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Breakouts and induced fracture basics: Don't make these common mistakes!
 
 

Contents

Introduction  
Breakout parameters  
Wellbore stress, breakouts, and induced tensile fractures  
The relationship between earth stress and wellbore stress  
A common mistake: Equating breakout orientation with principal stress orientation  
Why is The Mistake so common?  



Introduction

  THE PROBLEM:  Many practicing earth scientists are grossly confused about what breakouts represent. These folks think that:
  • The present-day minimum principal earth stress is always horizontal.
  • The breakout axis is always parallel to it so that the breakout azimuth (the compass azimuth, not the relative azimuth) directly tells you the orientation of that stress.


  • Unfortunately, these ideas are completely wrong except in two special cases. In the general case the breakout azimuth has a complex relationship or no relationship to the orientation of the minimum earth stress.

    WHY IT MATTERS:  These misconceptions have had serious consequences:
  • The misconceptions lead to the conclusion that hydrofracs will run perpendicular to the breakout axis. This error can and has resulted in entire oil fields being drilled with the wrong pattern.
  • Many software vendors have written log interpretation software that only reports breakout plunge-azimuth and ignores the plunge-angle of the breakout axis. Presenting only the plunge-azimuth is incorrect because giving only the plunge-azimuth of a breakout is like giving only the dip-azimuth of a bed. Both are 3D objects so you can't just randomly pick one angle to present and expect the output to be meaningful.


  • This web page reviews key earth-stress concepts and explains the basics of breakout and induced fracture interpretation.

     CONTENTS 


    Breakout parameters: What to measure

      Breakouts are indentations in the wellbore wall that form when present-day earth stresses cause rock failure. The mechanics of breakout formation are discussed on another page of this website.

    Breakouts nearly always form simultaneously on opposite sides of the wellbore. Although breakouts form over depth intervals, they are measured in wellbore-perpendicular cross-sections at discrete depths. Figure 1 shows such a wellbore-perpendicular cross-section.

    Breakout orientation is the most important breakout parameter. Figure 1 shows a perpendicular cross-section of a wellbore with breakouts. The breakout orientation is the orientation of the line that connects the centers of the breakouts on each side of the wellbore. Note that the axis orientation is a three-dimensional orientation. In other words, the axis has a plunge-azimuth (the horizontal angle which is given as a compass azimuth) and a plunge-angle (the vertical angle, which is usually measured downwards from horizontal and given in degrees). If your image log analysis software doesn't measure both of these angles (and most packages do not) then you should ask the company that makes it why they are providing you with such a shoddy product. The rest of this webpage explains why it is so important to measure the full three-dimensional orientation of the breakout axis.

    Breakout width is the angle subtended by the breakouts, as shown in Figure 1.

    Breakout depth is the distance from the original wellbore wall to the deepest part of the breakout, as shown in Figure 2.

     Figure 1. 

     Figure 2. 


     CONTENTS 


    Breakouts and induced tensile fractures form at constant azimuths in the wellbore.

      A breakout orientation DOES NOT necessarily indicate the orientation of the present-day minimum principal compressive stress. Instead it indicates (to a first approximation) the orientation of the minimum borehole-perpendicular component of the present-day stress field.

     Figure 3.  Wellbore-perpendicular cross section showing breakouts, induced tensile fractures, and the maximum and mimimum wellbore-perpendicular components of the present-day earth-stress field.

     Figure 4.  Schematic image log showing the geometry of breakouts and induced tensile fractures.

     

    Breakouts (dark gray rectangles in Fig. 4) and induced tensile fractures form in the compressive and tensile quadrants on both sides of the wellbore. Arrows (Figures. 3 & 4) indicate the centers of the tensile and compressive quadrants. Black lines on colored sine waves (Fig. 4) indicate induced tensile fractures that are inclined to the wellbore.

    When the minimum principal stress is perpendicular or nearly perpendicular to the wellbore:

  • Induced tensile fractures form parallel to the wellbore (parallel to the dashed lines in Fig. 4).
  • When the minimum principal stress is inclined to the wellbore:

  • As always, the tensile fractures form in the tensile quadrants.
  • The tensile fractures are inclined to the wellbore. (Blue and yellow sine waves in Fig. 4.)
  • The relationship between the induced tensile fracture orientation and the principal stress orientations is complex.
  • If the tensile fractures form ahead of the bit, then they may be present on both sides of the wellbore (black lines on yellow sine wave). More commonly these fractures develop behind the bit so that separate fractures develop independently on both sides of the wellbore (black lines on blue sine waves) because the two sides of the well are mechanically decoupled.



     CONTENTS 


    The relationship between earth stress and wellbore stress

      The three principal stresses (colored arrows) define a triaxial ellipsoid, which can be represented by the three stress ellipses in the principal planes, as shown. This figure shows a thrust-faulting regime, in which the minimum principal stress is vertical. Wells fail in response to the stresses in the wellbore-perpendicular plane.

    Wells drilled parallel to one of the principal stress axes are special cases because the wellbore-perpendicular plane is a principal plane. A principal plane is a plane that contains two of the three principal stresses. As shown in the diagram, there are three such special cases. In these cases the breakouts reflect the orientation of either the minimum or intermediate principal stress, as shown by the dashed lines.

     Figure 5. 

      A well drilled at a general angle (dark green cylindrical line) fails in response to the stresses in the wellbore-perpendicular plane (brown). The stresses in this plane are a complex combination of all three principal stresses. The shape and orientation of the stress ellipse in this plane is given by slicing the 3-dimensional stress ellipsoid in this orientation.

     CONTENTS 


    A common mistake:  Equating breakout orientation with principal stress orientation

      Colored arrows indicate the three principal stresses. Light gray parallelogram represents a horizontal plane. Dashed arrow represents the breakout axis. A wellbore (dark green cylindrical line) fails in response to the stress-state in the borehole-perpendicular plane (brown ellipse.) To a first approximation, the breakout axis (dashed double-headed arrow) indicates the orientations of the minimum and maximum stress components in this plane (double arrows). The breakout azimuth (gold line) is the horizontal component of the breakout axis orientation.

     Figure 6. 

      Note that the breakout azimuth is different from the principal stress azimuths. In the case shown here, the minimum principal stress is vertical so it has no horizontal component and the breakout azimuth reveals nothing about its azimuth.

    The breakout azimuth is at a high horizontal angle to both the intermediate and maximum principal stresses, but shows no obvious relationship to them. Because hydrofracs always run perpendicular to the minimum stress once they have propagated a few wellbore diameters, any hydrofracs driven from this well will run horizontally, not perpendicular to the breakout azimuth.

    The drawings shown in this discussion are based on the first-order assumption that breakout orientations are strictly related to the orientation of the well relative to the stress field in the rock volume around the wellbore. In detail, the relationship between the breakout azimuth and principal stress orientations is complex and also depends on the orientation of bedding, the rock properties, and the rock and wellbore fluid pressures because these affect the stress state of the wellbore wall.


     CONTENTS 


    Why are these mistakes so common?

      HISTORY: These misconceptions have the following origins:
    1. Once upon a time all wells were vertical so that breakout azimuths were always horizontal and there was no vertical-angle to be measured for the breakout axis. This is why the logging companies never gave the vertical-angle as output in their software and were/are so slow to change.
    2. About 60-70% of the Earth's crust is in a normal-faulting stress state in which the minimum and intermediate stresses are horizontal. Another 15% to 20% of the crust is in a strike-slip faulting stress state in which the minimum and maximum stresses are horizontal, so if you randomly sink vertical wells around the world then the misconceptions give correct results most of the time.
    3. Traditionally, the main use of breakout data was to predict the orientation in which hydrofracs would run. Hydrofracs always run perpendicular to the minimum principal stress, so the misconceptions gave good frac predictions most of the time.
    4. Petroleum geology was/is dominated by technological expertise developed in the U.S.. Most of the Appalachians, Texas, the Gulf Coast and a lot of California are in a normal-faulting stress state. Most of the rest of California is in a strike-slip faulting regime. So the misconceptions worked just fine in the traditional U.S. oil patch areas in the days when wells were always vertical.
    5. Until recently, most geologists, engineers and log-analysts didn't learn geomechanics as part of their university education.
     
     

    Go to Page 1.1.3  to learn more about regional earth-stress regimes.

     
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