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Image Credits: Calin Tatu/shutterstock.com
Fracking is a shorthand industry term for the process of hydraulic fracturing (also known as a ‘frac job’ or ‘frac unit’). The process involves breaking down rocks with a fluid, often a water mixture. If the water mixture becomes trapped in a rock, it can become highly pressurized; partly due to the overlying weight of the rock and partly due to the tight space in which it is confined. The fluid becomes so pressurized that it can cause breaks in the rock containing it.
In geological parlance, fractures will propagate throughout the rock once the principle stress exceeds the tensile strength of the rock. Fractures will preferentially form along weaker parts of the rock, such as bedding planes. Hydraulic fracturing is a natural geological process, with some examples being veins and dikes. This natural fracturing is also important in the mining industry, as the hydrothermal fluids that create the fractures can precipitate minerals and metals of commercial significance.
Shale and Fracking
The rock of choice for artificial fracking is often shale. Shale is a fine-grained sedimentary rock, meaning that spaces between the grains are very small. This means that if any gas works its way into these gaps, it is very difficult for it to get out again. As a result, the rock essentially becomes a gas reservoir. Although shale has a low permeability, it often has a high volume of pore space, meaning that a considerable amount of gas can collect over geological timescales.
Unlike other rock types containing gas, which generally release it slowly over time, shale is tightly packed, so the rock needs to be artificially broken to attain the precious natural gas inside.
The Fracking Process
The process of artificial hydraulic fracturing can either exploit existing cracks in the rock or create entirely new ones. The basic process starts with the creation of fractures in the shale using small explosives. Once these fractures are in place, a mixture of water, sand, and chemicals is pumped down into them to widen the gaps.
The water is the main driving force for forming fractures, and up to several million gallons of water may be needed to initiate fracturing. Various chemicals are added to the water to enhance productivity in different ways, for instance reducing friction, controlling pH and protecting equipment. Sand is used as a ‘proppant’, a compression-resistant particle that can hold gaps open.
The gas in the rock can then escape, flow back up the pipe and be processed for commercial use. The process is commonly implemented parallel to the rock beds, but it can also be carried out perpendicularly to the shale layers. The development of horizontal drilling was a significant breakthrough as it meant the ‘pay zone’ could be very large across the rock. 3-D modeling of the shale strata is now used to calculate where the multiple drills need to be placed to fracture the rock.
The well is encased in cement to prevent any contamination of the groundwater with either the natural gas or the chemicals in the solution.
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A schematic diagram showing the basic processes involved with commercial fracking. Image adapted from www.bbc.co.uk/news/uk-14432401
Examples Of Fracking
The first foray into fracking was in 1947 by Floyd Farris and J.B. Clark of Stanlind Oil and Gas Corporation. However, this mining technique particularly took off in the early 1990s, allowing some of the largest natural gas reservoirs to be exploited.
The first place where these new techniques were successfully implemented was the Barnett Shale of Texas. Since then, several huge gas fields have been developed in the USA, including the Fayetteville Shale, Arkansas, the Marcellus Shale of the Appalachians and the Haynesville Shale in Louisiana.
Hydraulic fracking has been present in the UK since the late 1970s, with around 200 onshore gas and oil wells created since the 1980s. Fracking has become increasingly controversial and politicized. In 2011, two minor earthquakes occurred in Lancashire after an exploration using high-volume hydraulic fracking. Despite the Royal Society and Royal Academy of Engineering expressing no concern over the safety risks associated with the fracking process, fracking was stopped in 2011. A number of UK governments are opposing fracking until more research is undertaken to determine its impact on the environment.
Environmental concerns do not stop at the potential damage caused by earth tremors. Fracking also uses a lot of water, which is transported to fracking sites at significant cost. There is also concern over carcinogenic gases being released through drilling, with a risk of contaminated water supplies also triggering controversy.
Summary
Although controversial, fracking is a popular method of extracting gas and oil. Naturally, there are concerns that forcing fractures to occur could have significant and dangerous environmental impacts, with major concern being raised over the increased risk of structural damage through earthquakes and landslides, and increased health risks from carcinogenic substances released through drilling contaminating local water supplies. However, licenses to carry out fracking are being issued to a number of energy companies, claiming that fracking could boost employment rates and investment in their countries.
Sources and Further Reading
This article was updated on the 13th March, 2019.
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