a hydraulic fracture is formed by pumping fracturing fluid into a wellbore at a rate sufficient to increase pressure at the target depth (determined by the location of the well casing perforations), to exceed that of the fracture "gradient" (pressure gradient) of the rock. the fracture gradient is defined as pressure increase per unit of depth relative to density, and is usually measured in pounds per square inch, per square foot, or bars. the rock cracks, and the fracture fluid permeates the rock extending the crack further, and further, and so on. fractures are localized as pressure drops off with the rate of frictional loss, which is relative to the distance from the well. operators typically try to maintain "fracture width", or slow its decline following treatment, by introducing a proppant into the injected fluida material such as grains of sand, ceramic, or other particulate, thus preventing the fractures from closing when injection is stopped and pressure removed. consideration of proppant strength and prevention of proppant failure becomes more important at greater depths where pressure and stresses on fractures are higher. the propped fracture is permeable enough to allow the flow of gas, oil, salt water and hydraulic fracturing fluids to the well.