Via EIA: “For decades, hydraulic fracturing had been referred to as an unconventional completion technique, but over the past 10 years it has become the technique by which most natural gas is produced in the United States. Based on the most recent data from states, EIA estimates that natural gas production from hydraulically fractured wells now makes up about two-thirds of total U.S. marketed gas production. This share of production is even greater than the share of crude oil produced using that method, where hydraulic fracturing accounts for about half of current U.S. crude oil production. Hydraulic fracturing, often in combination with horizontal drilling, involves forcing a liquid (primarily water) under high pressure from a wellbore against a rock formation until it fractures. The fracture lengthens as the high-pressure liquid in the wellbore flows into the formation. This injected liquid contains a proppant, or small, solid particles (usually sand or a manmade granular solid of similar size), that fills the expanding fracture. When the injection is stopped and the pressure is reduced, the formation attempts to settle back into its original configuration, but the proppant keeps the fractures open. This allows hydrocarbons to flow from the rock formation back to the wellbore and then to the surface.”