Monday, 12 August 2013
Issue 8: Fracking can and will cause earthquakes
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.