Hydraulic fracturing is a technique used to crack rock underground, usually for the purpose of extracting oil or gas. It involves injecting a fluid down a well under massive pressure, in order to force open cracks in the targeted rock. Conventional hydraulic fracturing has been used since the 1950s to create small cracks, just around well-bores, to speed the flow of oil or gas into the well. Typically modern hydraulic fracturing of a conventional oil or gas well might use between 50,000 and 200,000 gallons of nitrogen foam or cross linked gel to propagate cracks a few feet from the wellbore. The aim is to speed up the flow where the oil or gas bottlenecks as it enters the wellbore and it usually will not have effects beyond that.
More recently “massive slickwater hydraulic fracturing” has been developed for the purpose of extracting gas from more much impermeable rock formations, such as shale. Unlike conventional extraction where the oil or gas flows though the permeable formation and one well can drain a large area, unconventional extraction requires large numbers of wells which each drain a small area (40-80 acres). Hydraulic fracturing of a typical shale gas or oil well requires the use of around 5 to 10 million gallons of slickwater (a mixture of water, sand and chemicals) to fracture the rock in up 40 stages along a mile or more long horizontal. This aims to propagate fractures hundreds of feet from the wellbore and similar parallel horizontals are usually situated to drain adjacent areas.
See our articles If Fracking Has Been Happening Since 1947 What Is There To Worry About? and Fracking Nukes: Counting The Kilotons for more details on the scale of unconventional hydraulic fracturing.