Fluid pressure inside hydraulic fractures causes their propagation. The potential energy of this fluid provides the energy needed for the creation of fracture surfaces. Theoretical analysis of hydraulic fracturing has shown that fluid pressure needed for fracture extension decreases as the fracture becomes longer. Experimental work on two-dimensional fractures show the same trend in laboratory specimens. It also shows that obstructing the natural fracture growth will result in an increase in the fluid pressure. In field operations hydraulic fractures will encounter many obstructions as they propagate. The resulting fractures will therefore have irregular shapes. Obstructions which are overcome earliest during the treatment are the least severe. A relatively severe obstruction can completely restrict fracture growth at its vicinity, thus increasing the possibility of inducing propagation into one of the adjacent formations. A severe restriction to fracture growth increases the fracturing fluid pressure. This enhances the possibility of extending secondary fractures. The smaller the difference between at least two principal stresses, (e.g.; at shallow depths), the higher the chances of creating secondary fractures.
Fluid pressure inside a hydraulic fracture provides the energy needed for its extension. During its propagation, the hydraulic fracture encounters various restrictions to its growth, each of which will have its effect on fracturing fluid pressure. It is the purpose of this paper to discuss some of the common causes for fluid pressure changes, and, the effects they will have on the fracture. Suppose the fluid pressure inside the hydraulic fracture is constant. Any obstruction in the path of fluid flow inside the fracture will result in increases in p and consequently borehole pressure. A likely source of obstruction to fluid flow is the proppant, which may be bridged in the fracture, thus reducing the open channel for flow of fluid.
