1. The SPE Foundation through member donations
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
April 2015

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

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

4. Geomechanics and Hydraulic Fractures
Generalized Hooke’s Law:
(basic geomechanics)
Where: e=Strain (DL/L)=normalized deformation
s=Stress (Force/Area)
E=Young’s modulus (Stiffness)
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….
For Hydraulic Fracturing:
e=s/E
Where: w =Fracture width (deformation)
L =Fracture half-length
Dp =Net pressure
x =Distance from wellbore
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

5. Geomechanics and Hydraulic Fractures
Generalized Hooke’s Law:
(basic geomechanics)
s=E/e
Normalized Stress
Increasing normalized distance from fracture face
For Hydraulic Fracturing:
½ Fracture
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.
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

6. “Stress Shadows”: Single Hydr. Fracture
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.
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

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

8. DShmin ~f(height) – Single Stage
The magnitude and depth of the change in Shmin into the formation is controlled by width - f(net pressure) - and height (PKN-type fracture).
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

9. Stress Shadows = DShmin
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.
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

10. Stress Changes Around a Hydr. Fracture
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.
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

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

12. Shmin Profiles – Dual Hydr. Fractures
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.
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

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

14. Stress Shadows Along the Wellbore
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.
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

15. Stress Shadows: ISIP Field Data
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.
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

16. On Potential Hydraulic Fracture Rotation
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).
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

17. Frac Rotation - Field Evidence
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)
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

18. Stress Shadows and Cluster Fracturing
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.
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

19. Stress Shadows and Cluster Fracturing
“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.
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

20. Stress Shadows and Shale Plays
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.
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

21. Stress Shadows and Natural Fractures
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.
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

22. Hydraulic Fractures and Shear Stresses
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.
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

23. 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.
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

24. DShmin Contours Ahead of the Tip
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.
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

25. Tip Movement for Overlapping HFs
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.
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

26. Zipper Frac Stress Shadows: Well#1, HF#2
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

27. Conclusions and Comments
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.
Stress Shadows & Hydraulic Fracturing - Nagel SPE DL
April 2015

28. The SPE Foundation through member donations
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
April 2015