Abstract

An outstanding question in the design of a hydraulic fracturing treatment is how fractures, which often initiate in non-ideal orientations under the influence of near-field stress concentrations, propagate outwards and turn towards their ideal orientations under the far-field stress conditions. Using a fully coupled 2D/3D Finite Element hydraulic fracturing model in GEOS, we simulate fracture initiation and propagation for three distinct cases. First, we consider the growth of a simple bowl shaped fracture near a free surface. The numerical results are compared against recently published experimental results and are used to calibrate the model response. Second, we consider the growth of an infinitely long axial fracture from a vertical wellbore for a range of stress states, fracture orientations, and fluid characteristics. Finally, we consider the case where a discrete axial fracture from a horizontal wellbore propagates outward and transitions to a transverse fracture.

1. INTRODUCTION

In this paper, we consider the mechanisms that govern the initiation and growth of hydraulic fractures from an uncased horizontal wellbore. This is a challenging problem because the solution extends over a wide range of scales in solid and fluid dynamics. At the onset of the problem, the fluid in the wellbore is slowly pressurized until the resulting tensile hoop stress triggers the growth of an axial fracture in the surrounding rock. Shortly after initiation, the stored energy in the wellbore drives rapid axial fracture growth in the near-wellbore stress regime and the fluid flow is dominated by viscous forces. As the fracture increases in height, its rate of growth slows and it begins to encounter the far-field stress regime, which may favor azimuthal fracture propagation. At some distance, the fracture will tend to curve towards its new ideal orientation, or simply open a new branch in the azimuthal direction. Finally, the fracture will enter a more regular stage of azimuthal growth that is controlled by the pumping rate, fluid viscosity and/or rock toughness.

The behavior of this type of hydraulic fracture is well understood at very early times, where it effectively is a half-penny shaped fracture propagating in the near-wellbore stress regime, and at very late times, where it is approximated by a PKN or penny shaped fracture propagating in the far-field stress regime [1, 2, 3]. However, at intermediate times, there is very little experimental or theoretical work available to describe its behavior.

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