Borehole breakdown pressure with drilling fluids - II. Semi-analytical solution to predict borehole breakdown pressure

The fracture initiation and propagation experiments discussed in Part I revealed that three distinct zones exist around a fracture tip. They are the fracture process zone, the non-invaded and the dehydrated gel zone. These three zones significantly increase wellbore breakdown pressure and fracture propagation resistance under a relatively large in situ stress. With the assumption that wellbore pressure is sealed at the inlet of the dehydrated mud zone, a semi-analytical model of a fractured, inclined borehole is developed for a general plane strain condition. The model predicts that when drilling fluid is used as an injection fluid, borehole breakdown pressure is highly dependent on the formation Young's modulus, the in situ stress, the wellbore size, and type of the drilling fluids, although conventional theories have not included all these.