What is Fracking?

Mohammed Aly Sergie, and James McBride, in a backgrounder for the Council on Foreign Relations, write:

Hydraulic fracturing, or fracking, refers to two processes used to extract natural gas trapped in shale formations. The first step is to drill down to the sedimentary rocks, sometimes as far as ten thousand feet, then drill sideways for a mile or more. This horizontal drilling has been widely practiced since the 1980s to extract conventional oil and gas.

Then, since shale gas is trapped in dense layers of rock and can't flow through the well by drilling alone, producers must deploy hydraulic fracturing—first introduced into commercial practice in 1947 and effectively combined with horizontal drilling in the late 1990s—which pumps millions of gallons of water, sand, and chemicals at high pressure to open fractures in the rocks, allowing oil and gas to flow. Fracking is used in nine out of ten natural gas wells in the United States.

Horizontal drilling and hydraulic fracturing have allowed producers to tap into large tight oil, shale oil, and shale gas deposits—some of which span multiple states—in the United States, such as the Barnett, Permian Basin, and Eagle Ford shales in Texas; the Fayetteville in Arkansas; the Haynesville in Louisiana; and the largest, the Marcellus, which spans Pennsylvania, New York, and Ohio. Then there is the massive Bakken formation, centered in North Dakota, which has remade the energy map in the United States and around the world, becoming a top global producer and producing more than one million barrels per day of oil in 2014...

Though basic fracking techniques have been available for decades, technological breakthroughs in the 1990s paved the way for newfound commercial viability in the shale sector, as CFR Senior Fellow Michael Levi recounts in his book The Power Surge. 

[T]ight oil, which is extracted from shale and other rock formations, has been the fastest-growing segment of U.S. crude production, hitting nearly 4.5 million barrels per day by the end of 2014, up from nothing in 1999.