Abstract
Breakdown pressure of induced hydraulic fractures from circular boreholes is investigated experimentally utilizing a novel, two-dimensional (2-D) fracturing cell. The objective of this study is to investigate how breakdown pressure in fracturing experiments is affected by borehole notch properties that include notch permeability, notch length and its orientation relative to the applied far-field stresses. We show that test specimens with an initial borehole notch have markedly lower breakdown pressures compared to boreholes with no notches. However, we demonstrate that if the borehole notch is made impermeable to the injection fluid by plugging the notch with a flexible adhesive, the breakdown pressure of the specimen with an impermeable notch is virtually the same as a specimen without an initial notch. Also, the breakdown pressure decreases as the borehole notch becomes longer and increases as the notch deviates from the direction of the applied maximum far-field stress. The measured breakdown pressure values obtained in our laboratory experiments are compared with theoretical predictions from analytical solutions available in the literature for the effect of notch length and orientation.1. INRODUCTIONIn hydraulic fracturing, breakdown pressure is the peak fluid pressure at which a borehole fractures or breaks down. For a circular borehole with two symmetrical opposing cracks or notches at a certain orientation, theoretical estimation of breakdown pressure (Pb) can be obtained from Equation 1 based on fracture mechanics (Paris and Sih 1965; Abou-Sayed et al. 1978; Jin et al. 2013).(Equation)Where KIC is fracture toughness, α is the crack length, Rw is the borehole radius, G(α/Rw) and F(α/Rw) are tabulated functions, Shmin and SHmax are the far-field minimum and maximum horizontal principal stresses, respectively, and θ is the angle between the crack and SHmax. Equation 1 predicts a higher breakdown pressure as θ increases and generally a lower breakdown pressure as α increases (Jin et al. 2013). These initial cracks can be viewed as a two-dimensional (2-D) representation of cylindrical perforation cavities in the formation from a cased wellbore, or pre-existing natural fractures that intersect the wellbore.