Drilling horizontal wells and multi-stage hydraulic fracturing have become essential for economically producing oil and gas from unconventional resource plays. Selection of appropriate intervals for perforation clusters (in cased holes) is key for a successful fracturing job in horizontal wells. An inappropriate interval may lead to high breakdown pressures or even inability to breakdown which can prevent that part of the well from contributing to the production. There are two approaches commonly used in selection of perforation cluster intervals, geometric approach and engineered approach. In most cases where geometric staging was adopted, only 30% of the stages contributed to 70% of production. Engineered staging where perforation intervals are selected based on minimum horizontal stress along the lateral has proven to be more effective and increased production contribution of each stage. However, we found this approach can be effective only in normal stress regime. In other stress regimes, e.g. strike-slip and reverse stress regimes, breakdown pressures were found to be higher in intervals of lower horizontal stress contrary to what we typically see in normal stress regime.
We observed that breakdown pressure depends on the differential stress acting on borehole rather than one single stress, higher the difference between the two stresses (Δσ) acting on the borehole lower the breakdown pressure is. We also observed that well azimuth moving away from the minimum stress azimuth lowers the breakdown pressure in normal stress regime, however, in non-normal stress regimes, this effect is opposite. Therefore, for regions where non-normal stress regimes are dominant, conventional engineered approach in which stages are selected based on minimum horizontal stress cannot be applied. We present in this paper a new and more effective approach where a balance between the lowest minimum stress and minimum breakdown pressure is taken into account to design completions that favor both initiation and fracture extension in complex stress regimes. We demonstrate case studies from Saudi Arabia's unconventional plays where this approach has been applied successfully.
Theory and Methodology
Mechanical behavior of rock depends on its inherent physical properties (stiffness, Poisson's ratio, strength, etc.) and the in-situ stress field to which it is subjected to. In-situ stresses are principal stresses acting in three orthogonal directions, vertical and two horizontal stresses (σv, σHmax, and σhmin) referred to as the largest, intermediate and smallest principal stresses. To fully describe the state of stress at a point in the subsurface, it is necessary to determine the magnitudes and orientations of these three principal stresses.
