Extended-reach wells present difficult drilling challenges, which if inadequately understood and addressed can yield significant downside risks and extensive non-productive time (NPT). These challenges are mainly due to complex well designs that combine high-deviation and extended-reach wellbores with difficult geology and hostile environments. Understanding the challenges and developing solutions are important to deliver the well with the proper casing specifications for production purposes. Geomechanically, due to their long reaches and high deviations, borehole instability and lost circulations are particularly dominant in the overburden shale sections of extended-reach and horizontal wells. However, a good understanding of the rock failure mechanisms and an innovative use of the wellbore strengthening techniques can mitigate these geomechanical challenges through integration with good drilling practices such as efficient equivalent circulating density (ECD) management and effective hole-cleaning strategies. In addition, the long open-hole exposure typically experienced in these wells can cause chemical, thermal and/or fluid penetration issues that can further complicate the difficult drilling conditions. These secondary influences further stress the importance of incorporating geomechanical understanding in drilling fluids formulation. This presentation focuses on the geomechanical challenges of drilling extended-reach wells. It highlights the need to integrate geomechanical solutions with appropriate drilling practices, particularly solutions based on good understanding of the intricate relationship between borehole stability, lost circulation, ECD, hole cleaning and bottom-hole assembly (BHA) optimizations in overcoming the drilling performance limiters. A case history will be presented as an example.

1. Society of Petroleum Engineers
Distinguished Lecturer Program
www.spe.org/dl
1
See Hong Ong
Integrating Geomechanics with
Operational Practices Improves Extended-
Reach Drilling (ERD) Performances

2. 2
 Industry Drilling Envelope
 ERD Well Drilling Challenges
 ERD Well Geomechanical Challenges
‒ Borehole instability
‒ Lost circulations
‒ Operational considerations
 A Case Study
‒ Offshore Eastern Canada
 Concluding Remarks
Presentation Outline

3. 3
Industry Drilling Envelope
Industry continuously
pushes the ERD envelope
Long reach + high
deviation + ‘trouble zones’
→ directional drilling
difficulties + complex casing
programs
Complex wellbores
present difficult drilling
challenges:
‒ Significant downside risks
‒ Extensive Non-Productive
Time (NPT)
What will the future bring
as we continue to push
the limits?
HD = Horizontal Departure
TVD = True Vertical Depth
MD = Measured Depth
ERD → HD / TVD ≥ 2
Source: K&M Technology
Exxon-Neftgas Z-43 (2013)
(TVD=15,900’, H=38,514’,
MD=41,667’)

4. 4
ERD Well Drilling Challenges
Typical
onshore or
shelf casing
designs
Deepwater or
ultra deep-
water casing
designs
Smith, K. (2009)
Borehole Instability
‒ Overburdens
‒ High deviations
‒ Tangent sections through ‘difficult’
formations and complex geology
Lost Circulations
‒ Narrow/low drilling margins
‒ Long reach wellbore
‒ Difficulty in managing downhole
pressures
Complex Casing Designs
‒ Equivalent Circulating Density (ECD)
management
‒ Well barrier integrity
‒ Production/injection requirements

5. 5
ERD Well Geomechanical Challenges
￭Borehole Failures
– Insufficient mud
supports
– Breakouts
• Pack-offs
• Sidetracks
￭Lost circulations
– Too high mud pressures
– Fractures
(natural/induced)
• Heavy mud losses
• Formation breathing
Sub-optimal Drilling
Practices
– Tripping
– Hole cleaning
– Barite sag
Wellbore instability refer to certain
patterns and levels of borehole failures
that are unacceptable for continuation
of drilling operations.

6. 6
Borehole Stability (I)
￭Effective mud column support is
fundamental
( ) 





+−+=
2
45tan 02'
1
γ
σ pw PPUCS
Mechanical Instability
Gradishar, J. et al (2014)
Abdul Azim, S. et al (2011)
￭Long open-hole exposure →
time-dependent failures:
– Chemical
– Thermal
– Hydraulic
￭Weak-plane failures (shale):
– Near bed-parallel drilling
– Up-dip vs. down-dip vs. cross-dip
– Mud weight + sealing capacity

7. 7
Borehole Stability (II)
Traversing Faults
￭Brittle Failures Near Fault
– Brecciated rock pieces
– ‘Rubble’ zones
￭Across Faults
– Stress rotation
– Lithology changes
￭Fault Drags
– Bedding dip and dip direction
changes
– Weak plane failures
Normal to bedding
Optimal trajectory

8. 8
Borehole Stability (III)
Chemical Instability (Shale)
￭Managing Stability
– Control swelling/pore pressure
van Oort, E. (2003)
￭Minimize:
– Pore pressure transmissions
– Loss of mud support
￭Mitigation strategies:
– ‘Exploit’ osmotic processes
– Maintain high membrane efficiency
– Seal shale micro-fractures

9. 9
￭Geomechanically, severe losses:
– Propagation of fractures
The Basics
– Narrow/low drilling margins:
• High deviations
• Geo-pressured formations
• Severe depletions
‒ Compounded by:
• Cooling
• ECD vs. borehole stability
Lost Circulations (I)

10. 10
Lost Circulations (II)
￭FB/fracture charging:
– Simultaneous fracture
initiation + propagation
Formation Breathing (FB)
￭Wellbore Strengthening (WS)
– Extend the narrow margin
– WS strategies:
• Isolation
• Hoop stress enhancement
– WS enhances:
• Fracture initiation pressure (FIP)
• Fracture propagation resistance
(FPR)

11. 11
Operational Considerations (I)
￭ Borehole instability:
– Mud weight + casing setting depth +
wellbore trajectory
￭ ECD managements:
– Circulating/tripping/reaming + mud
rheology + annular x-section
￭ Hole cleanings:
– Cuttings bed + large cavings +
enlarging borehole
– Agitation + ROP + surveillance
￭ Bit and BHA designs
– Impact on ROP and wellbore quality
– MSE response → vibrations
General
Source: PetroWiki
￭ Barite sag:
– Variations in mud density
– Well control, pack-offs, lost circulation
ROP= Rate of Penetration
BHA = Bottom-Hole Assembly
MSE = Mechanical Specific Energy

12. 12
Operational Considerations (II)
￭Hole cleaning:
– Pump rate + pipe rotation +
controlled ROP
Gradishar, J. et al (2014)
Best Practices (Gradishar, J. et al)
￭Maintain downhole pressure
supports:
– Best monitored by PWD
￭Sweeps/short trips:
– Avoid
￭Tight hole/over-pulls:
– Tested by hole cleaning cycles
– Avoided back reaming if possible
PWD = Pressure While Drilling

13. 13
Case Study: Offshore Eastern Canada (SPE 140318*)
￭HMDC’s OPA2 and ST1:
– Severe enlargements →
12¼” tangent section (800
)
– BHA left in the hole
– P&A for re-evaluation
￭Unsuccessful attempts:
− Bed-parallel drilling
− Lithology changes across
faults
− Geological structural
complexity
− Hole cleaning in enlarged
wellbore
*Hibernia Well Overcomes Challenges to Further Extend Worldwide Extended Reach Drilling (ERD) Envelope
Woodfine, M. et al (2011)
Woodfine, M. et al (2011)

14. 14
￭Instability problems:
– Correlated well with hole
enlargements and ledges at two
faults
– Severe hole enlargements were
observed in shale drilled near
bed-parallel
Strategies to Minimize Instability
Case Study: Offshore Eastern Canada (SPE 140318*)
Woodfine, M. et al (2011)
￭Use highest possible mud weights for
stability
– consistent with no lost returns
￭Maximize the attack angle
– Low attack angles in competent rocks

15. 15
￭Optimize wellbore
trajectory
– Rocks of similar
mechanical strength on
opposite sides of faults
￭Choose a wellbore
trajectory:
– Minimizes the probability of
encountering weak strength
(anisotropic) shales
￭Drop angle in 8-½” hole:
– Lower mud weight for
stability (ECD limitations)
Strategies to Minimize Instability
Woodfine, M. et al (2011)
Case Study: Offshore Eastern Canada (SPE 140318*)

16. 16
Concluding Remarks
￭Extended reach and high deviation combined with
complex geology (trouble zones) manifest challenging
drilling conditions.
￭Besides high NPTs, these challenges often result in
modifications of casing programs.
￭New paradigms in well designs must integrate
geomechanical understanding (borehole instability, lost
circulations) with good drilling practices (hole cleaning,
ECD management etc.).
￭Geomechanics provide only partial solutions!
Interdisciplinary approach incorporating continual
redesign concepts to systematically overcome drilling
performance limiters is key!

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Thank You!
Please contact me with any additional
questions/comments at:
seehong.ong@bakerhughes.com

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