Hydraulic fracturing (also hydro fracturing, hydro racking, fracking or fraccing) is a well-stimulation technique in which rock is fractured by a pressurized liquid. The process involves the high-pressure injection of 'fracking fluid' (primarily water, containing sand and other proppants suspended with the aid of gelling agents) into a wellbore to create cracks in the deep-rock formations through which natural gas, petroleum, and brine will flow more freely. When the hydraulic pressure is removed from the well, small grains of hydraulic fracturing proppants (either sand or Aluminium oxide) hold the fractures open.
The first use of hydraulic fracturing was in 1947 but the modern fracturing technique, called horizontal slick water fracturing, that made the extraction of shale gas economical was first used in 1998 in the Barnett Shale in Texas. The energy from the injection of a highly pressurized hydraulic fracturing fluid creates new channels in the rock, which can increase the extraction rates and ultimate recovery of hydrocarbons.
Proponents of hydraulic fracturing point to the economic benefits from vast amounts of formerly inaccessible hydrocarbons the process can extract. Opponents point to potential environmental impacts, including contamination of ground water, risks to air quality, 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 scrutiny internationally, with some countries suspending or even banning it.
During this process, methane gas and toxic chemicals leach out from the system and contaminate nearby groundwater.
Methane concentrations are 17x higher in drinking-water wells near fracturing sites than in normal wells.
Fracturing in rocks at great depth frequently becomes suppressed by pressure due to the weight of the overlying rock strata and the cementation of the formation. This suppression process is particularly significant in "tensile" fractures which require the walls of the fracture to move against this pressure. Fracturing occurs when effective stress is overcome by the pressure of fluids within the rock. The minimum principal stress becomes tensile and exceeds the tensile strength of the material. Fractures formed in this way are generally oriented in a plane perpendicular to the minimum principal stress, and for this reason, hydraulic fractures in well bores can be used to determine the orientation of stresses. In natural examples, such as dikes or vein-filled fractures, the orientations can be used to infer past states of stress.
Scientists are worried that the chemicals used in fracturing may pose a threat either underground or when waste fluids are handled and sometimes spilled on the surface.