When a hydraulic fracture is created, the interface between the fracture and the formation may encounter some permeability reduction either intentionally (leak-off additives) or unintentionally (for example, relative permeability effects, non-breaking gel). This is modeled by assuming the presence of a skin on the fracture face as shown below.
The skin on the fracture face causes an additional pressure drop that tends to negate some of the benefits of the fracture. Sometimes this skin on the fracture face improves with production or time (clean up or temperature effects). Cinco-Ley and Samaniego (1987 and 1981) have defined sf as:
Even a small skin on the fracture face, sf < 1, can cause a substantial change in the shape of the derivative curve. Typically, sf ranges from 0 to 1.