The Barnett Shale is a natural gas deposit that stretches over a dozen counties in northern and eastern Texas. It is the second largest producing on-shore domestic natural gas field in the United States after the San Juan Basin in New Mexico and Colorado, covering an area of around 5,000 square miles (13,000 square kilometers). Although the Barnett was discovered in the 1950s, it wasn't succesfully drilled until the 1980s when technology evolved to do so. The field is proven to have 2.5 trillion cubic feet of natural gas.
Shale is a fine-grained sedimentary rock containing organic compounds from which liquid hydrocarbons such as oil and natural gas can be extracted. Worldwide deposits of petroleum products in oil shales are around 2.8-3.3 trillion barrels of oil equivalent.
Extracting natural gas from shale is more expensive than extracting it from wells because the natural gas has been absorbed into sedimentary rock and must be released through a complex heating process. As a result, natural gas extraction from shale is only profitable when energy prices are high. After reaching a peak of about $14 per million BTU (MMBtu) in 2008, natural gas prices have stabilized at around $4-5 MMBtu.
The natural gas stored in shale is locked into hard rock under immense pressure, often deeper than conventional gas wells. Current shale-gas extraction uses a technique called hydraulic fracturing, often known as "fracking." A well is drilled through the upper strata into the shale bed, generally 1.5-6km below the surface, and a mixture of water and solid particles - often sand, sometimes manmade ceramic beads - is pumped down into the shale at pressures up to 100 MPa (145,000psi). This fractures the rock and the solid particles hold the crack open.
Much of the Barnett Shale is underneath the cities of Dallas and Fort Worth. A technique called horizontal drilling has allowed natural gas drillers to access the shale under urban and residential areas. However, researchers at the Jackson School of Geosciences have warned that natural gas drilling could put pressure on water supplies, endangering drinking water. With some 80 new wells being drilled every month, demand for water is already rising steeply.
There is also concern that fracking could itself be harmful to drinking water. Drills sometimes have to go through drinking-water acquifers and there have been cases of water being contaminated; in April 2010, the state of Pennsylvania banned one company, Cabot Oil & Gas (COG), from drilling, after combustible gas found its way into water supplies and a water well exploded. Shale-gas drilling in Colorado has also been controversial, with a documentary called "Gasland" showing people igniting gas coming off their household water supplies.
During 2008, the Barnett Shale produced 1.396 Tcf (trillions of cubic feet) of natural gas. The top ten producers of natural gas were:
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