Hydraulic fracturing is the propagation of fractures in a rock layer caused by the presence of a pressurized fluid. Hydraulic fractures form naturally, as in the case of veins or dikes, and is one means by which gas and petroleum from source rocks may migrate to reservoir rocks. However, oil and gas companies may attempt to accelerate this process in order to release petroleum, natural gas, coal seam gas, or other substances for extraction, where the technique is often called fracking or hydrofracking. This type of fracturing, known colloquially as a frack job (or frac job), is done from a wellbore drilled into reservoir rock formations. The energy from the injection of a highly-pressurized fracking fluid creates new channels in the rock which can increase the extraction rates and ultimate recovery of fossil fuels. When done in already highly-permeable reservoirs such as sandstone-based wells, the technique is known as well stimulation. Operators typically try to maintain fracture width or slow its decline following treatment by introducing a proppant into the injected fluid, a material, such as grains of sand, ceramic, or other particulates, that prevent the fractures from closing when the injection is stopped. Consideration of proppant strengths and prevention of proppant failure becomes more important at deeper depths where pressure and stresses on fractures are higher.
Distinction can be made between low-volume hydraulic fracturing used to stimulate high-permeability reservoirs, which may consume typically 20,000 to 80,000 gallons of fluid per well, with high-volume hydraulic fracturing, used in the completion of tight gas and shale gas wells; high-volume hydraulic fracturing can use as much as two to three million gallons of fluid per well. This latter practice has come under scrutiny internationally due to concerns about the environmental impact, health and safety, and has been suspended or banned in sensible countries.