, Arkansas region, and in central and southern Oklahoma.”
USGS cites research by other scientists linking the Arkansas earthquakes to deep waste water injection wells.
Fracking opponents have already begun to characterise these findings as “fracking causes quakes” but the reality is more complicated.
First it is not the fracking itself which seems to be causing increased seismic activity. Any mining or oil/gas production activity involving fluid injection into underground formations can set off small earthquakes.
Tremors have been associated with coal production and coal-bed methane, and they may be caused by underground injection of waste water produced from conventional oil and gas wells just as much as the pressure pumping used to fracture unconventional wells.
If carbon capture and storage (CCS) programmes eventually inject large volumes of liquid CO2 into saline aquifers, un-mineable coal seams and other rock formations, that too will cause tremors, which is one reason many environmental groups have been careful not to oppose injection and pressure pumping entirely.
Second, most of these “earthquakes” are very small. Earthquake is an emotive word associated with mass destruction of buildings and loss of life. There is no suggestion tremors caused by fracking and other mining processes have caused any damage remotely like this.
“To our knowledge, hydrofracturing ... rarely creates unwanted induced seismicity large enough to be detected on the surface with very sensitive sensors, let alone be a hazard or annoyance,” wrote researchers at the Earth Sciences Division at Lawrence Berkeley National Laboratory.
While an earthquake of magnitude 2.3 or even 3.0 sounds serious, the surface shaking and amount of energy released is tiny. The surface shaking generated by the worst quake at Blackpool (2.3) was 10,000 times smaller than in the earthquake which devastated large parts of Christchurch, New Zealand, in February 2011. The amount of energy released was 1 million times smaller.
There is no simple relationship between the magnitude of a tremor and the intensity with which it is felt by local inhabitants. Small, but shallow, earthquakes can be felt quite strongly while deep but large earthquakes may not be felt as much.
But USGS suggests an earthquake with a magnitude of 1.0-3.0 would only be felt by a few persons (it would register just 1 on the 12-point Modified Mercalli Scale). An earthquake of 3.0-3.9, which is similar to the worst associated with mineral extraction, would be felt as a weak or noticeable vibration indoors especially on the upper floors of buildings (it would register at 2-3 on the Modified Mercalli Scale).
Vibrations would be similar to the passing of a heavy truck and many people would not recognise it as an earthquake.
The UK government review suggests 3.0 is probably the worst that could be expected from fracking, based on previous experience from coal mining. But to create a margin of safety, it recommended operations should be halted and remedial action instituted immediately if events of magnitude 0.5 or above are detected.
It also recommends the main frack should be preceded by a smaller pre-injection and monitoring stage, and future operations should include effective real-time monitoring of the locations and magnitudes of seismic events to provide an early warning of trouble.
Based on the scientific studies so far, however, fracking will not cause homes and other buildings to collapse or suffer significant damage. With appropriate safeguards, it is a safe procedure that poses no more risk than coal mining or developing geothermal, both of which are seen as perfectly acceptable hazards.