A matter of Frack

So, who’s here for a dry and technical account of industrial mining techniques? Please try to contain your excitement. If you’re good, I’ll even include some diagrams.

Hydraulic fracturing is an old technique. As I expect is the case with many of you, it came to my attention during around 2010 with media coverage of oil and gas prospectors Caudrilla drilling exploratory well in Lancashire (UK). They intended to apply recently developed techniques to access hydrocarbons which were previously economically unviable. The results of these explorations, the public reaction and how this shaped the UK’s policy on fracking, will be examined more thoroughly in a future post. For now, please don your PPE and be sure to stay with the group.

This is what a frack looks like. Borrowed from here

Hydraulic fracturing is the use of pressurised water (or other fluids) to increase the porosity of rock. Water is injected down a borehole (the hole from the surface, also called the well) and the pressure is increased until it overcomes the structural integrity of the rock. This is manifested in the formation of various small cracks, radiating from the borehole. The purpose of this is to increase the exposed surface area of rock, and to give the gas (or oil, or water, although we’re primarily concerned with gas in this series) more gaps to migrate into the borehole for collection. Suspended in the fluid are tiny “proppants” such as sand particles, which prop open the cracks once the pressure is released, otherwise the ambient pressure would cause them to close again. The advantage of this is that very “tight” hydrocarbon reservoir rocks, which would otherwise be reluctant to give up their charge, can be stimulated into producing more of the good stuff. Conventional hydrocarbon sources are generally porous rocks, those with plenty of escape channels for hydrocarbons to migrate through. It’s only with recent advances that it’s become economically viable to extract hydrocarbons from the less forthcoming rock types such as shale, which is generally very finely grained and tightly packed.

Fundamentally, these are the important ingredients for hydraulic fracturing, but as with virtually every other industrial technique, some of the world’s finest minds have endeavoured to optimise the heck out of it.

To this end, a variety of chemicals are added to the fluid, to modify its properties and increase its efficacy. These include gelling and foaming agents, which help to keep the proppants in suspension, as well as various classes of chemicals to, for example, prevent deposition of dissolved compounds. While it may be slightly terrifying to think of an intangible concoction of chemicals being injected into the ground in vast quantities, it’s worth considering that only around 0.05% of the injected fluid is made up of these chemicals. However 0.05% of between 7600 and 18,899m3 of fluid is still pretty vast. The important questions are whether they can end up in the surrounding environment or the water supply, and what sort of harm they could do if they did. Furthermore, it’s worth considering whether any of the hydrocarbons which are released from the fractured rock are likely to end up where they shouldn’t.

As I previously insinuated, fracking is not a new phenomenon. The technique was developed in the 1940s and has been used extensively, in order to stimulate low permeability rocks into giving up their precious hydrocarbons, or to simply increase the yield of the not-so-low permeability rocks of more conventional sources. The recent explosion of fracking has resulted from techniques which allow for previously nonviable sources to be tapped more effectively (referred to as “unconventional” hydrocarbon sources).

Horizontal drilling has been used to increase the area of contact between mining wells and the hydrocarbon bearing rock. To understand why, we need to talk about rocks. Hold onto your hats, people.

Horizontal layers of rock. Picture of the Lias
Shale Group, taken by me near Lyme Regis in 2014

One of the fundamental principles of geology is superposition; when rock forms from compaction (a process called diagenesis), it tends to be oriented the same way as the ground (that is to say, relatively flat for the most part). When more rock forms, it tends to form in the same orientation, superimposed on top of that previous rock. This results in the “layer cake” pattern that you might see in cliff faces, where many layers have been serially compacted into layers of rock. These layers are referred to as beds, and lots of geologically similar beds are referred to as a formation. The upshot of this is that hydrocarbon bearing rocks tend to form vast subterranean sheets, horizontally oriented. Consequently, a horizontal mining well can follow the sheet and maximise the amount of contact with the hydrocarbon bearing rock.

So, let’s recap:

• Hydraulic fracturing is the use of a pressurised liquid to force open cracks in a non porous rock.
• Proppants are little “support strut” particles which are forced into the cracks, to hold them open once the pressure is released.
• Chemical additives are used to change the properties of the water, making the process more effective.
• Unconventional hydrocarbon sources are increasingly getting the fracking treatment, where previously it would not have been economically viable.
• Horizontal drilling allows for maximum exchange between the formation containing the hydrocarbon and the mining well.
• I stole that “layer cake” comparison from the Hitchhiker's Guide to the Galaxy.

I don’t suggest you go out and attempt your own frack based upon the information contained in this post, as it’s a very brief overview of the aspects concerned, but it should serve as a starting point for our more thorough investigation of some of the specific issues surrounding the process, and the reasons for social and political reactions to this expansion of fracking in recent years.

Next time:

• Where does all that water come from?
• Where does all that water go?
• Will it kill my cat/make me sterile?
• Who do I blame if my taps catch fire?