INTERPRETATION OF HYDRAULIC FRACTURING BREAKDOWN PRESSURE

Breakdown pressure is an important parameter obtained during hydraulic fracturing stress measurements. It is thought that the maximum horizontal stress can be calculated from the breakdown pressure, the minimum principal stress and the properties of the rocks. On the other hand, breakdown is a complex process. The breakdown pressure is rate-dependent, size-dependent, fracture fluid-dependent and sigma3-dependent. As a result, many breakdown models prevail. This paper evaluates the various models experimentally and theoretically. The analysis shows that the classical breakdown model, the poroelastic model, the shear failure model and the point stress model cannot explain the observed abnormally high breakdown pressure. The fracture mechanics model is promising. The breakdown pressure appears to correspond to the onset of unstable fracture propagation. It should be stressed that the fracture criterion for the unstable fracture propagation is K(I) - K(Ic) = 0 and partial derivative(K(I) - K(Ic))/partial derivative L greater-than-or-equal-to 0, not only K(I) - K(Ic) = 0. Introduction of partial derivative(K(I) - K(Ic))/partial derivative L greater-than-or-equal-to 0 makes it possible to explain several phenomena such as rate-dependent, size-dependent fracture fluid-dependent and sigma3-dependent responses and abnormally high breakdown pressures.