Abstract
Multi-stage hydraulic fracturing in horizontal laterals is a key technology in the development of Unconventional plays around the world wherein many hydraulic fractures (on the order of hundreds) are initiated that can greatly increase the surface area for flow and, correspondingly, well production. Stress profiles, both near-wellbore and far-field, play an important role in the initiation of these hydraulic fractures as well as hydraulic fracture growth.
Field measurement of the minimum stress component is often done using DFITS, mini-fracs and leak-off tests, which are very scarce and rarely done on sequenced vertical layers or along horizontal laterals. With the development of dipole sonic tools in the 1970s, log-based, continuous stress profiles (‘Frac logs’) have been used in vertical wells for hydraulic fracture design. Commonly, the minimum horizontal stress, Shmin (often mistakenly called the ‘frac gradient’) has been estimated by assuming no lateral strain (a uniaxial strain assumption) within the earth (sometimes called ‘Eaton's equation’), wherein Shmin is simply a function of Poisson's ratio (from the dipole sonic log), vertical stress, pore pressure, and Biot's constant (also computed from the dipole sonic log). There are fundamental shortcomings to this approach for estimating stresses, essentially related to the change in elastic properties through geological time and plastic deformation, which essentially promotes stress release - not elastic stress buildup. Additionally, tectonic stresses were not originally considered, and Eaton's formulation was extended to include tectonic strain effects as well as the effects of elastic anisotropy (commonly vertical transverse isotropy, VTI).
For horizontal wells, the need for accurate stress measurements is even more critical than for vertical wells; however, Poisson's ratio values obtained from dipole sonic tools run in a horizontal lateral cannot be used to generate a profile of stress along the wellbore as done in vertical wells. The focus of the numerical study presented in this paper was on the limitations and inaccuracies of computing log-based stress in horizontal laterals.
The results presented in the paper provide important insight into the variation of in-situ stress along horizontal laterals as well as the associated change in breakdown pressure. As estimated breakdown pressure has become a common metric for perforation placement, the results of the presented work will impact common horizontal well completion designs in Unconventional Plays.
