Mike Stephenson, British Geological Survey

March 2012

Methane natural gas is an important part of Britain’s energy mix and will continue to be so in the future. As old coal and nuclear power stations shut down, gas could provide flexible and reliable backup supplies of electricity to complement increased renewable energy. Gas is the primary fuel to heat homes in Britain, and will likely remain so until well into the 2020s. Gas is also a relatively clean fuel whether used for heating or by power stations, and could be very clean when used in association with carbon capture and storage technology.

But we have to get the gas. 2011 was a landmark year for Britain because for the first time the country imported more methane natural gas – whether piped from Norway or shipped from Qatar – than was pumped from Britain’s offshore gas fields. Importing gas is fine if you have the amount you need, and a steady supply helps to keep prices stable. But supply can be affected by unforeseen international events. So it makes sense to have ‘home-grown’ gas.

This is why Britain is waking up to the idea of shale gas for power and fuel into the future. Shale is the most common sedimentary rock, and Britain has a lot of it in northern England, the Midlands, Wales and southern England. Shale is soft so often isn’t seen at the surface, but shale of various ages underlies much of the country.

The British Geological Survey’s (BGS) first area-based assessment¹ of the amount of shale gas that might be present in these areas came up with a fairly large figure of 150 billion cubic metres (BCM), which is about half of Britain’s estimated reserves of conventional natural gas and about one and a half year’s worth of gas at present rates of usage. However other unpublished estimates for parts of the country are much larger.² To reduce uncertainty, BGS (commissioned by the UK Department of Energy and Climate Change) is working on a new volumetric-based estimate which may be available within the year.

The problem is, however, how to get the gas out. Britain is a crowded island full of people who are fond of their surroundings and concerned for the quality and care of the environment. The key extraction technology, hydraulic fracturing (‘fracking’), has had a very bad press in the last year, and has been blamed for causing earthquakes. The public also worry about contamination of groundwater by methane and/or chemicals from fracturing fluids.

As any shale gas engineer will tell you, you need to crack the shale to release the gas. A simple well without hydraulic fracturing will not release much gas. The shale itself is very rich in organic matter but the gas which is generated from the organic matter, can’t move easily in the rock because it is fine grained and impermeable. So hydraulic fracturing is generally essential.

Some of the public’s worries are no doubt justified. Badly managed hydraulic fracturing (though not related to shale gas) has recently been shown to have contaminated water wells in Wyoming.³ One of the problems is that it’s very difficult to get reliable independent information, and there are lots of vested interests. So peer-reviewed science has a role in deciding what the real risks are.

Most geologists think that methane or fracturing fluid contamination of aquifers is unlikely because of the great difference between the depths at which hydraulic fracturing activities are usually carried out and the aquifers from which we get our water. Put simply, there’s a lot of hard, impermeable rock between the rocks being hydraulically fractured and the aquifer. However there are relatively few peer-reviewed studies of methane contamination during shale gas hydraulic fracturing, and rather problematically, there are few baseline studies of amounts of background biogenic or thermogenic methane in groundwater.

Biogenic methane is usually generated by bacteria, and isn’t usually associated with deep shale. Thermogenic methane, generated by heat acting on the organic matter in shale, is usually deep but sometimes occurs naturally in shallow aquifers. Showing that methane in a water well is thermogenic (when the δ¹³C of the C in the CH₄ is above about -50 ‰) might be one way of telling if a deep hydraulic fracturing operation is leaking, but you have to know what the baseline natural levels of methane are as well.

It’s a little known fact that many of our aquifers in Britain contain methane – biogenic and thermogenic.⁴ Knowing how much is natural – so that you can distinguish it from possible leaked methane – is only possible if you’ve measured baseline levels. This is why the BGS is working on a baseline survey at the moment.

It’s well known that hydraulic fracturing causes earthquakes - usually infinitesimally small - because they are used by geologists to track the progress of a fracturing operation. However the two earthquakes caused by hydraulic fracturing in Blackpool recently (of magnitudes 1.5 and 2.3) were larger than the operating company expected. Some areas of Britain are quite used to natural earthquakes of this size, or earthquakes caused by old mine workings, but they came as a shock to the people of Blackpool.

The larger quake on the 1st April 2011 was felt by more than 50 people, but the energy released was quite inconsistent with the damage that was claimed for the earthquake. The seismology, including the matching seismological traces, told BGS seismologists that the two earthquakes were generated in the same area underground, and in the same way.⁵ The coincidence in time between the earthquakes and the hydraulic fracturing operations suggests they resulted from high pressure water finding its way into small pre-stressed faults which then moved slightly.

Earthquakes can be monitored during hydraulic fracturing in quite a sophisticated way. For risk mitigation, a ‘traffic light ’ system can be used. The operator would monitor seismicity, and if any of the myriad small tremors exceeded a threshold magnitude the ‘red light’ would come on and the operations would be stopped immediately to avoid causing a larger earthquake which would be felt by the local population. The operator would also have to avoid hydraulic fracturing close to known active faults. Proposed mitigation options are detailled in the "Geomechanical Study of Bowland Shale Seismicity".⁶

Because the technology to construct wells and manage subsurface operations is mature there is little reason to believe that shale gas extraction involves greater risks than in conventional hydrocarbons extraction. Reassurance can be gained by monitoring seismic activity during hydraulic fracturing and through long-term monitoring of the condition of nearby aquifers.

The British Geological Service Shale Gas Project website provides more information on shale gas in the UK.⁷

¹ https://www.og.decc.gov.uk/UKpromote/onshore_paper/UK_onshore_shalegas.pdf

² www.guardian.co.uk/environment/damian-carrington-blog/2011/sep/23/cuadrilla-shale-gas-uk-energy

 ³ www2.epa.gov/region8/pavillion

⁴ Gooddy, Daren; Darling, George. 2005 The potential for methane emissions from groundwaters of the UK. Science of the Total Environment, 339. 117-126. http://www.sciencedirect.com/science/article/pii/S0048969704005467

⁵ earthquakes.bgs.ac.uk/research/events/BlackpoolMay2011.html

⁶ www.cuadrillaresources.com/wp-content/uploads/2012/02/Final_Report_Bowland_Seismicity_02-11-11.pdf

⁷ www.bgs.ac.uk/shalegas/