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ABSTRACT Predicting the near-well pathway of a hydraulic fracture is important for understanding development of tortuosity that can impact on injection pressures, proppant placement and production rates. In this paper, a 2D fully coupled hydraulic fracture model is used to study how these pathways depend on both wellbore geometry and starter fracture sizes. Non-circular wellbores and cased circular wellbores are studied and taken to correspond to a wellbore with breakouts and with a pressure mismatch between the wellbore and fracture, respectively. The initial starter fracture length for cased wellbores is varied to study its effect on fracture path. The formation is assumed to be elastic and impermeable and the fluid is incompressible and Newtonian. The numerical results show that, for the conditions considered in this paper, the fracture reorientation and path can be predicted for each non-circular wellbore by the value of a dimensionless parameter that has previously been shown to control fracture paths for fracture initiation and growth from circular wellbores. For the cased conditions with a reduced or zero pressure acting on the wellbore walls, the initial fracture length is also an important factor affecting fracture trajectory. As the low pressure wellbore results in the radial stress becoming the minimum principal stress, a fracture initiating from a short flaw will orbit around the wellbore, while the initiation from a longer flaw allows the fracture to escape the stress conditions near the wellbore to eventually follow the far-field maximum principal stress direction.
1. INTRODUCTION
There are a variety of reasons why non-circular wellbore cross-sections may be formed. Possible causes include the mechanical action of the drill string on the wellbore after drilling and the localized failure (breakout) induced by in situ stress concentrations exceeding the compressive strength of the rock. Such breakout processes under high horizontal deviatoric stress result in a non-circular wellbore shape. In vertically deviated wells, rock failure may occur adjacent to the start of a sidetrack, and soft or poorly consolidated rocks can be washed out from weak formations. The stress concentration along the wall of such a non-circular wellbore is therefore not only dependent on the in situ stresses, but also on the wellbore shape. Most of the published work [1-6, among others] deals with circular wellbores, and there has been little work reported on fracture initiation and growth from non-circular fluidfilled wellbores. These studies either treated the starter fractures as stationary and with given width, or assumed the fracture trajectories a priori. The effect of fluid viscosity on pressure variation was not dealt with in these studies. Hydraulic fracture initiation and growth from a noncircular wellbore will differ from initiation from a circular wellbore owing to the change in stress near the well, although the ideal conditions are useful to generate classical analytic solutions. The accurate description of the fracture growth process under complex borehole conditions is important not only for fracture design, but also for interpretation of well logs used for in situ stress estimation.
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