What the frack? – What is Fracking.

The video below and a podcast from Science Vs. has inspired me to write this blog about fracking. As a geoscientist I will try and explain what fracking is and point out some pros and cons.

Fracking is a term that pertains to the hydraulic fracturing of rock to free up hydrocarbons. Hydraulic fracturing is basically a way to induce cracks at great depths within a rock under the immense pressure of a fluid usually a water chemical mixture with sand and clays. Once the cracks are created the grains will keep the cracks from sealing shut.

The diagram (made by me) on the left shows a well being drilled into a layer of rock which holds the hydrocarbons of interest. This rock layer is known as the host rock. In the case for fracking the host rock tends to be in rocks that formed at the bottom of an ocean many million years ago. Since hydrocarbons are less dense than water and the surrounding rock, without a rock that acts as a seal the oil or gas would just seep towards the surface. The rock that acts as a seal is known as the cap rock. The other rocks are not important in this schematic. Once the host rocks characteristics like width, length and thickness have been established you can drill a vertical hole down to the level which you want to kick it off to create a horizontal well. The reason for drilling a horizontal well is so that you can have access to much more of the rock.

Once the the horizontal wells are drilled (usually more than one is drilled), we frack to increase porosity and the permeability of the host rock to allow the hydrocarbons to flow more easily. The diagrams below explains what goes on when you frack a rock and what permeability and porosity actually mean in the context of fracking.

Fracking in the pore spaces. Brown fluid is the hydrocarbon of interest. a) shows the fluid being injected into the horizontal well. The pore space is occupied by the the hydrocarbon fluid of interest. The amount of pore space is the porosity. Higher porosity means more volume to hold potential fluids. The lack of greater gaps between the grey grains prevents the hydrocarbons to mobilize. b) shows the fractures which not only happen on a macro scale but also on a macro scale, the spaces between the pores mentioned in a) are now better connected and form fluid pathways. The degree of interconnectivity between the pore spaces is the permeability. High permeability means fluids can more easily move through the rock. You can see that the light grey fluid flushes out the little droplets of brown hydrocarbon. (image made by me)

Why are we only doing this now?

We have known about hydraulic fracturing since the 1940 and it is also heavily been used in the geothermal energy sector to increase the surface area of hot rock bodies underground. Most of the early fracking was focused in North America, ~87% of all fracking was carried out there in 2011.

We’ve exhausted most of the easily extractable hydrocarbon reservoirs. Fracking has enabled us to further extend the lifetimes of existing wells and gain access to previously un-exploitable reservoirs. The original pressure can only produce a certain percentage of hydrocarbons sometimes only yielding 30% of the total resource using conventional extraction methods.

The geology in which fracking occurs is in tight gas shales like the Bakken formation in the US and Canada. The porosity and permeability is so low that 30 years ago you would have been ridiculed for thinking it was ever possible to extract anything out of these formations.

The chemicals used in the operations are a trade secret and there are more than 700 solutions in use in the industry. The solutions are likely to contain solvents and thinners to get the hydrocarbons out of the pore spaces and into the well. Acids may also be introduced into the solutions to target certain minerals like calcite to further increase the permeability.

Many people associate recent human induced seismic activity with fracking. However, the actual fracturing process is not responsible for this. The likely contender is the injection of waste water as this occurs over a longer period of time than the fracking process and more fluid is injected into the ground than with fracking. Waste water injection is common in all forms of oil extraction, even conventional extraction. For more details on this refer to the USGS website on this topic.

So should we worry about re-injecting all these chemicals back into the ground?

In short, no. Oil and gas companies have been doing this for decades and are held to very high standards by the government. Geologists and hydrogeologists are employed and understand the ground in far more detail and know where the risks are. No project can come into fruition if the safety of a water source is ever in jeopardy. However there are some rare cases where negligence has lead to contamination of ground water (refer to the podcast here), but with anything in the world negligence in any industry can lead to problems for society.

Fracking gets at a resource that would otherwise be left in the ground and with us having to continue bridging the gap between fossil fuels and renewable energy we need a cheap source of energy. Much of what is extracted is in the form of natural gas which burns much cleaner than other fossil fuels like coal or oil (natural gas is used for heating houses and in the kitchen for cooking). It still produces CO2 as a result of burning. The extraction process also leads to liberation of methane. On average 3% of natural gas that is extracted escapes in the form of methane into the atmosphere. Read my other blog for more on CO2 and methane.

Done correctly fracking is short term solution to reduce pollution and bridge the gap between fossil fuels and renewable energy.