<P> Hydraulic fracturing began as an experiment in 1947, and the first commercially successful application followed in 1950 . As of 2012, 2.5 million "frac jobs" had been performed worldwide on oil and gas wells; over one million of those within the U.S. Such treatment is generally necessary to achieve adequate flow rates in shale gas, tight gas, tight oil, and coal seam gas wells . Some hydraulic fractures can form naturally in certain veins or dikes . </P> <P> Hydraulic fracturing is highly controversial in many countries . Its proponents advocate the economic benefits of more extensively accessible hydrocarbons . Opponents argue that these are outweighed by the potential environmental impacts, which include risks of ground and surface water contamination, air and noise pollution, and the triggering of earthquakes, along with the consequential hazards to public health and the environment . </P> <P> Increases in seismic activity following hydraulic fracturing along dormant or previously unknown faults are sometimes caused by the deep - injection disposal of hydraulic fracturing flowback (a byproduct of hydraulically fractured wells), and produced formation brine (a byproduct of both fractured and nonfractured oil and gas wells). For these reasons, hydraulic fracturing is under international scrutiny, restricted in some countries, and banned altogether in others . The European Union is drafting regulations that would permit the controlled application of hydraulic fracturing . </P> <P> Fracturing 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" (Mode 1) 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 . Also see a general review paper on the mechanics of fracking . </P>

Where does the sand for fracking come from