Seismicity: a case of the shakes

Earthquake risk is seen as a potential show stopper. The issues are actually quite subtle, writes Simon Inglethorpe

The Lancashire village of Weeton, just north of the M55 several kilometres to the west of Blackpool, sits above a large and potentially valuable energy resource. Natural gas is locked deep below within the Bowland Shale, a brittle faulted rock more than 300 million years old.

The presence of unconventional gas explains the exploratory drilling by Cuadrilla Resources at Preese Hall outside the village in 2011. This led to something unusual and unexpected happening several kilometres underground that spring.

Two man-made earthquakes – subsequently linked to hydraulic fracturing (fracking) at Preese Hall – stopped UK shale gas exploration for 18 months, as scientists, regulators and the government determined what had happened and what to do about it.

The magnitude 2.3 seismic event of 1 April 2011, and the much lower magnitude 1.5 event that followed on 27 May, were puzzling, although not without precedent.

No earthquakes had been reported before at any of the other 200 UK onshore oil and gas wells previously fracked.

In fact, the Preese Hall events meant that the Bowland Shale became one of only three shale gas fields globally where there is definite evidence of hydraulic fracturing causing seismicity (see figure).

Fracking-induced seismic events above magnitude one

Natural earthquakes

Geologists were always doubtful about the risks to humans and property.

Strathclyde University’s Professor Zoe Shipton says: “The magnitude 2.3 event in Blackpool is like a lorry going past your house. In fact, the British Geological Survey can’t measure below magnitude two in towns because of the traffic.”

The power of the Preese Hall tremors is also well within the range of natural UK earthquakes. Lancashire itself has witnessed larger past earthquakes, the magnitude 4.4 Lancaster earthquake in 1835 being one example.

Geologists measure the power of earthquakes using the Richter scale. Each whole- number increase on this marks a 32-fold increase in energy release. So a magnitude five event is a 1,000-times more powerful than one of magnitude three, for instance.

A magnitude five earthquake takes place in the UK once in twenty years on average – and is severe enough to be felt by everyone who happens to be nearby. But historical records suggest that the UK has never experienced a natural earthquake above this size in the past 600 years.

This means that an underground shift in the rocks in the UK is only ever likely to cause a few chimneys to topple and loose plaster to fall.

Geological detective work

Determining exactly what happened at Preese Hall has involved painstaking geological detective work.

British Geological Survey (BGS) seismologist Dr Brian Baptie explains the difficulty: “Identifying causative faults is difficult as they are small – a few hundred metres in size – and may be shifting by a maximum of a couple of centimetres. Also, from what we know of these rocks at the surface, there’s likely to be lots and lots of these types of faults in the Bowland Shale at depth.”

The entry of fluid pumped underground at Preese Hall into an existing fault emerged as the most likely earthquake cause in Cuadrilla’s November 2011 summary report.1

“The flow of fluid from the well along a bedding plane into a nearby fault – located within a few hundred metres of the well – is more likely than the direct interception of a fault by exploratory drilling,” says Baptie.

A sideways rock slippage along a steeply inclined and westward-dipping fault is a probable explanation.

For Cuadrilla’s consultants, what happened at Preese Hall was a worst-case scenario because of the unusual combination of fault characteristics required. The fault needed to be primed to move (“critically-stressed”), be able to accept a large quantity of fluid and be brittle enough for a failure to cause an earthquake.

A crude estimate in Cuadrilla’s report put the probability of a repeat earthquake as low as one in 10,000 – due to the unlikelihood of encountering faults with all three characteristics again.

A subsequent independent geological review ordered by DECC agreed that hydraulic fracturing at Preese Hall was the cause.2 But it took a different view about a likely recurrence.

“We believe it not possible to state categorically that no further earthquakes will be experienced in a nearby well. The analyses [in Cuadrilla’s report] failed to identify a causative fault and it is entirely possible that there are critically-stressed faults elsewhere in the basin.”

Operating conditions play a key part in triggering seismicity. Fluid pressure build-up within the shale next to the well is particularly critical.

It is therefore not surprising that the two fracking episodes linked to the Blackpool tremors injected larger volumes of fluid with little or no flow back to the surface.

Doing exactly the same thing again in the Bowland Shale could trigger a repeat performance, says Baptie, a co-author of the Preese Hall review.

“It’s quite likely that a similar earthquake would be induced if Preese Hall operating conditions were repeated at another exploratory well down the road.

“Only a small fluid pressure perturbation in a similar critically-stressed fault is required to trigger this.”

Risks from well integrity

The Blackpool tremors left Cuadrilla’s test well intact and there is no evidence that it came close to failure.

The Preese Hall review found that “the integrity of the casing and the cement in the upper completion has not been compromised”. The magnitude 2.3 event did, however, deform the lower perforated part of the well casing by more than 1.3cm at a 2-3km depth.

The Environment Agency has assessed the environmental risks from possible borehole damage and loss of containment as a consequence of seismic activity.3 It foresees minimal impacts on aquifers and ecosystems in most cases.

 “Low-level seismic activity in the oil, gas and geothermal sectors is well known. The levels of seismicity means that impacts are generally minimal – though there are circumstances in which consequences could be more severe.”

The agency has set the overall environmental risk for ‘high volume’ hydraulic fracturing at a medium level because of this (see p9).

Permission for fracking to resume began to look inevitable last year due to the positive findings of the Royal Society and Royal Academy of Engineering review.4

The review’s chair Sir Robert Mair said: “We conclude that the earth tremors associated with hydraulic fracturing are not significant and are, in fact, very minor and lower than natural seismicity in the UK, and also lower than mining. The potential for groundwater contamination is greater through faulty well construction.”

The go-ahead for fracking to resume from climate and energy secretary Ed Davey came with caveats. He imposed new controls on shale gas exploration to guard against future earthquake risks.

Central to the new operational safeguards is ‘traffic-light’ seismic monitoring to halt operations should hydraulic fracturing trigger an event of magnitude 0.5 or above.

Baptie advised DECC on this and explains: “A high level of seismic monitoring will be required initially – certainly for the next few fracks in the Bowland Shale – due to the current lack of scientific data in the UK. It might be possible to adapt and fine-tune the low [magnitude 0.5] traffic light threshold over time.”

A DECC-approved ‘fracking plan’ is another consent condition. This will require test injections of small volumes of fluid initially, with immediate flow back, to avoid repeating the well pressure conditions linked to the Preese Hall tremors.

Micro-seismic monitoring of hydraulic fracture growth during operations is another DECC stipulation.

Earthquake size

Allowing shale gas exploration to resume also required a firm answer to a more general question: how big an earthquake could be triggered by hydraulic fracturing?

The Preese Hall review thought the answer was about magnitude three, based in part on the history of coal-mining-induced earthquakes in the UK.

The Royal Society and Royal Academy of Engineering review agreed.

“There is emerging consensus that the magnitude of seismicity induced by hydraulic fracturing would be no greater than three – felt by few people and resulting in negligible, if any, surface impacts.”

A similar conclusion was also reached by the first global review on the subject published in Marine and Petroleum Geology this spring.5 This examined almost 200 published accounts of earthquakes triggered by hydraulic fracturing since 1929.

Lead author, Professor Richard Davies of Durham University explains: “Earthquakes caused by mining can range from a magnitude of 1.6 to 5.6, reservoir-filling from 2.0 to 7.9 and waste disposal from 2.0 to 5.7.

“By comparison, most fracking-related events release a negligible amount of energy roughly equivalent to or even less than someone jumping off a ladder onto the floor.”

“Of the three fracking-related quakes that could be felt, even the largest ever, in the Horn River Basin in Canada in 2011, had a magnitude of only 3.8. That is at the lower end of the range that could be felt by people.”

Davies is confident this is the worst hydraulic fracturing earthquake.

“I’ve never heard of any unpublished evidence showing fracking is capable of inducing seismic events above magnitude 3.8, either on- or off-record. We would never have excluded such data from our paper, even if it was in the form of a personal communication.”

Avoiding active faults will be a major priority for shale gas operators when exploration resumes.

Professor Davies highlights one way of doing this: “Data such as the international World Stress Map project hosted by Potsdam’s Helmholtz Centre can tell us whether the alignments of faults in an exploration area are optimal for reactivation. The most important thing for hydraulic fracturing to avoid is fluid injection into large-scale, tectonic faults.

All the attention devoted to fracking conceals what is potentially the greatest seismic risk from shale gas development: the injection of wastewater underground.

William Ellsworth of the US Geological Survey gives one example of the possible consequences: “There’s evidence that an earthquake of magnitude 5.6 that occurred in Oklahoma in 2011 was induced by wastewater injection. It destroyed 14 homes, sent several people to hospital and caused millions of dollars of damage.”

A review by Ellsworth of injection-induced earthquakes was published in July’s Science.6

“Wastewater is tricky because there are over 30,000 wells in the United States. And it appears that only a handful of them are problematic.”

The large volumes being pumped into the ground is partly to blame. Wastewater is known to have triggered earthquakes in the Barnett Shale, Texas, for example, where volumes above 24,000m3 per month were injected.7

Underground injection

This makes perfect sense to Professor Davies: “The energy released by a seismic event relates to the area of the fault which caused it. So yes, pumping more fluid into a fault – by causing a larger slip – can produce a greater magnitude earthquake.”

But Davies does not think similar risks will arise here. “Unlike the US, the wastewater seismicity problem doesn’t apply in the UK as underground injection is not allowed – except as a specific depletion and recovery measure in offshore oilfields.”

UK shale gas operators have already been advised to recycle and reuse wastewater where possible. Surface disposal of wastewater instead of underground -injection would also avoid unnecessary seismic risks.

UK shale gas and the environment

A special report, sponsored by RPS(energy and environmental consultants)

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