Shale Gas

Shale gas is natural gas produced from shale. Shales have insufficient permeability to allow significant fluid flow, therefore most shales are not commercial sources of natural gas. Shale gas is one of a number of “unconventional” sources of natural gas. Other unconventional natural gas sources are coalbed methane, tight sandstones, and methane hydrates.

 
 











Shale has low matrix permeability, so gas production in commercial quantities requires fractures to provide permeability. 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. Horizontal drilling is often used with shale gas wells, with lateral lengths up to 10,000 feet within the shale, to create maximum borehole surface area in contact with the shale.

Shales that host economic quantities of gas have a number of common properties. They are rich in organic material (0.5% to 25%), and are usually mature petroleum source rocks in the thermogenic gas window, where high heat and pressure have converted petroleum to natural gas. They are sufficiently brittle and rigid enough to maintain open fractures. In some areas, shale intervals with high natural gamma radiation are the most productive, as high gamma radiation is often correlated with high organic carbon content.

Some of the gas produced is held in natural fractures, some in pore spaces, and some is adsorbed onto the organic material. The gas in the fractures is produced immediately; the gas adsorbed onto organic material is released as the formation pressure is drawn down by the well.

Shale gas areas are often known as resource plays (as opposed to exploration plays). The geological risk of not finding gas is low in resource plays, but the potential profits per successful well are usually also lower.

From Wikipedia, the free encyclopedia