What is shale gas?
Shale rocks are generally dense and
black-coloured, formed from mud deposited at the bottom of past oceans,
now solidified into rock. This mud is rich in un-decayed organic matter -
that's what gives shales their black colour. When heated, the organic
matter is transformed into oil and gas.
Once it has
formed, some of this oil and gas is able to move out of the shale
layers, rising through overlying strata, where it may become trapped in
sandstone or limestone layers. This oil and gas is what we consider to
be 'conventional' reservoirs, where we have usually looked for oil.
it has always been known that much of the oil and gas formed during
burial remains behind, trapped in the shale layers. Compared to
sandstones and limestones shales tend to have lower porosity, making it
harder for the oil and gas to move about through the shale. This means
that it is harder to extract gas from shale than from conventional
reservoirs where the fluids can flow more freely.
how is shale gas extracted?
Firstly, a well is sunk, which
travels vertically through the overlying strata. When it reaches the
shale layer, it turns sideways, drilling horizontally. Modern wells are
capable of drilling over 10km horizontally: such wells are called
'extended reach laterals'. This part is absolutely no different to
Once the horizontal well is
drilled, it must be hydraulically stimulated, or as it has become known,
"fracked". The horizontal well is "fracked" in portions, stage by stage
every few hundred meters or so, meaning that a 2km lateral well might
need 10 to 20 stages. For each stage, the targetted section of the well
is sealed off, and water is pumped down at high pressure. This water
will contain about 1% chemicals that make the stimulation more
efficient: surfactants (basically like detergent) that make the water
more 'slippery', so less pressure is needed to pump it; and
viscosifiers, that help the water carry proppant (more on that in a
The pressure of the water is
sufficient to open up pre-existing fractures in the rock, and to create
new ones. These fractures are important, because they provide a pathway
for the gas to flow out of the shale rock and into the well. Above I
pointed out that it is difficult for oil and gas to flow through shale
rocks. It is the fractures that allow them to flow into the well.
Towards the end of the stage, proppant is pumped with the water.
Proppant is usually just sand and gravel, although ceramic beads can
also be used. The proppant is pushed into the fractures that have been
created, literally 'propping' them open, ensuring that the gas can
continue to flow.
Each stage takes a few hours of
pumping, so to "frack" all the stages of a lateral might take a week.
Once that is completed, the well is ready for production. A small
'Christmas Tree' valve is placed on the top of the well, and the gas may
continue to flow for years without any further intervention.
suddenly a lot of media fuss about fracking. It must be a new
Wells were first hydraulically stimulated
in the 1940s. It has been a standard tool in a driller's toolbox for a
long time. Horizontal wells are in fact far newer, really only becoming
common in the 1990s. Some would argue that the current developments have
more to do with horizontal drilling than they do hydraulic stimulation.
However, even with these developments, many geologists felt that it
would never be possible to extract gas from shales at economic rates.
Mitchell, a Texan, persevered through the 1990s, improving the
technique, to show that it was in fact possible, and the shale
revolution was born. Since then, a drilling boom exploded as American
companies realised they didn't have to look abroad, or in the deep and
treacherous waters of the Gulf of Mexico, because huge volumes of oil
and gas were to be found under Texas, Colorado, Pennsylvania and now 30
other US states.
So hydraulic stimulation, or
"fracking", is a very well established technique. Horizontal drilling,
just as essential to shale development, but much less talked about, is
newer, developing in the 1990s.
However, to extract gas
from shales requires more fractures than in a conventional reservoir,
as they are initially less permeable. Therefore, the volumes of water
being injected for shale are typically larger than that used previously
in conventional reservoirs. So while current developments are not using
new technology, it does represent a scaling up of that technology.
heard about scary-sounding chemicals contaminating water supplies. Is
Opponents of fracking sometimes talk about the 500
toxic chemicals needed to "frack". I don't think any stimulation needs
500 different chemicals: that's probably the total number used in the
history of the technique, not the number used for a single stimulation.
two main chemicals used are surfactants (found in most
soaps/detergents) and viscosifiers (typically guar gum, used in many
food products). While not hideously toxic, I wouldn't want to drink
water contaminated with surfactants and the like (who'd want to drink
the sink-water after they've done the washing up). In addition, the
water injected during "fracking" comes into contact with the deep shale
rocks. These sometimes contain heavy metals and salts, which may also
enter into the "fracking" fluid.
The most obvious way
for these chemicals to enter groundwater is if they are spilled on the
surface. There is an example from Louisiana where 17 cattle died after
undiluted KCl was allowed to spill off of a drill pad. Many of the
reported issues in the US are due to the ponds used to hold the waste
pools lined with plastic. The lining of these ponds has been known
to tear, or if it rains heavily they can overflow. Ponds like this are
not allowed in the UK for exactly these reasons. In the UK waste fluids
must be stored in double-lined steel tanks: this is a major difference
between drilling in the UK and the USA.
of fluids on the surface should be standard practice for all oil and
gas operations - you can read here all the precautions Cuadrilla have
taken at Balcombe, layer upon layer of protection to ensure that no
substance on the drill site is allowed to leak.
secondary concern, of course, is what happens when the fluid is injected
into the ground. "Fracking" takes place well below the water table,
typically at depths of 2 - 4km (most potable ground water is found
within a few hundred meters of the surface). So there is a lot of solid
rock between where the fluid is injected and any potable water.
the injected fluids to contaminate groundwater, two things are
required: (1) a path (such as faults and fractures) along which these
fluids could migrate, and (2) a force to push these fluids along this
If there were an easy path allowing fluids to
move upwards, then the oil or gas, more buoyant and more mobile than
water-based "frack" fluids, would have already travelled through these
paths during the 200 million years of geologic history for which the gas
has been trapped. Therefore, the fact that the gas is still trapped
there to start with tells us that such paths are unlikely.
for a driving force to push the fluids along such a pathway, should it
exist? The water-based "frack" fluids, with their various additives, are
of a similar density, or perhaps even more dense, than the brines that
fill non-gas-bearing rocks at these depths, and of course much denser
than oil and gas. Therefore, they will tend to sink downwards, rather
than rise upwards. There is no driving force to push the "frack" fluids
back towards the surface.
So we have no pathway, and no
driving mechanism, to cause injected "frack" fluids to rise upwards
towards potable groundwater sources. This was the conclusion of a recent
study into the possibility of hydraulic communication between shale
layers are depth and shallow groundwater bodies, finding it "physically
That's all fine in theory, what about
the evidence? Well, a number of studies have been done on water quality
in shale areas. Only one, a recent paper by an Arlington group
has suggested any kind of link between drilling and contamination by
"chemicals". They found a correlation between arsenic and selenium
levels and proximity to natural gas wells. They are uncertain in their
conclusions, however, as there are a number of possible causes for their
observations. Moreover, they do not link their observations with any
communication of "fracking" fluids from depth - if drilling is to blame,
they believe is likely to be due to vibrations from drilling operations
that agitate old, rusty water wells. This agitation of old rusty metal
is the most likely source of the observed metals.
majority of water quality studies have found no evidence of
fracking-related chemicals in groundwater: some (one from the
Duke team, one by Molofsky
et al) found no evidence of any effect whatsoever on water quality.
from the Duke team found evidence for methane contamination (see
below), but no evidence for "fracking" fluids.
also seen videos of a tap catching on fire? Is methane leakage a
Perhaps the most dramatic footage in the famous
Gaslands film is the scene where the farmer can set his tap water on
fire. This is caused by methane gas contaminating groundwater. Methane
itself is not toxic in any way, although if it builds up in large enough
quantities it presents a fire hazard.
There are two
potential ways that methane trapped at depth in shale rock could get
into shallow groundwater - through a pathway in the rock, or through a
gas well. As above, if there were an existing pathway through the rock
to the surface, then the gas would have already taken that route during
the millions of years that it was otherwise trapped. Unlike "fracking"
fluids, methane is buoyant and mobile. In drilling a well, a new
potential pathway is created for methane to get to the surface.
wells are drilled, they are lined with several concentric layers of
steel, called the casing. When all is as it should be, the gas flows up
the middle of the well, inside the casing, to the surface. The casing is
fixed into place with a layer of cement that fills the small gap
between the casing and the rock.
If there are gaps or
cracks in this cement then methane can move up through this gap
(sometimes called the annulus) towards the surface. Of course, the
crack/gap has to run all the way from the shale layer to the surface.
Well bore integrity has long been known as a potential problem for all
oil and gas wells, for both conventional and shale reservoirs.
have been definite examples where poor casing/cement has lead to
methane migration into shallow groundwater. Dimock, Pennsylvania, is
probably the best-known. The company involved was cited for a number of
violations of drilling regulations, and fined heavily. The wells have
since been repaired, and methane levels have fallen back to below safe
minima. Casing and cement is something the industry has been working
with for a long time - there are monitoring tools that can be used to
check that there are no gaps or cracks in the cement, and, as at Dimock,
it is possible to repair problematic casing.
majority of methane-in-groundwater complaints come from Pennsylvania.
This perhaps inevitable, because methane occurs very commonly in
groundwater in PA. There are a number of natural ways that can lead to
methane in groundwater. Of course, that means that determining when gas
drilling is to blame, and when it is natural, can be problematic.
know at Dimock the gas was drilling-related. But how common is this
problem. There have been three main studies here, two by the Duke team,
one by Molofsky et al.. The Duke team
studied the Fayetteville shale, Arkansas, and did not find any
evidence for drilling-induced methane contamination.
they examined the Marcellus (Pennsylvania), they found evidence for
drilling-induced methane contamination. Yet this paper has come in for
substantial criticism for two main reasons: the number of samples
analysed (only 160ish), and the apparent non-randomness of where the
samples were taken from.
To address this, Molofsky et
al conducted a much wider sampling regime (over 2,000 samples). They
found that if you lived near a gas well, there was a 3 - 4% chance of
finding methane in your groundwater. However, if you lived in an area
with no drilling, there was also 3-4% chance of finding methane in your
groundwater. When you look regionally, whether or not you are in a
drilling area doesn't appear to affect the probability of groundwater
What can we conclude from this?
Well integrity and methane leakage is an important issue for the
industry, one that it needs to keep on top of. We've seen at Dimock that
if a company takes shortcuts, and violates regulations, this can be an
issue. Importantly, there are ways to check cement integrity once a well
has been drilled, and ways to repair problems.
question is, how widespread is this issue? The data from Molofsky et al
appear to show that it is likely to be a few isolated incidents, rather
than a widespread problem. Reviews
by the US Groundwater Protection Council have come to a similar
I've heard that 5% of wells fail
immediately, and that 50% fail eventually?
This is a statistic
often cited by opponents of drilling looking to highlight the
methane leakage issue discussed above. The statistic comes from a paper
by Schlumberger examining wells in the deep Gulf of Mexico, and
particularly the chart on page 2. Firstly, it's worth noting that this
'paper' is basically an advert by a company selling well repair
solutions, so it's in their interest to 'big-up' the stats as much as
More importantly, what do we mean by 'well
failure'? In the context of shale gas extraction, we surely mean that a
well that is allowing methane to leak into shallow groundwater. This is
where the use of the above statistic is somewhat disingenuous. The
statistics in the paper are for sustained casing pressure, or SCP. This
is where a portion of the annulus remains pressurised when it shouldn't
be. This is absolutely not the same as a well leaking - leaking well
will probably experience SCP, but that doesn't mean that SCP indicates a
Equally disingenuous is the fact that
these stats come from deepwater Gulf of Mexico wells. Drilling is a lot
more challenging when there is a couple of kilometers of water between
your rig and the ground (as the Deepwater Horizon accident showed). It's
only in recent years that drilling technology has advanced to enable us
to drill there at all.
It's not a fair comparison to
link wells drilled in the GoM with onshore shale wells, the drilling and
casing of which is no different to the thousands of conventional
onshore wells we've been drilling for almost a century. It'd be like
using the number of crashes in an F1 race to predict how many accidents
there'll be on the M25. Deepwater GoM wells are at the limits of our
technology. Shale gas wells are far more mundane.
really want to understand how common well casing issues might be in the
UK, surely the best place to look is at our many current onshore wells.
There have been over 2,000 wells drilled onshore UK. Whether on not
they are "fracked" or not has little bearing on casing integrity. These
2,000 existing onshore wells will be no different to shale gas wells.
I'm not aware of any complaints of casing integrity issues or methane
contamination from any of these existing sites.
fracking also trigger an earthquake in Lancashire. Is that
It did. In 2011, when Cuadrilla "fracked" a well near
Blackpool, two small earthquakes were triggered. Both were very small,
at the limits of what humans can feel, and would have caused a similar
amount of shaking to an HGV driving past your house.
a fact not often appreciated that everything we do in the subsurface
carries a small risk of triggering an earthquake, whether it be coal
mining, conventional oil/gas, geothermal, hydroelectric. Even quarry
blasts are basically man-made earthquakes. Shale gas is no different,
there will always be a small risk of triggering small earthquakes.
However, this risk is small: only one of the hundreds of thousands of
"frack" stages in the US has triggered an earthquake. Any quakes
produced will be too small to cause actual damage. DECC have said that
every future "fracking" site will require seismic monitoring, and Bristol
University have currently deployed seismometers at the Balcombe site
(even though they're not planning to "frack" at this stage).
know that some forms of hydrocarbon extraction lead to subsidence
issues. Could that be a problem for shale gas?
When you remove
material from the ground, a space is created that is sometimes filled
by the overlying material subsiding into the gap. This is particularly
true for coal mining, which can cause severe subsidence.
shale rocks are dense, with low porosity. That is why they need to be
"fracked" to get the gas out. Because of this, they are usually
mechanically strong enough to support themselves once the gas is
removed. As such, subsidence is not expected to be an issue during shale
gas extraction. The Barnett shale in Texas is the oldest shale field:
production started over 10 years ago. There has been no measurable
subsidence during this time.
How much water will be
Several million gallons of water, or 2 - 5,000 cubic
meters of water are needed for each well. That's a couple of
olympic-sized swimming pools. That sounds like a lot of water. However,
it's important to keep that number in context. The average golf course
can easily use this much water a week in summer months. Similarly (and a
lot more shockingly), UK water systems leak over 3 billion liters
(3 million cubic meters) per day. So if we our utilities were to
improve leakage by 1%, we'd have enough water to "frack" 30 stages every
In all but the driest places, shale gas
development doesn't pose a strain on water resources. Moreover, water
abstraction is regulated in the UK. If demands on water resources are
too great, the Environment Agency will not provide a license to abstract
water, and utilities will not provide it.
about air pollution, is that a potential issue?
water contamination, this is one of the hot issues for shale extraction,
because it involves people's health. There have been a number of
regional-scale air quality surveys that do not find any evidence for
drilling-related air quality issues,
including in the Barnett shale and in
Pennsylvania. In fact, the Pennsylvania report shows significant
improvements in air quality, mainly because coal-fired power stations
are being replace by gas power stations.
localised studies have found the occasional issue, mainly
it seems with compressor stations rather than drilling sites.
However, even these studies have concluded that "the screening results
do not indicate a potential
for major air-related health issues associated with the Marcellus Shale
Fracking might be ok in the wide-open spaces of the
US, but surely there's no space for it in the UK?
that shale gas extraction is easier in unpopulated areas. Opponents of
shale gas often show images of the Jonah gas field,
where the land is covered with wells. This is actually a conventional
gas field, drilled in the 1990s, before horizontal drilling had taken
off. The benefit of horizontal drilling is that you don't need nearly as
many well pads.
The truth is that the drilling
industry is very adaptable. Sure, if you give them a big empty space and
tell them you can drill all over it, then they probably will. However,
they can cope in far more constrained conditions when it is necessary.
Perhaps the best example of this is Dallas-Forth
Worth. This is the 9th most populated city in America, with a
population of over 6 million. Yet the Barnett shale runs right
underneath, and it's being drilled. By using long lateral wells,
drilling sites can be squeezed into urban and suburban areas without
taking up much space at all.
Does the UK have much
of a record for onshore drilling?
The UK onshore industry does
not have a very visible profile. However, it is there: we produce 100
million cubic meters of gas per year. Over 2,000 onshore wells have been
drilled in the UK. Of these, about 200 have been "fracked". I've
plotted a map of all the wells here.
In some places, such as Beckingham
Marshes, they manage to squeeze a
lot of wells into not-very-much space, without upsetting local
The UK onshore industry has been very good at
staying out of sight, and very good at making sure they do not pollute.
If we want to predict what shale gas development in the UK will be like,
the first place to look should be the current onshore industry, which
does a very effective job working with local communities.
heard that the methane leaks mean that shale gas is actually worse for
climate change than coal. Is that true?
Burning coal for
electricity produces approximately 3 times as much CO2 as natural gas
does. Therefore, switching from coal fired to natural gas fired
electricity should represent a significant benefit. CO2 emissions in the
US have dropped significantly as gas has replaced coal as a source of
However, natural gas production has the
potential to release methane to the atmosphere. Methane is also a potent
greenhouse gas, so if shale gas extraction emits a lot of methane, it
could counteract the benefits of reduced CO2. This was the premise of a
paper by Howarth et al., which has garnered a lot of publicity.
However, this paper has come in for a raft of criticism, as most other
studies on the subject have indicated very clear benefits from switching
from coal to gas. No
Hot Air lists some of the scathing comments about the Howarth
study funded by the EU Commission concluded that, with respect to
greenhouse gas emission, domestic shale gas production has a similar
footprint to imported gas (that has to be compressed and shipped to
Europe from the Middle East), and a significantly better footprint than
Even if it's better than coal, gas is still a fossil fuel.
Surely we should be focusing all our efforts on renewable electricity?
we're on to a more serious criticism of shale gas development. We know
we already have more fossil fuel reserves than we can safely burn
without causing catastrophic climate change, so why are we bothering to
look for more?
My answer to this comes in two parts:
firstly is to point out, as above, that for a unit of energy produced,
shale gas emits less than half as much CO2 as coal. If we are to avoid
climate change, there must presumably be an upper limit to the rate that
we can emit CO2. If the treat of climate change requires us to ration
our CO2 emissions, it seems obvious that we should choose the fuel
source that gives us the most energy for that ration of CO2 emission.
That fuel source, by a significant margin, is gas.
correct that we should leave a large portion of our current fossil fuel
reserves in the ground. That portion, however, should be coal.
Moreover, renewable energy currently requires flexible backup sources -
an abundance of gas provides this.
My second argument
relates to how we get to a point where our CO2 emissions are reduced. A
common call is that 'we should be investing in renewable energy
sources'. This is absolutely true. However, in order to invest, you need
to have money to invest. Given our current economic struggles, it
becomes harder and harder to politically justify renewable energy
receiving public money, either as a supplement on bills or as a direct
subsidy from the treasury.
If the economy improves,
there will be more money available to invest. In my opinion, we should
be ring-fencing a proportion of the taxes made on shale gas development,
in order that they be re-invested into renewables and/or next-gen
nuclear. This would ensure that in the short term we reduce our CO2
emissions by replacing coal fired power, but that in the long term
investment continues in alternative energy sources, such that they will
be ready as soon as possible.
That's the theory, how
does this bear up to reality? Well, Texas is the undisputed home of
shale gas, with the Barnett, Eagleford and Haynesville shale plays. Yet,
perhaps surprisingly, Texas
is also one of the leading states in terms of renewable energy, and
the renewables boom has occurred at pretty-much the same time as the
shale boom. In Texas, at least, a booming shale gas industry has gone
hand-in-hand with booming renewables, rather than competing with each
Will shale gas have an impact on my gas
This is somewhat uncertain, and as a
geoscientist I'm probably straying outside of my main area of expertise.
recent report commissioned by the government suggests prices could
fall by 25%. However, other reports have suggested it would have less of
However, it's important to look at the
economic impacts beyond consumer gas prices. At present, we expect to be
buying more and more gas from places like Norway and Qatar. That is
money that leaves the UK economy for good, never to be seen again. It
creates no jobs, and pays no taxes.
In contrast, a UK
shale gas industry would provide jobs for UK workers. It's true that
some of those jobs would be specialists, attracting high-payed
international workers. That is still beneficial to the UK economy,
because those people will live in the UK, and spend their money here.
But there are many lower-skilled jobs involved as well.
remember the manufacturing chain. For example, well pads need cement.
The casing is high quality steel, and each well needs several kilometers
of it. That means work for people who make cement and steel. Add in the
mulitplier effect, as more employment means more people buying things
in shops, eating in restaurants, staying in hotels, and it's clear that,
whatever the effect on the gas price, shale development will have a
significant impact on the economy.
Equally, any gas
produced will be taxed. That's money going into the public purse, to be
spent on schools, hospitals, or even wind farms. Public finances appear
to be somewhat short of cash at the moment. Given our current situation,
I don't think we can afford to be handing billions of pounds a year
over to Qatar to host an air-conditioned World Cup when we could be
reaping the economic benefits of shale gas development at home.