This paper presents a basis for interpreting fracture treating pressures which permits identification of periods of confined-height extension, uncontrolled height growth, and more importantly, a critical pressure. When a treatment reaches the critical pressure, fracture extension is significantly reduced, and a pressure (or screen-out) condition or undesired fracture height can follow. Example applications for data from five treatments are presented along with potential explanations for, and implications of, the critical pressure.
This paper will be limited to the discussion of hydraulically created fractures that are in the vertical plane. In addition, if these fractures are desired to be deeply penetrating, they must have confined or limited height growth.
There are two fundamentally different concepts for the propagation of a constant-height vertical-fracture which lead to very different results. One concept1 is that the fracture width is constant across the height of the fracture. This requires the assumption that the formation bed being fractured is independent of the beds above and below - that is, the beds can slip freely of one another at their boundaries. This assumption leads to the conclusion that the fluid pressure required to extend the fracture decreases with time.
The other concept, presented by Perkins and Kern,
2 assumes that there is no, or negligible, slip of boundaries along the horizontal planes which confine the fracture height. This assumption leads to the conclusion that the fluid pressure required to extend the fracture increases with time.