Yes. They can and will, but their size is extremely
unlikely to cause damage or concern.
What are Earthquakes?
Earthquakes happen when the Earth’s crust breaks in
response to a pressure being put on it. I often use the analogy of bending a
pencil. We know that progressive bending of a pencil by using the pressure of
one’s hands will eventually break the pencil, but we don’t know how dramatic it
will be or exactly where the pencil will break, or indeed when. That is the
problem of earthquake prediction.
Brittle and ductile
rocks
The pencil is brittle so its response to pressure is to
break. However, if the pencil were a roll of plasticine, there would be no
breakage; the plasticine would respond to the stress merely by deforming. This is called ductile behaviour.
Some rocks behave like the pencil, and some like the
plasticine.
Granite, sandstone, limestone and many other rocks are
brittle and behave like the pencil. Some rocks like salt and some shales are
ductile, while others have a little brittle behaviour and a little ductile
behaviour. Shales contain a high proportion of clay, which is so ductile it can
be moulded into pots, then becomes extremely brittle when dried and fired in a
kiln. Shales and clays in the crust are always water saturated and more ductile
than brittle.
Shales and shale gas
So shales are generally rather ductile. However, the
shales involved in hydraulic fracturing have to be a little brittle in order
that we can hydraulically fracture them. If they weren’t, the hydraulic
fracturing would not result in useful fractures.
There are many gas shales in the world that would be a
good source of gas, but are too ductile to allow hydraulic fracturing to occur,
and so we cannot produce gas from them. One of the prime purposes of shale as
exploration is to find out if the shales have gas in them. The other is to
ascertain if the shales are brittle enough to be hydraulically fractured. The ones that are, are often beneath thick layers
of more ductile shales often kilometres thick. This is a good thing, because it
means that the fractures induced in the gas shales cannot penetrate through the
overlying strata.
To envisage this, take a Mars bar from the fridge, unwrap
it and turn it upside down. Now treat it like the pencil. It will break, but
look carefully - the chocolate in the bottom layer, the one equivalent to the
shale gas, has fractures in it. It have deformed in a brittle way. However,
these fractures do not penetrate the caramel, which represents the overlying
ductile shale layers. Instead, the caramel bends to accommodate the pressure with
no fractures forming or penetrating it from the brittle chocolate.
This is what happens in a shale gas reservoir - hydraulic
fractures in the target shales do not travel far into the rocks above and below
the target because those rocks are more ductile and it is difficult to generate
fractures in them.
This experiment is particularly good as one can eat the
results.
So hydraulic fracturing breaks the target rocks, but the
fracturing is confined to them by the natural properties of the rocks.
What is the extent of
the fracturing?
Getting a rock to fracture by adding pressurised water is
extremely difficult. Even the biggest hydraulic fracturing rigs cannot provide pressures
greater than about 15,000 psi and that would produce fractures typically about
100 meters long and a few centimetres wide in a
typical gas shale. It would not be technically possible to double this, and it
is doubtful whether it would be economically useful to do so even if it were
possible. Rarely,
induced fractures can be up to 400 m long.
So, we would expect fracturing along a single horizontal
portion of a well to extend between 100 and 400 m meters up, down or less sideways.
The actual direction depends upon the depth and the pressures already in the
ground. Most shale gas deposits in the world, and all in the UK, exist at
depths greater than several thousand metres. Hence fracturing would be unlikely
to reach the surface.
It is possible to say, therefore, that the contamination
of potable water aquifers is not likely by direct contamination from hydraulic
fracturing of shale. (There are much greater dangers from the casing and
surface spills, but these are not the subject of this blog.)
What then about
pre-existing fractures?
The earth tremors from hydraulic fracturing cannot be
felt on the surface by humans or animals.
Why then were there two small
earthquakes (magnitudes of 1.5 and 2.2 with no injuries or damage recorded)
associated with hydraulic fracturing on the Fylde peninsula? In this case, the
hydraulic fracturing triggered pre-existing fractures to slip.
The Earth is full of fractures. Big ones are rare, like
the San Andreas fault in California or the Craven faults in Yorkshire. As the
size (length, width and offset) decrease, they become more common until you get
to a tiny scale.
Not all of these fractures are active, but all, counter-intuitively,
add to the strength of a rock by allowing it to deform slightly in response to
pressure without needing to break. Just imagine how easy it is to fracture a
piece of glass, yet it is almost impossible to create a fracture in beach sand
because the grains can move against each other, yet glass and beach sand are essentially
the same material.
Some fractures in the earth are just at the point of
failing. Let us take the analogy of pulling your finger across a polished
desk-top. Make sure that you apply a moderate downward pressure as you do so.
If you lick your finger first (remembering that you still have the remnants of Mars
bar on them!), your finger will glide smoothly across the table-top. However,
if the finger is dry, then the finger will probably cross the table top in a
series of movements with brief episodes of sticking in between. This is
stick-slip behaviour, and it is exactly what faults do: they move in response
to the applied stress (pulling your finger), but then sometimes become stuck.
It is friction between your finger and the table, but it is the interlocking of
nobbles on the fault surfaces, which is also a type of friction.
Eventually, continued stress in the earth will cause the
fracture to fail, just as continued pulling on your finger causes movement
again. Of course it may be a long time before such a ‘stuck’ fracture moves in
the earth, and the longer we wait, the more energy is built-up across the
fracture, and the bigger the earthquake, when it does move.
The question is can hydraulic fracturing trigger the
earthquake? The answer is Yes, just in the same way that the sudden lubrication
of the dry finger briefly stuck on the table-top would also start it moving
again.
Can hydraulic fracturing
cause pre-existing fractures to fail?
Yes, and it is precisely here that the main danger lies.
For such a thing to occur there would have to be:
·
A
pre-existing fracture,
·
on
the point of failure,
·
intersected
or influenced by a hydraulic fracturing well,
·
which
provided sufficient water pressure to trigger the fracture to fail.
Clearly the first thing to do would be to know where all
the fractures are in the earth and to avoid them. We do not know the
sub-surface well enough to do that. We know many major fractures, but some only
become apparent when a surprise earthquake occurs. The 2010/2011
Christchurch earthquakes (magnitude 7.0 and 7.1) occurred on an unsuspected
fracture.
Neither do we know whether the existing fractures are at
the point of failure. However, in the UK the state of the crust’s stress is
such that large earthquakes are extremely rare (only one over magnitude 6 in the
last 300 years, and that in the middle of the North Sea). The chance of
triggering a significant earthquake is correspondingly minimal.
Micro-earthquakes (magnitude less than 2) do occur during
and after hydraulic fracturing. We can monitor these events; their magnitude
and sub-surface position. The
UK government recommends that a ‘traffic light’ system is put in place
whereby any event of magnitude 0.5 or greater stops all processes, until the
data shows that further exploration is safe and that fracturing is not trending
in a fashion that might delineate a major pre-existing fracture or in the
direction of an existing aquifer.
What sizes of earth
tremors or earthquake are there?
The magnitudes of earthquakes are measured using a number
of different scales, the most common of which is probably the Richter (local)
magnitude ML. The scale is not linear. Each unit increase in the
scale represents ten times the earth movement and almost 32 times the energy
released (it is the energy that does the damage!).
Micro-earthquakes (tremors) associated with hydraulic
fracturing have ML<0.5 and cannot be felt at the surface even if
you are waiting for them, but can be monitored by sophisticated micro-seismic
equipment that can tell their size and position.
In fact micro-earthquakes (tremors) with ML<2
and cannot generally be felt at the surface.
Some people are sensitive enough to feel earthquakes with
2<ML<3 but there is no damage to buildings, and most can feel
earthquakes with 3<ML<4 with building damage rare and movement
of indoor objects as they would if a heavy lorry was passing.
So, tremors associated with hydraulic fracturing cannot
be felt at the surface and have energies about 330,000 times less than the
equivalent of a passing heavy lorry.
Moreover, if the process triggered a typical UK
earthquake by activating a pre-existing stressed fracture, we might expect it
to have a magnitude less than 4 (a
magnitude earthquake of this magnitude happens in the UK about once every 2
years).
Hence, though liky to be minor, these are more of a
danger. However, procedures are already in place to ensure that these should be
minimised.
What size of earth
tremors or earthquakes have been associated with hydraulic fracturing?
Over the last decade of intense shale gas production in
the states there are no documented cases of shale gas operations, whether
exploration or production, causing subsidence or earthquakes large enough to
cause damage at the surface.
However, there have been a few incidents of the
reinjection of fluids for their disposal, and in an irresponsible manner, causing
significant earthquakes. Such reinjection for disposal should never be
carried out in the UK as it is too dangerous. Even if carried out, it would
come under the precautionary guidelines and micro-seismic monitoring
requirements already in place.
Are there any guidelines
for good practice?
On the 30th of July 2013 the UK government
provided a set of hydraulic
fracturing guidelines and the UK Onshore Operators Group has issued Onshore
Shale Gas Well Guidelines that together address and control hydraulic
fracturing in the light of its potential to cause earth tremors. This document
makes it clear that:
·
Seismic
monitoring before, during and after hydraulic fracturing.
·
Immediate
cessation of activities should an event greater than ML=0.5 occur.
·
Scientific
review into the geophysical causes for the event and judgement whether further
hydraulic fracturing would be safe.
So, earth tremors are inevitable with hydraulic
fracturing, not just because of the process
itself, but the possibility that hydraulic fracturing may trigger pre-existing
stressed fractures. The size and likelihood for these to cause any danger to
humans or the built and natural environment is so extremely small as to be
truly negligible. Existing guidelines are specific and very conservative, and
if followed would provide a complete protection from any tremors capable of
causing disruption.
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