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Science Alert: Experts Respond

Fracking seen as likely trigger for minor quakes which hit Blackpool – report

Posted in Science Alert: Experts Respond on November 3rd, 2011.

A British report which found that fracking was the “highly probable” cause of two minor earthquakes in the Blackpool area during the northern hemisphere spring has attracted considerable media attention.

One tremor of magnitude 2.3 on the Richter scale hit the area on April 1 followed by a second of magnitude 1.4 on May 27. Fracking, or hydraulic fracturing, is used to extract gas from underground rocks.

Cuadrilla Resources, a British company exploring Lancashire’s shale gas resources, said an independent report it commissioned into the events had pointed to “strong evidence” that the two minor quakes and 48 weaker seismic events resulted from the injection of drilling fluids into shale rocks.

Cuadrilla suspended its fracking operations after the two quakes.The report suggested the events were due to an “unusual combination” of geology at the well site, coupled with the pressure exerted by fluid injection as part of the operations.

First, the gas well encountered a “pre-existing critically stressed fault” which “accepted large quantities of fluid”, and the fault was “brittle enough to fail seismically”. The two tremors were likely induced by “repeated direct injection of fluid into the same fault zone,” the report stated. It went on to say the probability of a repeat occurrence of a “fracture-induced seismic event” with similar magnitude is “very low”.

But the report also noted: “The observed events are already 2 orders of magnitude stronger than normally observed from hydraulic fracturing induced seismicity and if future stimulation treatments again induce seismicity, it is imperative that the maximum magnitude can be estimated”.

Similar technology is already used in New Zealand’s oil and gas sector (in Taranaki) and there has been public interest in its potential use for shale gas extraction in Southland, and in Canterbury.

The Science Media Centre contact New Zealand experts for their reaction to the report.

Dr Rosemary Quinn, Head of Petroleum Geosciences, GNS Science comments:

“There are many examples of activities that cause human-induced seismicity, of which hydraulic fracturing (or ‘fracking’) is just one. There is a large body of published data that indicates that seismic activity caused by fluid injection (for solution mining, hydrocarbon production, geothermal energy generation, hazardous and non-hazardous waste disposal purposes for example) cause small seismic events i.e. magnitude (ML) ? 3.9. The seismic activity associated with hydraulic fracturing is generally less than magnitude 2.0, with the magnitude and number of events depending on local geology, the pressure and duration of the fluid injection, and the injection-rate. The small-size of earthquakes induced by hydraulic fracturing (compared to those generated by other activities) is partly due to the fact that fluid injection is for shorter duration (days as opposed to weeks or years) than for other activities.

“This induced seismic activity will probably be minor compared to natural background seismicity. In New Zealand, for example, GeoNet records about 15,000 magnitude 2.5 and larger events in an average year. At the lower end of this scale, most people are unaware that anything is happening – a passing truck generates as much if not more vibration – so the effects of seismic activity that are typically induced by hydraulic fracturing would be hard to separate from the background level of seismicity.

“The fact that we experience natural earthquakes does not mean that we should be complacent: hydraulic fracturing operations need to be designed and monitored to ensure that they do not present undue risk to people and resources. The minimum pressures and flow-rates required to achieve the desired outcome should be used. If that is done, it is very unlikely that hydraulic fracturing operations will result in any noticeable seismic impact.”

Previous expert comments, collected by the NZ SMC earlier in the year, are available here.

Below are comments our colleagues at the UK SMC have obtained from UK scientists:

Prof Stuart Haszeldine, Geologist at the University of Edinburgh, said:

“It’s important not to confuse the engineering and borehole drilling, which can be undertaken anywhere, with the effects produced by the different geological settings deep below ground. The same rock type, with the same drilling, can produce very different results in different geological settings.

“This Bowland rock is a very hard muddy shale, which can slip very very easily along its bedding layers. Even a small increase of pressure by injecting water can produce that effect.

“This is because the UK geological setting is unlike that in the USA or many parts of Europe. Here the UK has lots of stress tensioning the rock, left over from glaciation burial and uplift, and even from just being on the edge of Europe after the Atlantic opening.

“That stress often needs just a small trigger to make a small earthquake. It’s also very hard to see where pre-existing faults are deep down in this location, so predicting how to stay away from future faults is very difficult indeed. Although this is a very thorough report, I would be very cautious before proceeding with more commercial drilling. More measurements of deep present-day stresses at different locations in the basin is needed to enable more accurate predictions.”

Dr Brian Baptie, Head of Earthquake Seismology at the British Geological Survey (BGS), said:

“The Cuadrilla report confirms the initial BGS conclusion that the earthquakes were probably caused by the injection of fluid (‘fracking’). It seems quite possible, given the same injection scheme in the same well, that there could be further earthquakes. This means that some form of mitigation strategy, linked to careful monitoring of any future activity is required.

“Cuadrilla are proposing a traffic light monitoring system with a threshold of a magnitude 1.7 earthquake for mitigation to be implemented. The proposed maximum acceptable earthquake of a magnitude 2.6 earthquake might, at a depth of 3 km, result in an intensity of shaking that would not be expected to cause any damage but would be widely felt by people indoors and out, and may displace objects on shelves. Earthquakes such as this result from very small movements on small faults that may be very difficult to identify.”

Prof Quentin Fisher, Professor of Petroleum Geoengineering at the University of Leeds, said:

“Although hydraulic fracturing always causes microseismic events (i.e. low magnitude earthquakes) I’ve never heard of large magnitude earthquakes being caused by hydraulic fracturing. Indeed, on a theoretical basis, there are reasons to believe that production of gas from shales could actually reduce the longer term risk of larger magnitude earthquakes. The magnitude of the events at Blackpool was very low – so much so that a passing train would be more noticeable.

“Many people are getting overly concerned about the environmental impact of hydraulic fracturing after watching GASLANDS. I would view GASLANDS in a similar way to “The Great Global Warming Swindle” – claims are presented in the form of a documentary but are not evidence-based. There isn’t actually any evidence to suggest water supplies have been contaminated due to hydraulic fracture formation. The famous examples of gas igniting from taps are probably caused by gas leakage along the casing of boreholes – not hydraulic fracturing. Such incidences can easily be avoided and if one is really going to ban shale gas production on the basis of such incidents one would seriously need to consider banning all on-land drilling because such incidents are just as likely to occur when drilling conventional gas reservoirs as they are shale gas plays.”

Dr Clifford Jones, Reader in Engineering at the University of Aberdeen, said:

“An important error to avoid in this discussion is that hydraulic fracture has come into being just because of the move internationally towards shale gas. The first hydraulic fracture in the North Sea was in the 1960s and hydraulic fracture has been used in conventional oil and gas production over the time since then.

“There are many currently producing shale gas deposits in the US and Australia and it will be to the UK’s loss if she does not develop such reserves of this as she has. Additionally to the shale gas on the Lancashire coast there is some, as yet untapped, in the UK sector of the North Sea near the Beryl field. The distinction between shale gas and conventional gas is that the latter occurs in much less permeable rock structures than the former making fracture for access necessary. It is believed that tremors due to activity including hydraulic fracture have occurred in the US. Of more concern is the entry of hydraulic fluid into drinking water. This is something else which will have to be noted when the UK develops her shale gas but will not preclude such development.

Prof. Andrew C. Aplin, School of Civil Engineering and Geosciences at Newcastle University, said:

“The report summary is clear and open. There appears to be reasonable evidence that the injection caused the earthquakes. However, the earthquakes are small (although large for a fracking situation) and did not cause surface harm; it is unlikely that a similar size seismic event will occur again, especially given the greater understanding of the subsurface which is only achieved after drilling. The report indicates that damage to rock was confined to the Bowland Shale and did not get anywhere near shallower drinking water aquifers.

“Remember that many thousands of wells have been fracked in the US and elsewhere over decades, including of course a large number in the last decade as a result of the interest in gas shales. No industrial process is without risk, but events which can directly related to fracking (earthquakes, subsurface escape of gas or drilling chemicals) are rare. Other issues include the integrity of drill casing, sourcing of the large amounts of water used in drilling and cleaning and disposal of water used in drilling and returned to the surface. All this can be handled within an appropriate regulatory framework.

“The social issue of whether the public want onshore drilling is a different matter, requiring public discussion. We should all realise that our lifestyles demand energy and that there are no easy answers to where that energy will come from. However, in my opinion we should not stop exploring for shale gas because of concerns over fracking.”

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