Numerical study of initiation pressure in hydraulic fracturing by dual criterion for non-circular wellbore

Abstract

In this paper, the DDM numerical method is used to calculate the initiation pressure and initiation length of the circular wellbore, or the further elongated and non-elongated ‘dog-ear’ breakout wellbore under different sizes and different stress states. An analysis is made about the transfer of the fracture initiation point for the further elongated breakout wellbore and its possible impacts. The numerical results show that for small-size wellbore ((Rwσt2)/KIC2 < 2), the initiation pressure is dominated by fracture toughness, and for large-size wellbore ((Rwσt2)/KIC2 > 6.4), the initiation fracture is dominated by tensile strength. There is only small difference between the initiation pressure of the ‘dog-ear’breakout wellbore and the circular wellbore. With further elongation of the ‘dog-ear’ wellbore or increase of the curvature radius around initiation point, the initiation pressure fracture along the direction of the maximum principal stress also increases in most cases. The further elongated ‘dog-ear’ wellbore causes the initiation point of the wellbore to deviate away from the direction of the maximum principal stress, and the existence of at least four initiation points tends to cause X-shaped fracture initiation around the wellbore, which may promote the further increase of the initiation pressure. The research in this paper provides a new possible version of multi-fracture initiation around wellbore and abnormally high initiation pressure during fracturing operation.