Abstract
Comprehensive stress analyses indicate that plastic yield, fracture initiation near or between packers or from a perforated hole are all possible during hydraulic fracturing. Furthermore a fracture can be created from one of the perforated holes located in deviated wellbore subject to three in-situ stresses regime. The corresponding fracturing pressure can be determined based on the orientation of this perforated hole. While a plastic yielding process on a wellbore or perforated hole surfaces may prevent a fracture from initiating, a fracture or fractures may initiate from a near packer region or at an oriented perforation when different completion strategies utilized.
1. INTRODUCTION
Staged fracturing strategy has been widely used in U.S. and Canada for low-permeability tight formations. Fractures or a fracture network may be initiated from an open hole or perforations. Depending on the layout of a horizontal well, the perforation orientations, clusters of perforations, and their distances to the packers in each stage, creations of fracture networks and fractures initiation are of great interests to engineers for productivity and surface control purposes. We normally calculate the tangential stresses near a wellbore or perforation for open and cased holes, respectively. Whether a transverse, radial, or arbitrarily angled fracture can be generated, if a fracture can be initiated at a specific wellbore location or from one of the perforations (in the middle, at the edge of the packers or from one of multiple perforated holes) remains unanswered. These issues must be addressed by considering the critical stresses at the wellbore and perforations once we shall have a clear pictures if plastic yield occurs prior fracturing, or which perforation is oriented at an optimum position referring to the in-situ stresses, and if the near packer regions are heavily influenced by the in-situ stresses. Both a 3D finite element model and an analytical poro-elastoplastic model are developed to analyze the corresponding effective stresses. These models focus on regions near the packer or the oriented perforations. They evaluate the plastic yielding and the stresses in these areas in order to determine the location and the pressure at which a fracture can be initiated or no tensile fracturing is possible at all due to plasticity.
