Hydraulic fracturing (fracking) is a technique that has been used for many years in the oil and gas industries for enhancing the productivity of wells.

Diagrammatic representation of hydraulic fracturing (fracking) to produce shale gas (Graphic by ProPublica/Creative Commons)

When applied to shale gas exploitation it involves the forcing of large volumes of fluid (99.5% water and sand, plus a mix of ten or more chemical additives) under pressure into a shale to fracture it to release gas. After fracturing, up to 40-50% of this fluid, and other fluids released from the rocks, returns to the surface (flow back) and is accompanied by large quantities of shale gas. At this pre-production stage gas may be collected or flared off (burnt). After a well has been completed (i.e. made ready for production) leaks may occur from equipment connected at the well head. Some gas may also escape during processing of the gas to make it ‘pipeline ready’. In addition, leaks undoubtedly occur during transport, storage and distribution of the gas.

The possible consequences of fracking are addressed below.

Minor earthquakes

Injecting water deep underground can release natural stresses within the rocks and lead to minor earthquakes. Such tremors have accompanied pumping waste down a borehole (e.g. Rangely, Colorado ca.1970), filling a new dam (e.g. Koyna Dam, India in 1961) as well as hydraulic fracturing. A number of minor earthquakes, the largest of which measured 2.3 on the Richter scale, occurred immediately following fracking in Lancashire in 2011. The largest known quake caused by fracking occurred in the Horn River Basin in Canada in 2011 and had a magnitude of 3.8 (causing about 32 times more shaking than the Lancashire example). A recent review (led by Durham University) of almost 200 earthquakes triggered by fracking concluded ‘Hydraulic fracturing is not an important mechanism for causing felt earthquakes’. Although generally minor and unlikely to pose a significant risk to life or property, such earthquakes can cause considerable local concern and even opposition to shale gas exploration. A slight concern exists if the well casing is damaged by an earthquake causing a leak to occur.

Pollution of freshwater aquifers

Water and chemicals used in fracking, and fluids and gas released from the rock, could potentially enter aquifers that supply drinking water. However, fracking usually takes place at depths greater than 1500m, where the temperature and pressure conditions have been sufficient to generate methane gas, whereas fresh water for drinking is usually found within the top 200m of the saturated rock zone. The considerable difference in depth (over 1000m) would normally be thought sufficient to prevent any mixing, subject only to the integrity of the well casing and its cement. A study in 2012 of the vertical extent of hydraulically induced fractures concluded ‘ ...   the probability of a stimulated …… hydraulic fracture extending vertically [more than] 350 m is 1%.

Nevertheless, it has been claimed that shale gas has leaked into drinking water supplies in the USA. Some experts have doubted that such occurrences could be caused by hydraulic fracturing, not least because the fracturing occurred at a much greater depth than the aquifer and, in one instance, even before the fracturing occurred. However, in at least one case, there appears to be conclusive chemical evidence that hydraulic fracturing was responsible.

The UK’s Energy and Climate Change Committee reported in 2011 that ‘There is no evidence that the hydraulic fracturing process poses any risk to underground water aquifers provided that the well-casing is intact before the process commences. Rather, the risks of water contamination are due to issues of well integrity, and are no different to concerns encountered during the extraction of oil and gas from conventional reservoirs.’

However, the risk of water contamination resulting from loss of well integrity would be reduced if, as was reported in 2012, the UK’s environmental regulator does not permit hydraulic fracturing below freshwater aquifers.

It appears that careful and detailed pre-drilling and post-drilling surveys of water quality and monitoring of drilling operations and installations by the regulatory authorities will be important factors in seeking to prevent pollution of freshwater aquifers.

More information on leaks from boreholes.

Air pollution

This section addresses potential air pollution and its effects on human health. The effects of fugitive (escaping) greenhouse gases on climate change are discussed here.

The first challenge is to know what potentially volatile chemicals are added to the fracking fluid (these make up at most 0.8% of the fluid). Fortunately disclosure of the constituents of fracturing fluid is already mandatory in the UK. The UK government also stated in December 2012 that ‘Licensees will be required to carry out a comprehensive high-level assessment of environmental risks, including risks to human health, and covering the full cycle of the proposed operations, including well abandonment’. Most additives are benign but others, such as benzene or toluene (if used), could cause chronic health problems at certain doses. The escape of vapours, known as volatile organic compounds (VOC’s), derived directly from the shale may represent a health threat.

Little is known about such effects. In the USA studies are afoot to provide a picture of whether air quality around wells has changed, as fracking in the region has intensified, and how, where and when pollutants could be affecting asthma patients or even domestic animals.

Water consumption and waste water treatment

Fracking requires a lot of water (on average about 15 million litres per fracking operation). However, up to 40-50% of this water flows back to the surface after fracking and may be re-used several times  Water treatment is not required if the flow back water is disposed of in a deep well (probably not allowed in Europe). On-site treatment for re-use in fracking removes most suspended solids, acid-producing bacteria and scaling materials like barium, calcium, iron, magnesium and strontium, which are likely to clog the well if returned to the gas reservoir. This treated water, largely free of suspended solids, is then mixed with fresh water and re-used. As everything is done on site, this option has negligible transport costs. The alternative is on- or off-site treatment for discharge as fresh water. The main objective here is to remove total dissolved solids, which can reach extremely high levels of both concentration and variability. This is done on-site by using mobile units or off-site at central treatment plants. Due to the large volume of water involved, transportation for off-site treatment is expensive as well as a matter of concern because of the number of vehicle movements involved.

Noise, traffic, visual impact etc

The UK Onshore Operators Group notes that drilling operations are expected to proceed for 24 hours a day, 7 days a week. This involves strong lighting and noise from electricity generators, heavy machinery and equipment handling in the drilling derrick.

Any drilling site will generate traffic not only from commuting to and from work but also from deliveries of equipment such as drill pipe, casing and consumables. The consumable that will involve the most traffic will be tankers bringing in water for the fracking process (assuming water is not piped in). Given that an average of 15 million litres (15,000 tonnes) of water is required to frack a well and that the maximum allowable tanker size in UK is 44 tonnes (carrying say 40 tonnes of water) this suggests that around 375 tanker trips will be required to service the first fracking operation. Additional water will be required for subsequent fracking operations, although some of the original water may be re-used. Even if water is re-used one or more times, eventually some of it may need to be taken off site for treatment (see previous sub-section) before it can either be dumped at sea or returned to the land surface. This might involve up to another 100 lorry trips. Clearly these movements of very large vehicles will have an impact on the local area and, if the site has poor access, may require road widening and straightening.

The visual impact of the site is unlikely to be significant, provided strong lights can be shielded and the drilling derrick can be largely hidden behind evergreen vegetation.

Well abandonment and afterlife

At the end of its productive life a well is usually ‘abandoned’ by the owner. This may include plugging the well with cement, to block off escaping fluids and gases. A 2012 Royal Society/Royal Academy of Engineering report recommended that ‘An Environmental Risk Assessment (ERA) should be mandatory for all shale gas operations. Risks should be assessed across the entire lifecycle of shale gas extraction, including risks associated with the disposal of wastes and abandonment of wells.’ One of the report’s top recommendations was that ‘Arrangements for monitoring abandoned wells need to be developed. Funding of this monitoring and any remediation work needs further consideration.’ The UK government’s statement of December 2012 said that it would act on these recommendations but, as at November 2013, it is not clear what arrangements will be made for monitoring abandoned wells.

More information on leaks from boreholes.

Last updated 13 Jan 2014

More information about shale gas and fracking