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Stress Shadows: How and Why They Can Affect Hydraulic Fracturing in Both Conventional and Unconventional Plays

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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.

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Stress Shadows: How and Why They Can Affect Hydraulic Fracturing in Both Conventional and Unconventional Plays

  1. 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. 2. Presentation Outline Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 2 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. 3. Geomechanics and Hydraulic Fractures Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 3 April 2015 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. 4. Geomechanics and Hydraulic Fractures Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 4 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. 5. “Stress Shadows”: Single Hydr. Fracture Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 5 April 2015 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. 6. Stress Shadow Basics Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 6 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. 7. DShmin ~f(height) – Single Stage Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 7 April 2015 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. 8. Stress Shadows  DShmin Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 8 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. 9. Stress Changes Around a Hydr. Fracture Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 9 April 2015 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. 10. Stress Shadows from Dual Hydr. Fractures Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 10 April 2015 When multiple hydraulic fractures are generated from a single horizontal as shown, the Stress Shadows interact and combine.
  11. 11. Shmin Profiles – Dual Hydr. Fractures Stress Shadows & Hydraulic Fracturing - Nagel SPE DL April 2015 11 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. 12. Stress Shadows from Dual Hydr. Fractures Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 12 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. 13. Stress Shadows Along the Wellbore Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 13 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. 14. Stress Shadows: ISIP Field Data Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 14 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. 15. On Potential Hydraulic Fracture Rotation Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 15 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. 16. Frac Rotation - Field Evidence Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 16 April 2015 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. 17. Stress Shadows and Cluster Fracturing Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 17 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. 18. Stress Shadows and Cluster Fracturing Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 18 April 2015 “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. 19. Stress Shadows and Shale Plays Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 19 April 2015 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. 20. Stress Shadows and Natural Fractures Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 20 April 2015 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. 21. Hydraulic Fractures and Shear Stresses Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 21 April 2015 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. 22. Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 22 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. 23. DShmin Contours Ahead of the Tip Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 23 April 2015 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. 24. Tip Movement for Overlapping HFs Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 24 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. 25. Zipper Frac Stress Shadows: Well#1, HF#2 Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 25 April 2015
  26. 26. Conclusions and Comments Stress Shadows & Hydraulic Fracturing - Nagel SPE DL 26 April 2015 • 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. 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

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