Shale gas

Shale gas is mostly composed of methane. Methane is ‘natural gas’ and is the gas used to generate electricity and for domestic heating and cooking. Shale gas is produced using technologies developed since the 1980s that enable gas to be recovered from rocks (mostly shale) which were previously considered to be unsuitable for extracting gas.

Conventional gas comes from a ‘source’ rock that was buried and heated at considerable depth (up to thousands of metres below the surface). Temperature increases with depth, and hydrocarbons, such as oil and gas, are released from the source rocks at different rates depending on how fast the rocks are heated. Due to the pressure underground, these hydrocarbons migrate upwards and may find their way into a porous ‘reservoir’ rock. If this is overlain by an impermeable ‘cap’ (or ‘seal’) rock the hydrocarbons become trapped. The hydrocarbons are extracted by drilling through the cap rock into the reservoir. These hydrocarbons, which can be relatively easily recovered, are known as conventional hydrocarbons and have been exploited for more than 100 years. North Sea gas is a conventional hydrocarbon. With ‘unconventional’ hydrocarbons, and shale gas in particular, the rock which contains the hydrocarbons is virtually impermeable and so acts as hydrocarbon source rock, reservoir and seal. The gas is both produced and trapped within the shale. It is only when the shale is drilled and artificially fractured that the gas is released from the rock and can be extracted. The process of artificially fracturing the rock is called ‘fracking’.

There are several different rock units within the geological sequence of the UK that have the potential to produce shale gas. The main interest is in the Carboniferous units, particularly the Bowland Shale Formation which occurs throughout parts of north-west and central and eastern England. Other units of interest occur in the Jurassic rocks of the Weald Basin, parts of southern and north-eastern England, and County Fermanagh Northern Ireland. There is a possibility that much older Lower Palaeozoic rocks in Wales and central England may be prospective in shale gas (although this remains to be confirmed as it has not been fully evaluated by exploration companies or the BGS). Exploration for shale gas in the UK (as with Europe in general) is in the very early stages and the actual prospectivity is not known.

Advances in technology, developed principally in the USA, have meant that gas can now be extracted from shales and other low permeability rocks. Transferring these technologies from the USA to the UK could give rise to shale gas production if gas occurs in the rocks in commercial quantities. After initial exploration of the shale deposits, a borehole is drilled into the shale horizon at a carefully selected site. It may be drilled horizontally to increase the volume of rock that can be accessed by the borehole. A process called hydraulic fracturing ('fracking') is undertaken. This involves pumping water into isolated sections of the borehole at pressures high enough to fracture the surrounding rock. Sand entrained in the water helps to 'prop' open the fractures, create permeability in the rock and allow the gas to flow into the borehole. Chemicals are also added to improve the efficiency of the fracking operation.

Shale gas extraction and fracking has received a huge amount of media interest. Just like any other industrial process there will be associated risks. Some of those relevant to shale gas include ‘induced seismicity’, such as the low magnitude earthquakes experienced in Lancashire in 2011. There is also the potential for groundwater and surface water contamination (see below for more infomation on potential impacts to groundwater). This may arise from surface activities that may lead to spills associated with the storage and mixing chemicals at the drill/fracking site or the storage/management of fluids that return to the surface from the borehole, the so-called ‘flowback and produced waters’. Other potential pathways for contamination of groundwater include poor well-design and well construction, and the migration of contaminants along natural pathways into overlying aquifers. Understanding the risks is a very important step in the design and approval process and very strict controls and regulations are in place to reduce the risks to an acceptable level.

The two main induced earthquakes in Lancashire in 2011 were very small. To put them into context, they were less powerful than some of the earth tremors that have been associated with coal mining in the 1950s and 60s and that occur today.

There is a broad regulatory framework that already exists. The use of fracking, shale gas technologies and associated activities is covered within the existing regulatory frameworks.

There are two potential impacts on groundwater. The first is associated with the supply and consumption of water for fracking as groundwater may be considered as a source of this water. The second is contamination of groundwater. In both cases the regulations that apply to shale gas extraction will require a detailed risk assessment before any authorisation or permit is granted. Before granting a permit the relevant regulatory authority (e.g. the Environment Agency in England) will need to be satisfied that the activity will not cause pollution of groundwater or lead to unsustainable abstraction. Once approved, monitoring of the environment will be required as part of permit conditions to demonstrate that no impact is occurring. To provide an independent environmental baseline against which this compliance monitoring can be compared, the BGS is undertaking a baseline survey of methane concentrations in groundwater ahead of any shale gas development as there is currently no UK baseline. The baseline study is not only restricted to methane. A wider range of chemical indicator parameters are also being measured and the results will supplement the data already published by the BGS and used to set groundwater threshold values (standards) for the EU Water Framework Directive.

Estimates of the amount of recoverable gas and the gas resources are variable. It possible that the shale gas resources in UK are very large. However, despite the size of the resource, the proportion that can be recovered is the critical factor. A better understanding of the shale gas resource, and the amount of gas that is potentially recoverable, will come from further geological research, such as that carried out by the BGS. If the amount of recoverable shale gas does prove to be large this will be a significant indigenous source of gas for the UK and may reduce our reliance on imported gas.