This document provides information on using borehole images to analyze in-situ stress indicators. It discusses how breakouts and drilling-induced fractures in borehole images can reveal the orientation of the present-day stress field. Breakouts typically occur parallel to Shmin and indicate the minimum horizontal stress, while induced fractures occur parallel to SHmax and indicate the maximum horizontal stress. The document also covers how tools like microresistivity imagers, acoustic televiewers, and LWD tools can be used to directly observe these stress indicators and make stress orientation measurements. Additional indirect evidence from hole shape, tool motion, and defocused image pads can also provide insight into the in-situ stress field.
2. Structure 4/2
Introduction
• Present day in-situ stress
acting on the borehole wall
may cause damage that is
seen in borehole images.
• Damage generally occurs
through shear fracturing
(breakout) and tensile
fracturing (drilling-induced
fractures), though other
failure modes may occur.
• The orientation of fractures
may be used to identify the
stress field orientation.
This data is very useful in well
planning, to ensure:
• optimal well placement
• drilling efficiency
• understanding of induced
fracturing direction and hence
drainage of reservoir
• well bore stability
• Avoidance or reduction of
sand production
3. Structure 4/3
Why does stress anisotropy exist in the subsurface?
In pristine undrilled formation
• Gravity (overburden stress)
• Present-day far-field tectonic stress
• Residual tectonic stress
• Transient stresses (ice weight during glaciation,
diagenetic stresses)
• Thermal stress
• Formation fluid pressure
After drilling
• Hoop stresses due to the presence of the borehole
• Thermal stresses as the drilling fluid temperature is not
in equilibrium with the formation fluid.
• Effective stresses due to a pressure differential between
the borehole and formation to prevent blowouts, etc.
5. Structure 4/5
In-situ stress information provided
by borehole images
Direct
• Feature recognition:
borehole breakout,
induced fractures
• Measurement of breakout
trend, width, length and
depth
• Measurement of induced
fracture orientation
• Geometrical relationship
between stress features,
bedding and rheology
Indirect
• Hole shape analysis from
caliper curves
• Modification of natural
fractures (propping)
• Lack of tool rotation in a
near-vertical well
• Defocussing of opposing
image pads
6. Structure 4/6
Borehole breakout
• Borehole walls may experience time-dependent shear
failure if geomechanical conditions are right
• Shear-planes form, enclosing a wedge of material that
becomes detached and spalls into the well bore
• Breakout enlarges, reducing the stress anisotropy
• Breakout may develop if the drilling fluid weight is
insufficient to support the hoop stress at the borehole wall
• More common in formations with low compressive
strength, especially mudrocks
Trends parallel to Shmin
7. Structure 4/7
Microresistivity images (STAR, FMI)
• Defocusing at breakout; smeared
• Difficulty in assessing edges of breakout in
larger holes due to gaps in images
• Shear planes sometimes imaged where stress
equilibration is not complete (below)
• Often restricted to weak rheologies,
terminating at bed boundaries (below)
Breakout examples in electrical image logs
Oil based mud images (OBMI)
• As microresistivity images
but drilling mud is resistive
and so breakout is pale
• Poorly imaged due to larger
measuring button than
WBM electrical tools.
All examples show
3 feet of formation (MD)
8. Structure 4/8
Breakout example in acoustic image
• Clear edges to breakouts
• 100% coverage so measurements
are accurate
• Prone to being degraded in inclined
holes with low-side furrows and
where the tool is decentralised
• Inclined holes tend to have en-
echelon arrays of breakout
segments as seen here
• Breakout may disappear at bed
boundaries where rheology changes
• Transit time image may allow
breakout depth to be calculated
using mud slowness data
Example shows static (left) and
dynamic (right) acoustic amplitude
over 3 feet of formation
9. Structure 4/9
Depth, ft
Dynamic RAB images
0 Gamma ray, API 100
Static RAB images
0
10
20
30
40
Breakout example in RAB LWD images
• 100% coverage so breakout trend, width
and length measurements are accurate
• Inclined holes have en-echelon arrays of
breakout segments as seen here
• As RAB images are taken behind the drill
bit, they sample breakout that forms soon
after drilling. If also taken during reaming,
the time sensitivity of breakout may be
studied
Example shows static (left) and
dynamic (right) RAB images
over 50 feet of formation
10. Structure 4/10
• Breakout axis
• Present day in-situ stress
anisotropy orientation
• Breakout width
• Magnitude of anisotropy,
lithology dependent
• Breakout depth (acoustic
image transit time only)
• Magnitude of anisotropy
Axis
Depth
Width
Borehole breakout measurements
11. Structure 4/11
• Borehole walls may suffer tensile failure if
geomechanical conditions are right
• Dilation occurs in mechanical units where drilling
fluid pressure exceeds the tensile strength of the
formation
• Induced fractures tend to be layer-bound
• More common in formations with low tensile
strength, especially sandstones and carbonates
• May develop alongside breakouts – if so, the
borehole wall is critically stressed and may be in
danger of collapse
Drilling-induced tension fractures (DITFs)
Strike parallel to SHmax
12. Structure 4/12
1077
1076
1075
Okabe and Hayashi, 2000. Fig 1
Vertical scale 2 metres
N E S W N
• Open fractures; conductive in water-base mud and
resistive in oil-base mud
• Comprises amalgamating segments that are bed-
bounded and coalesce as en-echelon arrays
• 180° separation about the wellbore
• Degree to which segments coalesce is dependent
upon stress anisotropy magnitude, rock properties
and the time the hole has been open
• May not be imaged due to gaps
between imaging pads
• Fracture segment lengths relate
to bed thickness
DITF examples in electrical images
Vertical scale 2 feet
13. Structure 4/13
• Irregular fractures
• En-echelon array (tendency in deviated wells
• Circumferential imaging leaves no gaps but fine
fractures may not be imaged due to the large
imaging footprint when compared to
microresistivity imagers - fractures must exceed
25 microns to have any likelihood of being
imaged
DITF examples in acoustic images
Vertical scale 2 feet
14. Structure 4/14
• Desiccation fractures form in
mudrocks due to the chemical
interaction of the drilling fluid with the
formation
• They are seen most frequently in
holes drilled with oil-base mud
• They occur as regularly-spaced arrays
of open fractures (here bright) that
have a similar paired-segment
appearance as DITFs but are often
enhanced into continuous fractures.
• Strongly influenced by bedding tilt and
so problematic as stress indicators.
• Common in Gulf of Mexico well bore
images seen by Task Geoscience.
Desiccation fractures
18. Structure 4/18
Can the in-situ stress indicators rotate?
Yes!
• If any of the components of the stress field
change, the indicators may rotate:
• The change in overburden stress through a section
may cause a change in the stress anisotropy.
• Residual tectonic stresses can influence feature
orientation, for example SHmax may rotate into
parallelism with fault strike.
• Different residual stress regimes may be encountered
in a section, e.g. in foreland basins, with a change
from compressional to extensional up-section. In
turn, this changes indicator orientations.
• Anisotropies (especially bedding) may be utilised,
such as shear along bedding planes. If bedding
orientation changes, the mode of hole failure may
change.
20. Structure 4/20
Part 1 discussion
• Breakout trends NE-SW, measured at the top of the
example at 055°-235°.
• Breakout is absent where the gamma-log shows that the
lithology changes subtly. Might the unit have higher
unconfined compressive strength? Is strain taken on a
shear plane bounding the layer?
• The breakout is oblique, swinging to circa 100°-280°.
This may happen against faults due to residual stress
and the presence of an anisotropy. Could the fractures
seen in the image identify a fault damage zone? Could
the interval of no breakout above be differentially
cemented? Is this artefact?
• SHmax runs NW-SE according to the overall breakout
trend.
21. Structure 4/21
Part 1 discussion
• A drilling-induced tension fracture is present below 399
ft. It terminates at a conductive fracture at 398.5 ft.
• The fracture strikes 140°-320°, NW-SE.
• The SHmax direction is therefore again NW-SE.
• Might the fracture accommodate some of the strain?
22. Structure 4/22
Hole shape analysis
• Tool arm radii from electrical imagers
• 4-arm tools give 2 orthogonal diameters – very low
resolution but tool centralised
• 6-arm tools give 6 independent radii – higher
resolution but tool may be decentralised
• Transit time images from acoustic televiewers
• Circa 180 samples per revolution – very high
resolution, accurate hole shape
• Caliper tools
• Feeler gauges with many dozens of independently
sprung fingers – very high resolution, generally only
run in cased hole
23. Structure 4/23
Cumulative caliper cross-plot
• Used for microresistivity logs where caliper
arms are references to P1AZ.
• Best with six-arm EMI/STAR data.
• xn = CALIn.sin(PnAZ) yn = CALIn.cos(PnAZ)
Hole shape plot
• Used for televiewer
transit-time logs (UBI, CBIL)
and specialised caliper tools.
• Provides a good idea of
hole shape.
Wireframe plot
• Stacked hole shape sections
producing 3D figure
Hole shape plots
24. Structure 4/24
Features identified in hole shape plots
• Key seating in a deviated well, where drill pipe
sits on the bottom of the hole and abrades a
furrow
• Hole spiral, common where down hole motors
are used
• Hole size reduction due to filtrate cake
• Hole ovalisation where the well bore is under-
gauge through swelling clays
• Borehole breakout
• Shear displacement
25. Structure 4/25
Modes of shear displacement
d
d’
d’’
Borehole geometry
result from shear
displacement along
existing fracture
(general case):
d = Displacement
d' = Strike component
d"= Dip component
Shear displacement
along strike of an
existing fracture
a = sinistral shear
b = dextral shear
Shear displacement
along dip of an existing
fracture
c = reverse
d = normal
a
c
b
d
Fracture plane
26. Structure 4/26
X35
X36
X37
Depth X36.0 m
Hole deviation 32.4 degrees
Hole azimuth 75.7 degrees
Slip 303.9 degrees N
230.7 degrees top
0.5 in.
TOP
N
Borehole Radius (inch)
–4 –2 0 2 4
S
4
2
0
–2
–4
Example of shear displacement in acoustic image
28. Structure 4/28
Effectiveness of tools for in-situ stress analysis
Direct observations Indirect evidence
Breakout
Induced
fractures
Hole shape
analysis
(calipers or
transit time)
Tool motion
(locked into
breakout)
Defocussed
of opposing
pads
Dipmeters
HDT, SHDT,
OBDT, HEXDIP
No No Yes Yes No
Microresistivity
imagers
FMI, FMS, STAR, EMI
Yes Yes Yes Yes Yes
Oil-base mud imagers
OBMI, DOBMI,
EARTHIMAGER
Yes No Yes Yes Yes
Acoustic televiewers
UBI, CBIL, CAST
Yes Yes Yes
Yes, due to
stabilisers on
tool string
N/A
LWD tools
RAB, ADN, APLS-Elite
Yes No No No N/A
Accuracy of direct
measurements
High
Moderate
Low
29. Structure 4/29
• Only vertical wells considered to this point.
• Inclined wells must be modelled using
geomechanical parameters and information on the
far-field stress regime to deduce breakout and
induced fracture sensitivity and likely prevalence.
Prior to this step, apparent principal stress directions
are in a plane perpendicular to the wellbore, not
horizontal and vertical.
• Hole inclination becomes the dominant influence,
with horizontal holes often failing through breakout
along the sides of the hole and induced fractures at
highside and lowside.
!
Effect of hole deviation upon in-situ stress field
31. Structure 4/31
Geomechanical applications
• Well bore stability characterisation
• Failure mechanisms (tensile & compressive shear)
• Full waveform provides dynamic/static elastic moduli;
calibrate against static lab tests
• Mud pressure envelopes (limits for compressive and
tensile failure) as function of depth for given planned
well traces
• Modelling of drilling-induced tension fractures
• Prediction of the orientation of induced fractures
following a ‘minifrac’ operation
• Predicting the effect upon fluid flow
• Critically stressed fractures and faults
32. Structure 4/32
Key inputs:
Sv magnitude
SH direction
Mud parameters
Far-field stress
regime
Well bore stability modelling
35. Structure 4/35
Extensional regime
Sh < SH < S
min
Strike-slip regime
Sh < SH
min S <
v
Compressional regime
Sh < SH
S <
v min
Shmin
Shmin
Sv
SH
max
SH
max
Sv
Shmin
Shmin SH
max
SH
max
SH
max
SH
max
Shmin
Shmin
Sv
s3
s1
s2
s1
s2
s3
s1
s2
s3
s2
s3
s1
s1
s3
s2
s3
s2
s1
max v max
max
Effect of stress regime on
induced fracture orientations
36. Structure 4/36
Effect of in-situ stress upon fluid flow
• Fluids more likely to flow along fractures that strike parallel
to SHmax as they have the lowest normal stress acting
across them.
• Fluids unlikely to flow well parallel to Shmin as they are
held closed by the in-situ stress field.
• Fractures oriented such that they experience a high ratio of
shear to normal stress, so-called ‘critically stressed
fractures’ lying 20-30° oblique to SHmax experience most
enhanced flow.
• Proved through measuring the orientation of fractures that
are seen to have enhanced flow from dynamic data and
comparing to in-situ stress features (e.g. Rogers 2003).
38. Structure 4/38
Part 2 discussion
• The fracture strikes ENE-WSW.
Fractures striking parallel to SHmax
may experience enhanced flow.
Indeed, a slight discordance can be
beneficial as fracture enhancement
will be dilational with a shear
component that may remove
asperities on the fracture surface and
hence allow smoother flow.
• This orientation is close to Shmin and
so this fracture is unlikely to be
enhanced by the present day in-situ
stress field. However, it may
experience shear and increase the
likelihood of the hole failing.
39. Structure 4/39
Part 2 discussion
• Balance the possibility of the hole failing against
possible increase in production.
• Fraccing and testing will greatly increase
geomechanical understanding.
• The thick open fractures will be readily opened
by fraccing as they strike close to Shmin.
• Careful choice of completions.
• Are the fractures connected or will they flow
deplete quickly?
