Hydraulic fracturing is the fracturing of rock by a pressurized liquid.
Some hydraulic fractures form naturally -- certain veins or dikes are examples.
Induced hydraulic fracturing or hydrofracturing, commonly known as fracking, is a technique in which typically water is mixed with sand and chemicals, and the mixture is injected at high pressure into a wellbore to create small fractures (typically less than 1mm), along which fluids such as gas, petroleum, uranium-bearing solution, and brine water may migrate to the well.
Hydraulic pressure is removed from the well, then small grains of proppant (sand or aluminium oxide) hold these fractures open once the rock achieves equilibrium.
The technique is very common in wells for shale gas, tight gas, tight oil, and coal seam gas and hard rock wells.
This well stimulation is usually conducted once in the life of the well and greatly enhances fluid removal and well productivity, but there has been an increasing trend towards multiple hydraulic fracturing as production declines.
A different technique where only acid is injected is referred to as acidizing.
The first experimental use of hydraulic fracturing was in 1947, and the first commercially successful applications were in 1949.
Mitchell is considered by some the modern "father of fracking" because he successfully applied it to the Barnett Shale in the 1990s.
As of 2012, 2.5 million hydraulic fracturing jobs have been performed on oil and gas wells worldwide, more than one million of them in the United States.
Uranium Energy Corporation is planning to use hydraulic fracturing to mine uranium.
Fracking for uranium involves injecting oxygenated water (to increase solubility) to dissolve the uranium, then pumping the solution back up to the surface.
Proponents of hydraulic fracturing point to the economic benefits from the vast amounts of formerly inaccessible hydrocarbons the process can extract.
Opponents point to potential environmental effects, including contamination of ground water, depletion of fresh water, risks to air quality, noise pollution, the migration of gases and hydraulic fracturing chemicals to the surface, surface contamination from spills and flow-back, and the health effects of these.
For these reasons hydraulic fracturing has come under international scrutiny, with some countries protecting it, and others suspending or banning it.
However, some of those countries, including most notably the United Kingdom, have recently lifted their bans, choosing to focus on regulations instead of outright prohibition.
The 2013 draft EU-Canada trade treaty includes language outlawing any "breach of legitimate expectations of investors" which may occur if revoking drilling licenses of Canada-registered companies in the territory of the European Union after the treaty comes into force.
Under Chapter 11 of the existing North American Free Trade Agreement, private companies can sue governments when new laws reduce expected profits from existing contracts, however in the U.K previous regulations have excluded hydraulic fracturing companies from potential costs from cleanup operations or the cost to the U.K taxpayer if such companies were to be made financially redundant.