Multistage hydraulic fracturing is an effective stimulation method as it opens communication between the wellbore and larger reservoir volumes. Nevertheless, there are cases where the formation cannot be fractured within the completion pressure limitations. This problem can be addressed by placing weak points, such as perforations and notches, so fractures could initiate there at lower pressure. This paper discusses this remedy applied in horizontal open holes that are highly deviated from the minimum horizontal stress.

In this study, cement grout block samples (24×18×18 in.) were prepared to cast precisely weak points and boreholes. The cured blocks were completed with 1.25×12 in. (diameter × length) long openhole section in the center. Hydraulic fracturing experiments were conducted inside a polyaxial load-frame to simulate the wellbore under strike-slip stress regime. Boreholes of two azimuthal deviations from minimum stress were considered: 0° (straight) and 35° (inclined). For each, two weak points were tested: V-notch and single perforation. Fracturing fluid was injected at low rate into the borehole. The obtained injection pressure trends were interpreted with open holes cored out to analyze the initiated fractures.

Both fracture initiation pressure (FIP) and fracture breakdown pressure (FBP) (i.e., the maximum) were selected for comparisons, as the former is not influenced by the fracture size nor by the fluid-rock interactions during propagation. FIP was estimated indirectly as deviation from linearity in the borehole injection pressure trend. The results revealed remarkably that the notch significantly reduces FIP and FBP, compared with the reduction by the single perforation, in both the straight and inclined boreholes. Each of the weak points delivered very similar FIP and FBP in straight and inclined boreholes. All the weak points initiated a fracture that is transverse to the borehole axis, which provides the advantage of 360° communication with the reservoir. It was noticed that the inclined borehole with one perforation initiated a transverse fracture with a distinct angle from the preferred plane. To summarize, even if the wellbore is drilled at a high angle of deviation from the minimum horizontal stress in the strike-slip regime, the notch still outperforms perforation in terms of FIP reduction.

This work presents experimental comparisons of hydraulic fracturing pressure reductions caused by notches and single perforation holes placed in horizontal open holes aligned with and highly deviated from the minimum horizontal stress. The experiments showed how notches outperform conventional perforation, and by this, provide insights for stimulation design of horizontal wells where trajectories were influenced by factors other than stimulation, such as geology and reservoir location.

You can access this article if you purchase or spend a download.