Unconventional reserves are situated in rocks of low permeability, which makes the gas difficult to access. In order to extract it, more advanced technologies are required than those employed in the extraction of conventional gas deposits.
The three most common types of unconventional gas are:
Shale gas is the most commonly known unconventional gas. Shale gas is found in shale deposits, which are made up of thin layers of fine-grained sedimentary rock, which are both the source and the reservoir for the natural gas. However, only formations with certain characteristics are viable for exploration. The International Energy Agency (IEA) estimates that there are currently 13 trillion cubic meters (tcm) of technically recoverable shale gas resources in Europe.
Tight gas is found trapped in low permeability sandstone or limestone formations, typically at depths greater than 3,500 meters below the surface. The viability of sandstone reservoirs is determined by their porosity (the open space between grains) and permeability (how easily fluid or gas moves through the rock).
The United States has been producing tight gas for more than four decades, and in 2010 it accounted for approximately 32 per cent of the country's unconventional gas output. Estimates for technically recoverable tight gas resources in OECD Europe in 2011 stood at 4 trillion cubic meters (tcm), according to the IEA. Tight gas has been produced in Europe for decades, notably in Germany.
On this website, we adhere to the international classification of tight gas within the group of unconventional gas, followed by e.g. the IEA. In some countries, like for example Germany, tight gas has been produced successfully for many years since the 1990s. Given the long experience with these reservoirs, many experts no longer categorize tight gas as unconventional gas.
Coal Bed Methane
Coal bed methane (CBM) is generated within and produced from coal seams that have very low permeability. CBM is formed by the decomposition of organic material found in coal that is too deep or too poor quality for commercial mining. Although occasionally found at shallow depths, CBM is often buried 1,000 to 2,000 meters underground. CBM is most often produced using vertical wells, although horizontal drilling is becoming increasingly common in some areas.
In order to extract the gas, water is pumped from the coal seam so as to reduce the water pressure that holds gas in the seam. CBM is not very soluble in water and easily separates from the coal seam as pressure decreases, allowing it to be piped out of the well separately from the water. The water removed during the depressurization process is then re-injected back into the subsurface.
The IEA has estimated technically recoverable CBM resources to reach 50 trillion cubic meters (tcm) globally, 2 trillion cubic meters (tcm) of which can be found in Europe.