Much is now made about “Stress Shadows” and their impact on hydraulic fracturing - particularly in multi-stage horizontal laterals commonly used in Unconventionals. Unfortunately, there is no standard definition of Stress Shadows and, as a result, there is much confusion over what they are and aren’t, and, most importantly, why they can have a significant impact on hydraulic fracturing operations. The goal of this presentation will be to address this confusion and more fully explain Stress Shadows and their impact. The creation of hydraulic fracture width during a stimulation generates a change in the stress field, which alters all three principal stresses as well as tip shear stresses. These stress changes are the Stress Shadows. As hydraulic fracture propagation is often controlled by the stress field, Stress Shadows may change the propagation path for subsequent hydraulic fractures or, as seen in cluster fracturing, propagation may be impeded completely. The presence of natural fractures and weakness planes can also affect, and be affected by, Stress Shadows. At the hydraulic fracture tip, shear stresses are generated that offer the potential to shear and open closed natural fractures and weak planes – and if opened, it may be possible to stimulate them. Equally important, behind the hydraulic fracture tip Stress Shadows increase the magnitude of all three principal stresses, which tends to close weakness planes making them more difficult to stimulate. Because of these effects and others, understanding and designing for the impact of Stress Shadows is often critical for stimulation optimization.

1. Stress Shadows:
How and Why They Affect
Hydraulic Fracturing in
Unconventional Shale Plays
Neal Nagel, Ph.D.
Chief Engineer, Oilfield Geomechanics LLC (OFG)

2. Presentation Outline
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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April 2015
I. The Geomechanics of Hydraulic Fracturing
II. Stress Changes from a Single Hydraulic Fracture
III. Stress Changes from Multi-Stage Hydraulic
Fractures and the Potential for Fracture Rotation
IV. Stress Shadows and Cluster Fracturing
V. Stress Shadows and Naturally Fractured Shales
VI. Hydraulic Fracturing and Tip Shear Stresses
VII. Stress Shadows and Multi-Well Completions
VIII.Conclusions

3. Geomechanics and Hydraulic Fractures
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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For Hydraulic Fracturing:
e=s/E
Where: w =Fracture width (deformation)
L =Fracture half-length
Dp=Net pressure
x =Distance from wellbore
2
1)( x
E
pL
xw 
D

Generalized Hooke’s Law:
(basic geomechanics)
Where: e=Strain (DL/L)=normalized deformation
s=Stress (Force/Area)
E=Young’s modulus (Stiffness)
E
σε 
Basic geomechanics says
that, for an elastic
material, deformation is
proportional to stress
divided by stiffness.
In hydraulic fracturing,
fracture width
(deformation) is
proportional to the net
pressure (stressing the
formation) divided by
stiffness.
But….

4. Geomechanics and Hydraulic Fractures
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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April 2015
Generalized Hooke’s Law:
(basic geomechanics)
s=E/e es  E
For Hydraulic Fracturing:
)(xwE s
Increasingnormalized
distancefromfracture
face
½ Fracture
NormalizedStress
Just as stress causes deformation,
deformation (i.e., fracture width)
causes a change in the stress field.
This was shown as early as 1946 by
Sneddon and others.

5. “Stress Shadows”: Single Hydr. Fracture
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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The generation of
fracture width causes a
change in the stress
field. Here, the
simulated increase in
the minimum horizontal
stress (DShmin) – often
called the “Stress
Shadow” – is shown in
cutaway view.

6. Stress Shadow Basics
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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April 2015
Just as hydraulic fracture width varies laterally and vertically, so does the
magnitude and shape of the change in Shmin (Stress Shadow).

7. DShmin ~f(height) – Single Stage
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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The magnitude and depth of the change in Shmin into the formation is
controlled by width - f(net pressure) - and height (PKN-type fracture).

8. Stress Shadows  DShmin
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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April 2015
Unfortunately, the common perception that the “Stress Shadow” is just a
change in Shmin is incorrect as all the principal stresses may change.
Furthermore, shear stresses are generated as well.

9. Stress Changes Around a Hydr. Fracture
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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From original stresses (A), a fracture
with small net pressure (B) shifts all
principal stresses higher. If the net
pressure is high enough (C), Shmin
and Shmax may reverse.

10. Stress Shadows from Dual Hydr. Fractures
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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When multiple hydraulic fractures are generated from a single
horizontal as shown, the Stress Shadows interact and combine.

11. Shmin Profiles – Dual Hydr. Fractures
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At close fracture spacing (A), there is
combining of the change in Shmin for
all heights. As the spacing increases
(B), the combined increase is smaller
until (C) there minimal stress
interaction between fractures.

12. Stress Shadows from Dual Hydr. Fractures
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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April 2015
When multiple hydraulic fractures are generated from a single horizontal, the
Stress Shadows interact and combine; however, timing does play a role as
shown.

13. Stress Shadows Along the Wellbore
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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April 2015
Along the horizontal wellbore, the increase in Shmin will be determined by
frac spacing and net pressure. For variable frac spacing (planned or not), the
change in Shmin will be highly variable.

14. Stress Shadows: ISIP Field Data
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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April 2015
It is difficult to get direct stress measurements along a horizontal, so ISIP is
used as a reasonable analog. As shown, field ISIP data clearly confirm the
generation of an increase in Shmin from toe to heel.

15. On Potential Hydraulic Fracture Rotation
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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April 2015
Owing to the nature of hydraulic
energy, hydraulic fractures propagate
towards the path of least resistance.
As the Stress Shadow from a hydraulic
fracture causes a variable change in
stress (above), subsequent fractures
will tend to grow away from the 1st
(right).

16. Frac Rotation - Field Evidence
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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Under certain
conditions, the stress
shadow will cause
fracture rotation.
What is the
significance for MS
evaluations??
This case from the
Vaca Muerta suggests
fracture rotation after
the 4th frac stage (S4)

17. Stress Shadows and Cluster Fracturing
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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April 2015
Because of the prohibitive cost of pumping into each perforation cluster,
multiple clusters are used per frac stage. If/when there is equal wellbore flow
diversion, the Stress Shadow greatly influences hydraulic fracture growth.

18. Stress Shadows and Cluster Fracturing
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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“Limited Entry” is a concept by which the number and size of perforation is
varied in order to achieve flow diversion within the wellbore. As shown, flow
diversion (w/o LE is no diversion) significantly affects hydraulic fracture
propagation from 3 and 4 cluster stages.

19. Stress Shadows and Shale Plays
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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As presented, the Stress Shadow responds to fracture width, which is a
function of net pressure. Net pressure is related to the resistance to fracture
propagation – as net pressure gets smaller, so does the Stress Shadow.

20. Stress Shadows and Natural Fractures
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When the natural fracture pattern is
extensive and conductive, its
orientation may dominate the flow of
fluids during a hydraulic fracture
stimulation (left).
For a dominate natural
fracture system not aligned
with the stress field (right,
above), the generated Stress
Shadow (far right) becomes
highly complex.

21. Hydraulic Fractures and Shear Stresses
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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Hydraulic fractures also generate significant shear stresses along their
leading edge; however, the dominate orientation of the shear stresses
changes depending upon position along the edge.

22. Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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April 2015
Tip Shear Area and Nat. Frac. Orientation
Because of the influence
of natural fracture friction
and the stress normal to
it, some orientations of
natural fractures are
more prone to slip due to
hydraulic fracture tip
shear stresses.
As shown, as natural
fracture orientation varies
from parallel (0°) to
perpendicular (90°) to the
fracture, the amount of
rock sheared changes.

23. DShmin Contours Ahead of the Tip
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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Most hydraulic fractures
propagate in tension,
which means at the tip
the earth compressive
stresses must be
overcome.
Ahead of the actual
fracture, the compressive
stresses are reduced –
which becomes an easier
propagation path for
approaching hydraulic
fractures.

24. Tip Movement for Overlapping HFs
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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April 2015
The inset shows
hydraulic fractures
propagating from
parallel wellbores in a
Zipper configuration.
The plot shows the tip
position as a function of
injection time.
Initially, the fractures propagate inwards and outwards uniformly; however,
at time T1 the tips sense the reduce stress region and accelerate towards
each other. Then, at T2, the inner tips stop and all propagation is outwards
due to Stress Shadows.

25. Zipper Frac Stress Shadows: Well#1, HF#2
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26. Conclusions and Comments
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
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• Stress Shadows are a real and easily explained geomechanical
effect of hydraulic fracturing.
• Colloquially, Stress Shadows are considered to be the change in
Shmin; however, Stress Shadows include the change in SHmax and
Sv as well as tip shear stresses.
• When hydraulic fractures are closely spaced, the increase in Shmin
is additive.
• Because the change in Shmin is not simply planar, hydraulic
fracture rotation is possible (particularly with high net pressures).
• Shear stresses are generated along the edge of a hydraulic
fracture, which may serve to shear local natural fractures.
• Multi-well Stress Shadows are very complex and not easily
predicted.

27. Primary funding is provided by
The SPE Foundation through member donations
and a contribution from Offshore Europe
The Society is grateful to those companies that allow their
professionals to serve as lecturers
Additional support provided by AIME
Society of Petroleum Engineers
Distinguished Lecturer Program
www.spe.org/dlApril 2015