Abstract
While numerous definitions of what makes Unconventional Plays ‘unconventional’ have been put forth, ranging from the reliance on horizontal wells and hydraulic fracturing technology to the often ultra-low permeability of the matrix, a key characteristic of Unconventionals is the impact of geomechanics on hydraulic fracturing in reservoir formations with natural fractures and/or weakness planes (e.g., bedding planes). Critical geomechanics effects include the influence of: 1) all three principal stress magnitudes; 2) anisotropy in the stress magnitudes; 3) the orientation of the stress field; 4) in-situ pressure (e.g., pressure within the natural fractures and weakness planes); 5) natural fracture and weakness plane frequency and connectivity; 6) natural fracture and weakness plane orientation relative to the stress field; 7) natural fracture and weakness plane mechanical properties (elastic properties such as stiffness as well as strength properties such as cohesion and friction coefficient); 8) natural fracture and weakness plane initial aperture; and 9) the interaction with operational parameters during a hydraulic fracture stimulation (e.g., rate, volume, and viscosity). In this paper, the authors review the importance and impact of these critical geomechanical parameters on hydraulic fracturing design and effectiveness in Unconventionals.
1. INTRODUCTION
Numerous definitions of what makes Unconventional Plays ‘unconventional’ have been put forth ranging from the reliance on horizontal wells and hydraulic fracturing technology to the often ultra-low permeability of the matrix (as low as single-digit nanodarcy). However, an important key to Unconventionals is the impact of geomechanics on hydraulic fracturing in reservoir formations with natural fractures and/or weakness planes (e.g., bedding planes), which has often made hydraulic fracture design and implementation both challenging and inconsistent in Unconventionals.
While commercial hydraulic fracturing has been conducted for more than 65 years [1], hydraulic fracture design has been based upon the assumption that equal half-length, bi-planar fractures will develop since a dominant control on fracture length is fracture height growth, which itself is dominantly controlled by the vertical profile of the minimum in-situ principal stress, Shmin [2]. This assumption is often valid because hydraulic fractures are created using hydraulic energy, which acts omni-directionally, ensuring that hydraulic fracture propagation always follows the path of least resistance, which itself is often dominated by a laterally-uniform stress field.
