Shale gas is a form of energy that is arousing increasing interest, mainly internationally, but also to some extent in Sweden. What is shale gas, where does it occur and how it is formed in nature? What is fracking and what are its greatest risks?
Shale gas is natural gas that is formed and trapped in bedrock consisting of shale. It is mainly dark shales, rich in organics, which have the potential to contain shale gas.
The gas mostly consists of pure methane gas. A distinction is made between shale gas formed through thermal degradation and gas formed through biological degradation of the shale's organic materials.
Thermally formed shale gas occurs at greater bedrock depths (> approximately 1 km), while biological degradation and gas formation occurs in more superficial shale formations, such as alum shale in Östergötland and on Öland.
Most of the shale gas extracted today, above all in the United States, is thermally formed in deep-lying dark shales.
Thermally formed shale gas
In order for a shale to contain thermally formed shale gas, it is necessary for:
- it to contain >2 % organic material and for the organic material to be of kerogen Type 2 so that gas can be formed.
- the shale to have a thermal maturity of between 1.1 and 3.5 (the value depending on the pressure and temperatures the bedrock is exposed to).
- the shale thickness to be >20 m for it to be economically and technically feasible to exploit the gas.
- the shale to be brittle and suitable to fracture.
- the overlying bedrock to have such a structure as to limit the propagation of cracks caused by fracturing.
- the shale to be deeper than 600 m, i.e. minimum depth for fracking.
- the shale to be sufficiently deep for large volumes of gas to be compressed and trapped in the shale, i.e. high pressure in the bedrock.
- the bedrock to have had a favourable geological development that has kept the gas from disappearing, for example in connection with mountain chain folding, fault movements and uplift.
Many shale areas meeting these requirements lie in mountainous areas, densely populated areas or in marine areas, which makes them impossible to exploit.
All in all, this means that the majority of dark shales either do not contain gas or are unsuitable for shale gas exploitation.
Biogenic gas is formed in organically rich shales or sediments at bedrock depths down to one or a few hundred metres. The bedrock has then not been heated to more than about 50 degrees, and the gas formed accumulates only in free form in the bedrock's pore spaces and is not as firmly trapped in the shale as thermally formed shale gas. The gas usually also consists of a high proportion of hydrogen sulphide and nitrogen in addition to methane. The accumulation of biogenic shale gas requires a natural supply of pore spaces, cracks, voids in the rock and a dense overlying bedrock. Gas volume relative to rock mass volume is also considerably less owing to the superficial occurrence of biogenic gas since the gas is then not as compressed as its deeper variant. Occurrences of biogenic gas can be exploited through conventional drilling without any “fracking”.
Is there potential for shale gas in Sweden?
In Sweden, dark shales, known as alum shale, of Cambrian age occur in parts of Skåne, Västergötland, Östergötland, Närke, Öland, the mountain chain and in the southern Baltic Sea. Except in Skåne and in the southern Baltic Sea, alum shale is found relatively superficially in the bedrock, at most down to a few hundred metres.
Alum shale is the shale that has most of the properties required for it to be able to contain gas. Within Sweden's borders, it is only in parts of its distribution area in Skåne where the shale is found at a suitable depth, has a sufficiently high thermal maturity, high organic content and thickness that makes it relevant for exploration. However, the bedrock in Skåne has been affected by major fault movements that has meant that the gas in the shale has probably not been retained. Drilling conducted by Shell in Skåne in 2011 confirms that the shale did not contain exploitable quantities of gas.
The gas found in the alum shale areas of Östergötland, Västergötland, Närke and Öland consists of biogenic gas. Locally, this shale can contain extractable quantities of gas, but from an international perspective, it is a matter of small volumes. Shale occurs in these areas at depths between 0 and 120 metres and has a maximum thickness of about 20 m.
On the Östgöta Plain, gas flows up out of the ground through cracks and crack systems, mainly in open pits where the groundwater no longer functions as a barrier. The gas coming from the alum shale is also largely accumulated in the Cambrian sandstone. A number of boreholes were drilled in the area in the 1930s. From one of them, drilled in 1934 a few kilometres outside Vadstena (Broby), it was possible to obtain 60 m³ of gas per day for a few months. Besides methane, the gas contains high concentrations of nitrogen and, in at least one test borehole (Granlund), there were also substantial quantities of toxic hydrogen sulphide (Westergård, 1940).
Relative to other countries in Europe, such as Poland, the potential for shale gas is assessed to be very small in Sweden, in particular regarding deep shale gas. Analyses of alum shale in Skåne have shown that the organic material is overmature, which has resulted in the shale not containing any extractable gas.
In addition to alum shale, the same areas, though in the Ordovician and Silurian bedrock, also have dark shales that may contain shale gas. In these younger shales, however, the organic content is considerably lower compared with the alum shale.
The occurrence of alum shale in the Swedish mountain chain has been affected by the Caledonian mountain chain formation that led to the shale very probably losing its content of shale gas. In addition, the shale is not sufficiently thick to be suitable for exploitation.
How is shale gas extracted?
The gas in the shale is trapped partly in organic particles and clay minerals and partly in microscopic pores as free gas and in solution together with formation water (the water contained in pore spaces and cracks in a sedimentary rock) and bitumen or oil. The fact that shale at great depths is relatively dense and has low permeability makes it very difficult to produce gas from it.
In order to increase the flow of gas to boreholes, high water pressure is used in the borehole to fracture up the shale and create cracks around the borehole to which the gas can flow. This method is called hydraulic fracturing and is used only at depths greater than 1000 metres. It is also known as fracking. In connection with fracturing, silica sand is injected to hold the cracks open. In addition to sand and water, about 1% of chemical additives are introduced to reduce friction in the cracks. Fracturing also requires large volumes of water.
The holes drilled for shale gas extraction are often more or less horizontal in the gas-bearing shale. The number of boreholes varies depending on gas content, bedrock density and depth. From each drilling site, several boreholes can be drilled and angled out horizontally in the shale. Many more boreholes are typically required than in traditional gas extraction.
In many cases, biogenic shale gas can be extracted without fracturing or stimulation of the borehole because superficial shales contain more naturally open cracks.
What are the risks of shale gas extraction?
The greatest risks associated with shale gas extraction are:
- That hydraulic fracturing can give rise to vibrations that create cracks in the bedrock, which in turn lead to damage to nearby groundwater reservoirs and properties.
- That the groundwater can become contaminated and that groundwater pressure and groundwater level may change.
- That the soil in connection with drilling and extraction is affected.
- That chemicals leak in connection with hydraulic fracturing.
- That drilling sites give rise to disruptive noise and transportation activities.
- Leakage of methane from boreholes, pipelines, valves, etc. Studies from the United States indicate that several per cent of the gas produced leak into the atmosphere.
Permits to explore shale
In Sweden, examinations pursuant to the Minerals Act and the Environmental Code are required both for the exploration and the extraction of shale gas. The Minerals Act imposes specific requirements on access to technical expertise and financial resources for those wishing to explore and extract gas occurrences.
Applications for permits under the Minerals Act are made to the Mining Inspectorate of Sweden, whose website contains further information on permit processes.
As of December 2012, there are seventeen exploration permits in Sweden under the Minerals Act that expressly relate to gas in alum shale areas. Five of these are on Öland, and the other twelve in Östergötland County. There are also additional permits authorising the exploration of alum shale as a whole. In one case, an exploitation concession (right of extraction) has been issued. This concerns an area situated in Mjölby and Motala municipalities designated Tornby K nr 2. Extraction in the area is not being considered at present.
Where are the biggest occurrences in Europe?
The largest estimated supplies of shale gas in Europe are in Poland. Supplies have also been estimated to exist in Central European countries such as Germany, Holland and Belgium and in the United Kingdom. France and Bulgaria have legislated against fracking, and other countries may follow suit.
Denmark has recently conducted a study of the potential of alum shale as a gas source, and two permits for drilling on Zealand and Jutland are under preparation.
There is still considerable uncertainty as to the potential of shale gas in Europe. There is often a lack of objective assessments.
The EU and shale gas
The EU is currently conducting investigations into the significance of shale gas and the risks associated with potential extraction, especially with regard to hydraulic fracturing. Work is under way to propose an explicit framework regulating the management of national permits and supervision of shale gas exploitation.
AEA Technology plc, 2012: Support to the identification of potential risks for the environment and human health arising from hydrocarbons operations involving hydraulic fracturing in Europe. European Commission DG Environment report, ref 07.0307/ENV.C.1/2011/604781/ENV.F1.
Andersson, A., Dahlman, B., Gee, D.G. and Snäll, S., 1985: The Scandinavian alum shales. SGU Ca 57.
Last reviewed 2020-11-10