Introduction to Shale Gas

Shale gas is natural gas produced from shale. Shale gas has become a very important source of natural gas in the United States over the past decade, and the search for shale gas has spread overseas leading to exploration in Canada, Europe, Asia, and Australia. Shale has low matrix permeability, so gas production in commercial quantities requires fractures to provide permeability. Shale gas has been produced for many years from shales with natural fractures but the recent shale gas boom in North America has been driven by modern technology in hydraulic fracturing to create extensive artificial fractures around (horizontal) well bores.

Shales that have economic quantities of gas generally have a number of common properties. They are rich in organic material, they are usually mature oil source rocks in the "gas window" - where heat and pressure have converted the oil to natural gas. The shales are also sufficiently brittle and rigid enough (with high quartz & carbonate) to maintain open fractures.

The gas produced from shale is held in natural fractures, some in pore spaces, and some is adsorbed onto the organic material. Initial gas production rates declines quickly as the gas in the fractures is produced immediately, the gas adsorbed onto organic material is then released as the formation pressure is drawn down by the well.