What is Shale Gas?
Shale gas is the name for natural gas trapped within sedimentary shale rocks1. Unlike conventional gas, shale gas does not accumulate in large volumes underneath impermeable “trap” rocks. It is therefore referred to as an unconventional gas, along with other types of unconventional gas resources, like coal-bed methane and tight gas.
Shale gas is the term often used to describe both tight gas and shale gas resources. The main difference between tight gas and shale gas, however, is that tight gas can be found in low permeability sedimentary rocks, such as sandstones, while shale gas rocks are usually impermeable2.
Shale gas reservoirs are usually called “plays” or “basins”, in contrast to the conventional gas “fields”. Within the impermeable shale rocks, the gas is trapped and sealed in unconnected fractures and pores. This requires specific extraction methods for its production. Hydraulic fracturing and horizontal drilling are used to increase the flow rate of the gas in shale gas rocks, which are usually wider than they are tall.
Although natural gas from shale rocks has been used since 1821 3, it was not until innovations in hydraulic fracturing (“fracking”) and horizontal drilling brought down the costs and allowed the resource to be mass-produced. In 2000 shale gas provided only about 1.6% of total U.S. natural gas production4 by 2013, it accounted for about 39% 5.
No other country has yet experienced a growth of the shale gas industry similar in scale to the U.S. This is largely due to the lack of infrastructure and the ongoing debate around the political, social and environmental consequences that such an industry could bring to many of the countries with proven reserves.
Hydraulic fracturing and the components of the hydraulic fracturing fluid are a primary source of controversy. Although the fluid is largely composed of water, about 0.5% contains chemical additives. This equates to large amounts of chemicals being used and millions of gallons of water needed for fracturing operations6. In addition, there have also been concerns linking fracking to enhanced seismic activity. In the wake of this debate, several countries placed local or nation-wide bans on fracking7.
Shale gas has higher GHG emissions than conventional natural gas, due to the additional need for energy input during extraction.