1. Effect of Drilling Variables on the
Characteristics of Fracture-Like
Breakouts in Two Reservoir-Type
Sandstones
by
Russell J. Sheets
A thesis submitted in partial fulfillment of the
requirement for the degree of
Master of Science
(Geological Engineering)
University of Wisconsin – Madison
(2004)

2. What are Borehole Breakouts?
Breakouts are diametrically opposed regions
of stress-induced borehole elongation that
are the result of compressive failure at the
borehole wall and extending along the h
springline.

3. Borehole Breakout Shapes
h
H H
h
Westerly Granite Berea Sandstone
V-shaped breakout Fracture-like breakout

4. Why are we interested in borehole
breakouts?
V-shape breakout orientation and dimensional
characteristics have been employed to estimate far-
field in situ stress conditions.
Fracture-like breakout length has been correlated to
the maximum principal stress with laboratory
experiments, but it has not been utilized to estimate in
situ conditions.
Furthermore, fracture-like breakouts appear to be the
result of compaction band formation.

5. • Determine the effect of several drilling
variables on fracture-like breakout
characteristics
• Characterize the failure mechanism that
produces fracture-like breakouts using
microscale analysis techniques, and
investigate the relationship between fracture-
like breakouts and compaction bands
Research Objectives

6. Presentation Outline
• Physical and mechanical rock properties
• Experimental procedure
• Fracture-like breakout failure mechanism
• Drilling variables affecting breakout shape
and dimensions
• Conclusions

7. Reservoir-type Sandstones:
Mansfield and St. Meinrad Sandstones
Property Mansfield sandstone St. Meinrad Sandstone
Mineral
composition
90% quartz 89% quartz
Primary
cementation
Sutured grain contacts Sutured grain contacts
Mean grain size 0.24 mm 0.13 mm
Effective porosity 27% 25%
Uniaxial
compressive
strength
19.3 MPa 38.8 MPa

8. Experimental Setup – Loading Frame
Biaxial
Load Cell
H,h)
Hydraulic
Cylinder
(v)
Drill
bit
Electric drillPotentiometer
Depth Gage
Rock
Specimen
Flow Meter
Load
Cell

9. 0
10
20
30
40
50
60
0
20
40
60
80
100
120
0 500 1000 1500 2000 2500 3000 3500
Stress(MPa)
DrillBitPenetration(mm)
Time (sec)
Loading cycle and drilling sequence
H
v
h
Bit
Penetration

10. Presentation Outline
• Physical and mechanical rock properties
• Experimental procedure
• Fracture-like breakout failure mechanism
• Drilling variables affecting breakout shape
and dimensions
• Summary and conclusions

11. Typical breakout shapes
Mansfield Sandstone St. Meinrad Sandstone
H H

12. Consistent
Breakout Width
Bi-cone bit R = 8.0 mm
0.06 l/s
H H
Various far-field
stress conditions
0.07 mm/s

13. St. Meinrad Sandstone
h
Undamaged specimen
Breakout
tip

14. h
Fracture-like breakouts result
from compaction bands
A
B

15. A
Porosity = 19%
B
Porosity = 24%

16. Drilling Variables
• Far-field stress magnitudes: ( h < v < H )
• Drill-bit type: bi-cone and diamond drill-bit
• Drill-bit size: 8.0 – 19.5 mm radii
• Drilling-fluid flow rate: 0.01 – 0.1 l/s
• Drill-bit penetration rate: 0.07 – 0.54 mm/s
• Drilling-fluid (mud) weight: SG = 1.00 to 1.05

17. Far-field Stress Effect –
St. Meinrad SandstoneBreakoutLength(mm)
H (MPa)
h, v
15
20
25
30
35
50 60 70 80 90 100 110 120
30, 40
40, 50
50, 60
60, 70

18. Drill-Bit Type Comparison
Diamond-Impregnated Coring Bit Bi-cone Rotary Drill-Bit
Radius = 16.0 mm

19. Drill-Bit Type Comparison -
Mansfield Sandstone
H
BreakoutLength(mm)
20
22
24
26
28
30
32
Bi-cone
Diamond
18
20
22
24
26
28
30
28 30 32 34 36 38 40 42
Bi-cone
Diamond
 (MPa)
h = 10 MPa
v = 20 MPa
h = 20 MPa
v = 30 MPa

20. Drill-Bit Type Comparison –
St. Meinrad Sandstone
H
BreakoutLength(mm)
 MPa)(
15
20
25
30
35
15
20
25
30
35
35 40 45 50 55 60 65
Bi-cone
Diamond
Bi-cone
Diamond
h = 20 MPa
v = 30 MPa
h = 30 MPa
v = 40 MPa

21. Drill-bit Size
Effect
h = 40
v = 50
H = 70 MPa
H
R = 8 mm
R = 11.5 mm
R = 16 mm
h
St. Meinrad Sandstone

22. Drill-bit Size Effect –
St. Meinrad SandstoneBreakoutLength(mm)
Borehole Radius (mm)
40, 50, 70
40, 50, 80
h, v, H
8
12
16
20
24
28
32
36
40
6 8 10 12 14 16 18

23. Drilling-fluid Flow
Rate Effect
h = 30
v = 40
H = 50 MPa
0.01 l/s
0.03 l/s
0.06 l/s
H
h
Mansfield Sandstone

24. Drilling-fluid Flow Rate Effect –
Mansfield Sandstone
15
20
25
30
35
40
45
50
0 0.02 0.04 0.06 0.08 0.1 0.12
30, 40, 50
20, 30, 50
20, 30, 40
BreakoutLength(mm)
Flow Rate (l/s)
h, v, H

25. Drill-bit Penetration Rate Effect –
Mansfield Sandstone
h, v, H
15
20
25
30
35
40
45
50
0 0.1 0.2 0.3 0.4 0.5 0.6
30, 40, 50
20, 30, 50
20, 30, 40
BreakoutLength(mm)
Penetration Rate (mm/s)

26. Drilling-fluid (mud)
Weight Effect
SG = 1.00
SG = 1.02
SG = 1.04
H
h = 40
v = 50
H = 90 MPa
St. Meinrad Sandstone
SG = 1.05

27. Breakout Tip Using
Drilling Mud (SG = 1.04)
Mud
cake
Breakout
tip
Mansfield Sandstone

28. How does drilling mud effect fracture-like
breakouts in field wellbores?
Results suggest that heavier bentonite drilling
mud will prevent fracture-like breakouts.
This is practical for an entire wellbore, except
for the production zone.
Flushing the mud cake from the production
zone with a less viscous fluid will cause the
damage zone to be removed, and induce a
fracture-like breakout.

29. Conclusion
Can fracture-like breakout length be used to
estimate the maximum far-field principal stress
magnitude?
Length is related to in situ stress magnitudes,
however several drilling variables alter this
dimension relationship.
If the drilling conditions are known and
simulated in the laboratory, then breakout
dimensions can be correlated to far-field
stresses and used to estimate in situ conditions.