What is shale gas?
Natural gas can exist in Devonian shale deposits. Devonian shales are formed from the mud of shallow seas that existed about 350 million years ago (during the Devonian period of the Paleozoic era). Shale is a very fine-grained sedimentary rock, which is easily breakable into thin, parallel layers. It is a very soft rock, but does not disintegrate when it becomes wet. These shales can contain natural gas, usually when two thick, black shale deposits 'sandwich' a thinner area of shale. Because of some of the properties of these shales, the extraction of natural gas from shale formations is more difficult (and thus expensive!) than extraction of conventional natural gas.
Shale can be the source, reservoir and the seal for the gas. Shale gas plays are classified as a “continuous” type gas accumulations extending throughout large areas, typically with low permeability and perhaps natural fractures.
Why is shale gas considered unconventional gas?
Shale gas is considered an unconventional source as the gas may be attached to or "adsorbed" onto organic matter. The gas is contained in difficult-to-produce reservoirs that require special completion, stimulation and/or production techniques to achieve economic production. Shale gas may also be contained in thin, porous silt, sand and beds interbedded in the shale. In this case, the gas is classified as ‘free gas’ and is produced along with the adsorbed gas.
How are shale gas wells drilled?
The same techniques used for conventional shallow gas development may also be used with shale gas drilling. Some shale gas development can be horizontally drilled which will help reduce the surface impact. However, not all areas may be conducive to horizontal drilling. Drilling pads, from which multiple wells are drilled, can also be used to reduce the area of surface
Shale gas has been produced for years from shales with natural fractures; the shale gas boom in recent years has been due to modern technology in hydraulic fracturing to create extensive artificial fractures around well bores.