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Distinguishing natural from induced fractures in image logs
 
 

Contents

Introduction  
The Rules  
Orientations of breakouts and induced fractures  
Petal fractures, centerline fractures, and petal-centerline fractures  
Schematic log images and cross-sections illustrating identification criteria  
Quiz  



Introduction

 

Induced tensile fractures are the most common and easily visible induced fractures observed in image logs and core. Induced fractures are easily distinguished from natural fractures in core by visually examining fracture surface morphology and the geometric relationships between the core and the fracture shape, origin flaw and propagation path (Kulander et al; 1979, 1990). Individual fractures cannot be identified positively as natural or induced based solely on the fracture trace in an image log. However, the origin of a group of fractures is determinate from image log data because natural fractures and induced fractures have different geometries relative to the borehole. If breakouts are present and if the natural fractures formed in a stressfield significantly different from the present-day stressfield, then the orientation of the fractures relative to the breakouts provides an additional (though less rigorous) criteria for distinguishing the two types of fractures. This webpage provides some simple rules for using image logs to differentiate natural fractures of all types from induced tensile fractures and to distinguish induced fractures that form after passage of the bit from petal and petal-centerline fractures which form ahead of the bit.

These other pages on this site provide useful background material for this page:

Breakout and induced fracture basics

Regional fracture orientations
 CONTENTS 


The Rules

  Rule 1: The stacking rule.  Induced fractures that do not completely cut the wellbore have a consistent orientation and tend to appear at the same azimuth in the image but natural fractures with a consistent orientation that do not completely cut the wellbore appear at different azimuths in the image. 

Rule 2: The aperture rule.  Induced tensile fractures are always open; natural fractures may be open or partially to completely mineralized or gouge-filled. 

Rule 3: The continuity rule.  The continuity of a fracture trace is not an indication of its origin. 

Rule 4: The orientation rule.  The orientation of a fracture relative to the in-situ (present day) stress is not an indication of its origin. 

Rule 5: The breakout rule.  Hydraulically induced fractures form in, and tend to be restricted to, the tensile quadrants of the wellbore wall, which are 90 from the breakouts. Petal fractures, another common type of tensile induced fracture, form ahead of the bit in what will become the compressive quadrants (where breakouts develop); poorly developed petal fractures tend to be restricted to the compressive quadrants. 

Rule 6: The symmetry rule.  Individual natural fractures are often symmetrically developed on opposite sides of the borehole; petal and centerline fractures are nearly always symmetrical; but hydraulic fractures are usually asymmetrically developed. 

Note: 
  1. These rules apply to sections of the wellbore where the in-situ stresses maintain a constant orientation relative to the wellbore. The in-situ stress can vary in orientation and magnitude along the length of the wellbore due to recent tectonic activity, lithologic changes and other reasons.
  2. This poster does not directly address induced shear fractures. However, Rule 1 is applicable to them.


 CONTENTS 


Orientations of breakouts and induced fractures

  Schematic cross-section of a wellbore showing the orientation of breakouts and induced hydraulic and centerline fractures relative to the borehole perpendicular in-situ earth stress components. Broken-out (missing) material shown in dark gray. In most of the world one of the three principal stresses is oriented vertically, which requires the other two to be oriented horizontally. However, inclined stressfields do occur, especially in tectonically active areas. Breakouts form in response to the minimum and maximum stress components that are oriented perpendicular to the wellbore. Note that these components may or may not be principal stresses depending on the orientation of the wellbore relative to the in-situ stressfield. Induced fractures tend to form perpendicular to the least principal stress, so that well-developed, borehole-parallel induced fractures form when 3 (the minimum principal stress) is oriented perpendicularly to the borehole. Hydraulic induced fractures tend to be inclined to the wellbore when 3 is inclined to the wellbore, although they may not form perpendicular to 3 in this case. Imagine that Figure 1 is a cross-section of a vertical well. In a normal faulting stress regime, min = 3 and max = 2. In a strike-slip faulting stress regime, min = 3 and max = 1. In a reverse faulting stress regime, min = 2 and max = 1.

 Figure 1. 


 CONTENTS 


Petal fractures, centerline fractures, and petal-centerline fractures

  Petal, centerline, and petal-centerline fractures form ahead of the bit during both coring and normal drilling operations. They normally extend beyond the final borehole diameter so that they can usually be correlated between core and image logs. The direction of fracture propagation is easily determined in core and is always downhole (towards the bottom of the screen in the images shown here). Petal fractures form just ahead of the bit and are due to excessive bit weight. Centerline fractures propagate ahead of the bit but probably within approximately 1/2 meter of the bottom of the hole. Centerline fractures are driven by a combination of mud pressure and bit-induced stresses.

Figure 2.  Photos of 4 inch (10cm) diameter core of the sandstone reservoir rock from an oil well in West Texas, U.S.A. showing petal, centerline and petal-centerline fractures. 
Figures 2a &2b.  Detailed photos.  The centerline fracture is the continuous fracture that bisects the core in these two detailed photos.

 Figure 2a. 


Petal fractures ("petal" as in "flower petal") are the curved fractures that begin on the edge of the wellbore and curve parallel to the centerline fracture in the center of the core. Petal fractures can form in isolation whether or not a centerline fracture is present (Figure 2a). Petal fractures that grow to become a centerline fracture or that join with a centerline fracture (Figure 2b) are termed petal-centerline fractures.

 Figure 2b. 


Borehole stress interpretations. Centerline fractures form perpendicular to the least principal earth stress. Well-developed centerline fractures form only when the least principal stress is perpendicular or nearly perpendicular to the wellbore. The orientation of centerline fractures gives the orientation of the least principal stress and indicates the tensile quadrant of the wellbore but the location of a centerline fracture is not an accurate guide to the location of the tensile quadrant because centerline fractures propagate ahead of the bit and do not accurately track the center of the well. In contrast, petal fractures always accurately mark the compressive quadrant of the wellbore wall because they form immediately ahead of the bit.


Figure 2c.  Approximately 6.5 feet (2 meters) of continuous core.  Note how the centerline fracture maintains a constant azimuthal orientation but but gently wanders from side to side, and does not stay centered on the core. Note that at the top of the photograph the centerline fracture is entirely within the compressive quadrant of the wellbore. The petal fractures consistently originate in the compressive qudrant, propagate toward the centerline fracture, and curve smoothly into it. Centerline fractures can start and stop as drilling conditions change or as the well enters different lithologies with different mechanical properties and stress states. The centerline fracture forks and dies near the bottom of this photograph, just below the last marked handling fracture. The fracture at the bottom of this photo is a petal fracture. This petal grew into another centerline fracture downcore.
 Figure 2c. 

Handling fractures Handling fractures in this core were caused by flexure of the core barrel when it was laid on the rig floor. Some of the handling fractures are marked with an H. Note that the handling fractures abut (terminate against) the petal and centerline fractures proving that the handling fractures are younger. The handling fracture at bottom left of this photograph nucleated on the tip of a petal fracture and propagated outward to the edge of the core.



Figure 3.  CBIL images of oil well in limestone from Oklahoma, U.S.A. showing petal, centerline and petal-centerline fractures.  The images show the amplitude ultrasonic pulses reflected from the wellbore wall by a rotating transducer. The images are compressed in depth (the vertical dimension). 

Figures 3a & 3b show respectively unrolled (map view) and rolled (synthetic core view) of CBIL images showing a centerline fracture that nucleates in a group of petal fractures, propagates downcore, and exits the core at a petal fracture that is concave uphole and propagated from the center of the core outwards. Petal fractures that are concave uphole are relatively rare.

Figure 3c is a detailed view of the origin of the centerline fracture but the look direction is opposite that of Figure 3c. Compare with Figure 2b.
 Figure 3a 
4016-4036 ft (1,224.1-1,230.2 m)



 Figure 3b 
4016-4036 ft (1,224.1-1,230.2 m)



 Figure 3c 
4012-4032 ft (1,222.9-1,229.0 m)





 CONTENTS 

Schematic log images and cross-sections illustrating identification criteria

  Natural fractures are distinguished from induced tensile fractures in image logs using gross geometric criteria. These criteria are expressed succinctly as the six rules for distinguishing natural from induced fractures in image logs. This section provides more detailed explanations and illustrations of these geometric criteria and provides criteria for distinguishing pre-drill from post-drill fractures. The principals are illustrated with schematic image logs and cross-sections similar to Figure 3.

1.  The most fundamental principle for discriminating between natural fractures and all types of induced fractures, tensile or shear:  Induced fractures are geometrically related to the wellbore but natural fractures are not.  This relationship causes the traces of natural fractures that do not cross the entire wellbore to appear at different azimuthal positions in image logs even if the fractures have similar orientations. However, the traces of induced fractures that do not cross the entire wellbore tend to stack in depth.

2.  The extent of a fracture is not an indication of its origin because:
  • Both natural and induced fractures can cross the entire wellbore or can be confined to a particular lithology.
  • The wellbore may incompletely sample a natural fracture.
  • Induced fractures may be confined to a particular part of the wellbore wall.
3.  The orientation of a fracture relative to the in-situ (present-day) stress is not an indication of its nature even though induced fractures are consistently oriented relative to the in-situ stress because:
  • Most natural rock fractures formed in response to paleostresses (ancient stresses) that are unrelated to the in-situ stress.
  • In tectonically active areas natural fractures may have formed recently or may even be forming at the present time so that induced fractures can form with the same orientations as natural fractures.
  • The paleostresses that caused natural fracturing may coincidentally have been oriented similarly to the in-situ stress.
  • Induced fractures are not necessarily parallel to the wellbore because sigma3 may not be perpendicular to even a vertical wellbore.
  • Petal fractures are never perpendicular to sigma3.
  • Hydraulic induced fractures are not necessarily perpendicular to sigma3 in the wellbore because the stress-state of the wellbore is complexly related to the relative magnitudes of the in-situ stresses and the orientation of the wellbore relative to them. A hydraulically induced fracture will become perpendicular to sigma3 as it grows away from the wellbore.
4.  Distinguishing pre-drill from post-drill fractures.

A.  Symmetrical fractures are usually pre-drill:
  • Natural fractures are always pre-drill. They are often symmetrically developed across the wellbore because natural fractures with apertures sufficient to resolve in image logs often have a minimum diameter equal to or greater than the wellbore so that fracture traces tend to be present on both sides of the image.
  • Petal, centerline, and petal-centerline fractures are usually larger than the wellbore so that they also show two symmetrical traces.
B.  Asymmetrical fractures are usually post-drill:
  • Hydraulic fractures form after passage of the bit due to excessive mudweight, swabbing (pistonnage), surging, water hammering, overdriving turbo-drills on startup and other reasons. Tensile fractures can also form by thermal contraction caused by cooling of the rock by the mud and for a variety of other reasons. Regardless of their origin, fractures that form behind the bit form in isolation because opposite sides of the wellbore are mechanically decoupled. Consequently, they usually are asymmetrically developed.


Schematic llustrations
 

Figure 4a.  

The well has randomly caught four wellbore-parallel fractures, which are color coded. Projections of the fractures are shown on the bottom of the core view at right to facilitate visualization.

If these fractures were induced, they would appear at the same azimuth and there would be 180 degrees between each trace.

These fractures have no particular relationship to the breakout orientations because they are natural.

Figure 4b.  

Fractures only developed in the blue lithology. In the image log view the yellow sine waves show fracture orientations, the black lines show the fracture traces. All of the fractures have the same orientation. The fracture traces are pressent at different azimuths in the wellbore because the well randomly samples a population of small fractures.

These fractures have no particular relationship to the breakout orientations because they are natural.

Figure 4c.   The stratigraphy, orientation of bedding, and fracture orientations are identical in this figure and Figure 4b. Because these fractures are induced they form at a consistent azimuth in the wellbore.

The lowermost fracture is the same in Figure 4b and this figure. If natural fractures were present in this orientation in this well, it would not be possible to determine if that particular fracture was natural or induced.

These fractures are induced so they develop only in the tensile quadrant of the wellbore, at 90 degrees to the breakouts.

Figure 4d.  

Schematic log images showing wellbore parallel (left) and inclined (right) post-drilling induced fractures.

The fractures are only developed in the tensile quadrant of the wellbore, at 90 degrees to the breakouts. The orientations of these fractues often can be used as a rough guide to the orientation of the minimum principal stress if the stress axis is not too far out of perpendicular with the wellbore, however it is usually difficult or impossible to make reliable orientation measurements of them in image logs because they are not symmetrically developed. A common interpretation error is to incorrectly correlate two such independent fracture traces (red sine wave). Note that two independent fracture segments can coincidentally lie on the correct sine wave (green sine wave).

Figure 4e.  

Petal fractures are induced, form in the compressive quadrants, and have a consistent relationship to the breakouts. However, the centerline fracture formed ahead of the bit and may wander around all over the image, although on average a well-developed centerline fracture will closely track the center of the tensile quadrant.

Petal fractures are curved so that they cannot be fit by a sine wave (colored sines).

Figure 4f.  

Petal fractures may only develop in one lithology.


 CONTENTS 

STOP!

 DON'T READ THE ANSWER UNDER THE IMAGE!
 

Look at this image and try to decide on your own: Are they natural or are they induced?

ABOUT THE IMAGE:  False-color STAR image (red: amplitude, green: microtopography, blue: resisitivity) from an oil well in granite. Depths in meters below an arbitrary subsurface datum.


 Figure 5 

 

ANSWER:  Image shows two natural fractures (A) and well-developed induced fractures arranged in parallel rows on opposite sides of the wellbore (B). Breakouts are absent. Fracture C is less easy to interpret because it crosses the entire borehole and is oriented parallel to the smaller induced fractures above and below it. The induced fracture immediately above C crosses into the compressive quadrant of the borehole, indicating (but not proving) that fracture C is also induced. These induced fractures appear to be post-drill, probably hydraulic, fractures rather than petal fractures because they are asymmetrically developed and nucleated in two parallel rows on opposite sides of the borehole rather than nucleating on one side of the well and propagating towards the centerline. Note that induced fractures abut both natural fractures, which shows their relative ages.



 CONTENTS 

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