Fracture characterisation

1. Structure 2/1
Image log & dipmeter
analysis course
Fracture characterisation

2. Structure 2/2
• Introduction
• Which fractures might be imaged?
• Direct measurement and description
of fractures from borehole images
• Ground-truthing with core or outcrop
• Analysis of results
• Data integration
Outline

3. Structure 2/3
The impact of fractures on reservoir performance
Good
• Fractured reservoirs can be
prolific (e.g. Asmari Fm, Iran)
• Open fractures may sustain
high flow rates (e.g. Ekofisk
Fm, Norwegian North Sea)
• Hydrocarbons found in
basement where otherwise
unproducable (e.g. granitic
basement in Vietnam)
• Closed fractures can
increase sweep tortuosity
and so improve overall
sweep effectiveness
• Sealing faults can produce
structural traps
Bad
• Sealing faults lead to reservoir
compartmentalisation and
unswept oil
• Closed fractures baffle
perpendicular flow
• Open fractures act as thief zones
within injectors – early water
breakthrough
• Dual porosity systems – initial
fracture contribution may be high
but subsequent matrix
contribution slow; injector
planning critical to later
production
• Fractures and faults may
adversely affect well stability

4. Structure 2/4
The goal

5. Structure 2/5
Where do images fit?
15
m
1
cm
Borehole
images
and
dipmeters
3D seismic
Core
Fault throw (m)
Cumulative
fault
density
(faults
per
km)
0.01
1
100
10000
10000
100
1
0.01
Real geology and
limitless resolution
but limited coverage
& hard to see large
structure
VSP
Fractures detected
down to 10s of mm
and imaged to 5
mm, runs of 1000s
of metres, allows
large structure to be
inferred
Fractures unlikely
to be detected; only
faults and fracture
corridors over 10
metres wide.
Provides large-
scale structure
Seismic at a finer
scale, sees smaller
faults but scale too
coarse to image
fracture network

6. Structure 2/6
Increased compartmentalisation
Permeability barriers, baffles
Increased communication
Permeability conduits
Add to understanding of structural trends which are
significant within the reservoir.
Generally un-resolved on seismic (<10m throw), but
seen in core.
GO
The importance of sub-seismic faults

7. Structure 2/7
Images & core
Fracture model
Reservoir simulation
mapped seismic
faults
Cemented
Partially cemented;
vuggy
Closed
Mudstone
shear
Open
Unlithified
breccia
Cement
fault seals
Phyllosilicate
fault seals
Cataclastic
fault seals
Cataclastic
Fracture types and properties
The impact of images

8. Structure 2/8
Data processing
Quality control
Fracture effect on flow
• Visual appearance
• Match with direct/indirect flow indicators
Fracture distribution
• Raw fracture density
• Corrected fracture density
• Fracture spacing
• Fracture distribution statistics
Image description
• Fracture identification
• Classification
• Orientation
• Additional attributes
Upscaling/prediction
• Damage zone widths
• Recognition of seismic and
subseismic scale faults
• Relationship to major structures
FRACTURED RESERVOIR MODELLING
Raw image
log data
Core
Log data
Stratigraphy
Seismic sections
and maps
Core
Mudloss
Sonic waveforms
Dynamic data
Stage 1
Stage 2 Stage 3
Fracture interpretation workflow

9. Structure 2/9
Which fractures might be imaged?

10. Structure 2/10
A fracture will be imaged if…
• It is broader than the minimum intrinsic tool
resolution
• It has sufficiently contrasting properties compared
to the host rock; density and/or textural contrast
for acoustic tools and resistivity contrast for
electrical tools
• Due to the presence of a mineral infill, reduction in
grain size and/or preferential cementation around
closed fractures
• Due to the difference in properties between the drilling
fluid and the host rock across open fractures
• It is intersected by the borehole

11. Structure 2/11
Resolving fractures
• Function of tool sample rate
• Sensor size (electrical tools)
• Beam spot size (acoustic tools)
• For electrical images operating under ideal conditions, resolution
approaches button size.
• Current distortion by strong contrast features has the following effects:-
• Features below the intrinsic tool resolution down to around 10 μm may
dominate the pixel response of a sample and therefore be detected but
their true width and location within the image pixel is unresolved
• Conductive fractures draw in current and so appear larger than they really
are, with a resistive halo surrounding the fracture
• Conductive fractures have a greater depth of penetration than the
surrounding matrix and so may be steeper than they appear from images
• Resistive fractures repel current and so appear smaller than they really are
• These issues may be reduced by running acoustic and electrical tools
together (STAR-CBIL and Earth Imager-CBIL)

12. Structure 2/12
Fracture sampling by well bore
Map view
Plan view

13. Structure 2/13
Direct measurements and observations
from borehole images

14. Structure 2/14
Fracture description from borehole images
Direct measurements and observations
• Location of fracture, measured depth
• Fracture attitude as dip and dip direction
• Fracture category based on characteristics listed below
• Interpretation confidence
• Tool response: resistive/conductive if microresistivity, high/low
amplitude and long/short transit time if acoustic.
• Morphology, e.g. irregular, vuggy, etc.
• Continuity: continuous, terminates within borehole, discontinuous
within borehole (discrete segments), etc.
• Apparent aperture & thickness
• Relationship to bedding and other fractures: cross-cutting
relationships, offsets, terminations, intersection orientations

15. Structure 2/15
Terminology
Fracture
Natural
Induced
Fault
Microfault
Joint
Failure plane accommodating
strain resulting from tectonism,
thermal stresses, compaction, etc.
Present in the pristine,
formation and relating to
geological phenomena
Formed in response
to drilling operations
and not geological
Fracture with no
offset of wall rocks
and often due to
dilation
Fracture with shear offset
displacing hanging- and foot-walls
Occasionally used to denote
faults with a small offset on
a centimetre scale

16. Structure 2/16
Fracture categories
Descriptive schemes
• Response only
high, amplitude fracture
• Response and offset
resistive microfault
• Response, offset and morphology
discontinuous low-amplitude fracture
thick continuous conductive fracture
Interpretive schemes
• Inferred aperture (caution!)
thick open fracture, cemented fracture
• Geological interpretation (following core calibration)
granulation seam, vuggy fracture
FIT FOR PURPOSE

17. Structure 2/17
Fracture description exercise – part 1
10 mins

18. Structure 2/18
Resistive fault
Resistive fracture
Conductive fractures
Resistive fractures?
M M
Continuous
Irregular trace
Width mm-cm?
Offset circa 8-10 cm
Splays (riedel/antiriedel?)
No displacement
Hairline
Regular trace (planar)
Discontinuous
Terminates at fault
Discontinuous
Layer-bound?
Form connected network
Variable width
Weak fabric
Resistive
Fracture description

19. Structure 2/19
Fracture morphologies in a carbonate reservoir
Luthi, 2000
Planar
Variable
width ‘blebs’
Bedding-
confined
Wide
conductive Breccia

20. Structure 2/20
• Resistivity images in a water-based mud system (traditional):
• Conductive (dark image) =/= open?
• Resistive (light) =/= closed?
• Resistivity images in an oil-base mud system
(new tools; Earth Imager, OBMI):
• Conductive (dark image) =/= closed?
• Resistive (light) =/= open?
• Acoustic Images:
• Low amplitude (dark) =/= open?
Check transit time image to confirm aperture
• high amplitude (light) =/= closed?
• Core calibration should be used to confirm type.
• Image logs can provide an interpretive insight only.
• Only dynamic data provide true insight into fracture “producibility”.
Open versus closed

21. Structure 2/21
Cemented
Partially cemented;
vuggy
Closed
Mudstone
shear
Open
Unlithified
breccia
Cement
fault seals
Phyllosilicate
fault seals
Cataclastic
fault seals
Cataclastic
Cemented
Cemented
Partially cemented;
vuggy
Partially cemented;
vuggy
Closed
Closed
Mudstone
shear
Mudstone
shear
Open
Open
Unlithified
breccia
Unlithified
breccia
Cement
fault seals
Cement
fault seals
Phyllosilicate
fault seals
Phyllosilicate
fault seals
Cataclastic
fault seals
Cataclastic
fault seals
Cataclastic
Cataclastic
This needs local calibration to
core!
Fracture Microfault Fault
Resistive FRAC RES MF RES FAULT RES
Mixed FRAC MIX MF MIX FAULT MIX
Conductive FRAC CON MF CON FAULT CON
Fracture classification schemes

22. Structure 2/22
The importance of ground-truthing
Resistive fracture swarm
Braided – may relate to shear?
Strain hardening?
Closed? Cemented?
Microfaults?
Granulation seams
Core or outcrop

23. Structure 2/23
Flow zone 2
Flow zone 1
Flowmeter data Acoustic images Manual dips
Vuggy fracture

24. Structure 2/24
3m
2m
Cataclastic fracture and microfault

25. Structure 2/25
2m 2m
Propped?
Open and part-open fractures

26. Structure 2/26
Conductive,
tension gashes
Resistive
cemented
limestone
Dissolution
seam
Stylolite-associated fractures

27. Structure 2/27
Flow paths around fractures
Resistive fracture
Current is repelled
away from the
fracture.
Current-lines at B are
compressed, giving
elevated resistivity
(low current).
Current lines at C are
more separated,
producing a more
conductive response.
Conductive fracture
Current is drawn into
the fracture.
Current is increased
across the fracture,
giving a conductive
response at b, B and C.
The fracture mid point
may therefore be higher
than the true fracture
location

28. Structure 2/28
• If fluid is flowing through fractures, then fracture
aperture (open width) influences flow rate. Flow rate
is proportional to the cube of the aperture
• Aperture may be measured and ranked from resistivity
images (although conductive fractures might not be
open) and acoustic transit time images
• Aperture readings may be misleading as fractures may
change in width along their trace, be damaged and
enlarged at their interface with the borehole wall, and
be affected by the effects of mud invasion. Readings
vary between the water, oil and gas legs and must be
corrected
• Aperture does not necessarily correlate with flow as
flow requires connected volume rather than isolated
fractures
0 mm
0.64 mm
1.27 mm
Fracture aperture assessment

29. Structure 2/29
b
xo
b
mR
cAR
W 
 1
W fracture width /mm
A excess conductance
(right)
Rm mud resistivity
Rxo formation resistivity
c,b tool-specific constants
derived from forward
modelling
Relationship between fracture width and excess
conductance due to the presence of the fracture.
dz
I
z
I
V
A bm
z
z
b
e
n
}
)
(
{
1
0

 
Ve voltage difference across tool
Ib button current, fracture
Ibm button current, matrix
z vertical position
0,n base, top
Luthi-Souhaité equation

30. Structure 2/30
Pros and cons of imaging technologies for fracture characterisation
WBM electrical imagers
• Highest intrinsic resolution so detects
finest fractures & mechanical layers
• Sees slightly into the formation
through the mudcake
• May be used to derive apertures
• Limited coverage; may miss some
fractures between the pads
• Gives a relative resistivity only
OBM electrical imagers
• Works in oil-base muds that are
commonly used for operational reasons
• Provides true resistivity values
• Limited coverage
• Low resolution as fewer, larger buttons
than WBM electrical imagers
• Poor in conductive formation e.g. shales.
Acoustic televiewers
• 100% borehole coverage so only
misses fractures unresolved
• High intrinsic resolution
• ‘Caliper image’ may see open fractures
• Allows reliable identification and
orientation of in-situ stress features
• Only sees the skin of the borehole wall
and cannot see through mudcake
LWD tools
• Macroresistivity devices see well into
formation, sometimes several depths;
may see behind hole artefacts
• Run close behind bit, before time
dependent damage occurs
• Used for geosteering horizontal wells;
may intersect steep fractures
• Generally lower resolution than wireline
tools by over 1 order of magnitude – will
only see broad fractures
Electrical tools are prone to stick and pull
Acoustic tools are poor in bad hole
All tools work best in smooth hole and fractures and faults often
facilitate spalling and thus hole damage; the tools often work least
well where the well bore is most fractured.

31. Structure 2/31
Fracture description exercise – part 2
10 mins

32. Structure 2/32
Consistent bedding
orientation; clear lamination
evident in SHDT images.
Parallel resistive fractures, thin
(under 1 inch), bed continuity
uncertain across fractures.
RESISTIVE FRACTURES
Probable mineralised fractures
and therefore likely to be
baffles. Four fractures in 8
feet – 0.5 fractures/foot
uncorrected density. Spacing
circa 0.5 feet.
Anomalously shallow bedding
at top of resistive unit.
Return to bed orientation at top
of image.

33. Structure 2/33
RESISTIVE FRACTURE
FMS of previous SHDT
example.
Consistent bedding
orientation; clear lamination.
RESISTIVE FRACTURES
(SHDT orientations dotted)
Parallel resistive fractures, thin
(under 1 inch), bed continuity
uncertain across fractures but
there may be bed offset.
Single conjugate fracture
present.
Probable mineralised fractures
and therefore likely to be
baffles. Seven fractures in 8
feet (main set)
– 0.87
fractures/foot uncorrected
density. Spacing
circa0.5
feet.
More fractures are identified
when compared to the SHDT;
conjugate feature not seen at
all in dipmeter.
All resistive and thus no
aperture likely; not stress
-
enhanced.

34. Structure 2/34
Analysing image-derived fracture data

35. Structure 2/35
Analysing the fracture data
Fracture display
• Tadpole plots
• Strike histograms
• Dip sticks (section view)
• Stereoplots
• Strike bars (map view)
• Data listings
• Fracture density curve
Fracture analysis
1. Separation into sets by
character and orientation
2. Borehole sampling bias
analysis
3. Data analysis by set
• Mean orientation (R&C)
• Fracture density (R&C)
• Damage zones or fracture
corridors
• Fracture spacing (C)
• Damage zone width (C)
4. Analysis by mechanical
stratigraphy
5. Likely geological significance
6. Construction of
conceptual model

36. Structure 2/36
Displaying fracture data
11000
11500
12000
12500
13000
13500
14000
14500

37. Structure 2/37
Coloured dots distinguish different fracture
Sets. Lines represent fracture set strikes.
N
15
30
45
60
75
90
15
30
45
60
75
90
15
30
45
60
75
90
15
30
45
60
75
90
15
30
45
60
75
90
15
30
45
60
75
90
Sets are defined on the basis of:-
• Fracture dip-type.
• Orientation (clusters on
stereonet).
• Association, i.e. conjugate sets
are identified, secondary
structures recorded (e.g. Riedel
shears) – may require core
calibration.
• Orientations recorded and effect
of borehole sampling bias
considered.
• Fracture spacing statistics
generated by fracture set, hence
key to defining subsurface
network.
Fracture sets

38. Structure 2/38
N
15
30
45
60
75
90
15
30
45
60
75
90
15
30
45
60
75
90
15
30
45
60
75
90
15
30
45
60
75
90
15
30
45
60
75
90
Orientation 88°/158°
Count 461
Type Discontinuous, conductive - set A
Sampling bias Low
Critically-stressed? Yes; evidence for propping, low angle to
SHmax
Interpretation Stress-enhanced open fracture; probably
mode-II, early; appears to be conjugate to
set B (orange) , subsequently rotated in
main tilt event.
0
20
40
60
80
100
120
140
160
180
200
Orthogonal fracture spacing
Frequency
Mean 0.30
Standard Error 0.03
Median 0.14
Mode 0.07
Standard Deviation 0.56
Sample Variance 0.32
Kurtosis 54.22
Skewness 6.27
Range 6.84
Minimum 0.00
Maximum 6.84
Count 461
Spacing statistics, set A
Fracture set statistics

39. Structure 2/39
Strike histogram
Green Set
Red set
q
q
Fracture plane
Borehole
Apparent Weighted
Weighted Apparent frequency
Frequency cos q
=
Borehole sampling bias

40. Structure 2/40
0
5
10
15
20
25
30
35
0 20 40 60 80
Angle between fracture and borehole (q)
Derived weighting factor
Bias correction factor
Weighting
factor
(1/cos
q)
q
=
84.2°
Weighting factor = 10
• Weighting to emphasise fractures that are not orthogonal to the well bore
• Cut-off applied to prevent fractures parallel to wellbore skewing the data-set
• Provides a better estimation of the true subsurface fracture network
Terzaghi correction factor

41. Structure 2/41
Vertical well Highly deviated well
No data girdle
No data cone
W E
No-data girdle

42. Structure 2/42
S
q reciprocal of corrected
fracture density
Fracture spacing
• Distribution of fracture spacing is used to infer larger-scale significance:
• Random
• Regularly-spaced
• Clustered

43. Structure 2/43
T
Fractures at
limits of
damage zone
q
0.001
0.01
0.1
1
10
100
1000
0.0001 0.001 0.01 0.1 1 10 100
Fault zone thickness, m
Fault
throw,
m
After Knott 1996.
et al.
Sandstone/sandstone
Sandstone/shale
Minimal throw estimates:-
• rule of thumb (e.g. fault throw = 2.5 x damage zone width for
brittle failure)
• drag zone analysis based on splines (for more plastic
deformation) and graphical cross-section approach.
Fault damage zone width

44. Structure 2/44
9500
10000
10500
11000
11500
12000
12500
13000
13500
14000
14500
Strike Histogram
Wellbore
deviation
85 degrees
to NW
Damage
zones
Fracture density data,
4000 ft horizontal well.
Fracture density shows
a series of broad zones
of red-set fractures,
representing fault
damage zones; and
narrower blue-set
zones. Difference in
apparent damage zone
widths is due to the
intersection angle with
NW-directed borehole.
FRAC
DEN
Fault damage zone width

45. Structure 2/45
Matrix block 5m x 5m x 3m
North
N-S red fracture set
5 m spacing
N-S blue fracture set
3 m spacing
E-W green fracture set
5 m spacing
Matrix block size

46. Structure 2/46
• Lithology – changes in fracture type, morphology
and density often occur at lithological boundaries.
Joints are layer-bound within mechanical units
• Curvature – fracture intensity is often proportional
to the degree of structural curvature due to outer-
arc extension and inner-arc compression
• Localisation – fracture intensity may be
proportional to distance from a fault, forming a
‘damage-zone’
• Present-day in-situ stress field; enhances some
fractures whilst closing others, causing an
apparent variation in intensity
Influences upon fracture distribution

47. Structure 2/47
• Well and fracture orientations
• Division into fracture sets (combining orientation
and fracture type information)
• Fracture density
• Core density calibration
• Borehole corrected fracture density
• Fracture spacing
• Fracture distribution
• 3D modelling
Fracture distribution analysis

48. Structure 2/48
Integration with other data-sets

49. Structure 2/49
Fracture description from wireline logs?
If thick, high-contrast fractures:
• Subtle in most conventional logs unless many inches thick and
highly contrasting fracture fill
• Separations of shallow and deep laterolog curves due to current
path effects
• Events on photoelectric effect curve in barite muds
• Spikes of increased delta-T sonic if fractures are large and open
• Cycle skipping in older-generation sonic logs
• Caliper events Full waveform sonic logs: Stoneley waveforms,
fast versus slow directions and magnitude of anisotropy from
shear sonic
• Nuclear magnetic resonance logs

50. Structure 2/50
• Use an array of receivers
which, under the right
conditions gather Stoneley
wave arrivals
• Stoneley tube wave
sensitive to lithological
changes, borehole
enlargements, faults and
open fractures
• Azimuthal anisotropy
(slow/fast directions)
identify in-situ stress
directions and fracture
strikes
Full waveform sonic data

51. Structure 2/51
8m
Stoneley waveforms

52. Structure 2/52
Integrated study of fractured carbonates
Luthi, 2000

53. Structure 2/53
Fracture description: what else can we use?
Core:
• Full geological description, ground-truth with resolution to electron
microscope scale but over limited intervals and it can be difficult to
see the larger structure
Dynamic data:
• Mud losses may identify open fractures
• Production logs may quantify producibility and fluid entry points
• Dual packer systems can be used test the producibility of imaged
fractures but are costly and time consuming
Seismic data:
• 3D volumes may identify, orientate and locate large faults and
fracture corridors but cannot image fine-scale detail
• VSP surveys may resolve features a few metres wide

54. Structure 2/54
Flow zone 2
Flow zone 1
Flowmeter data Acoustic images Manual dips
Calibration of log and core data

55. Structure 2/55
Integration of images and pressure data

56. Structure 2/56
000
Fractures per 10 ft interval
0 250
Resistive fracture
Conductive fracture
Cumulative fracture density
Depth
Production log data
(red – flow, blue – temperature)
0 100
Integration of images and spinner data

57. Structure 2/57
Time slice Seismic section
Filtered dip data overlaid on top of
seismic section. Green=bedding,
red=fractures
Rendered seismic horizon
Up-scaling to seismic data

58. Structure 2/58
Sealing or transmissive?

59. Structure 2/59
Juxtaposition Clay smear
Cemented/grain size reduction Open/vuggy
Fault seal types

60. Structure 2/60
Fault seal interpretation workflow
Fault zone recognised
on images as
discordant feature
Lithology difference
across fault?
JUXTAPOSITION
FAULT SEAL
CEMENTED
FAULT SEAL
GRAIN SIZE
REDUCTION
FAULT SEAL
CLAY LINED
FAULT SEAL
OPEN
FAULT
Fault is resistive
or high amplitude?
Fault is conductive
or low amplitude
Mud losses or
increased
transit time?
Fault shows
shale response on
open hole logs?
Argillaceous
lithology?
Strong image
response?
Cementation
indicators on
open hole logs?
Y
N
Y
N N Y Y
N
Y
Y
Y
N
N
N Y
N
Electrical responses assume water-base mud;
reverse resistive and conductive for oil-base mud
Adams & Dart 1998

61. Structure 2/61
4
feet
Borehole
Interpretive cross-section
Sandstone
Mudstone
Juxtaposition

62. Structure 2/62
Sandstone
2600 psi
Sandstone
3600 psi
Borehole
Cemented fault

63. Structure 2/63
2
feet
(60
cm)
Borehole
Offset
deformation
band
Internal
banded
structure
Grain size reduction (cataclasis)

64. Structure 2/64
Shale response
on openhole logs
Up hole
Interpretive cross-section
Shale-smeared
fault zone
Damage zone
Clay smear

65. Structure 2/65
Bedding
Major open fault
Bedding
Drilling-induced
tension fracture
2
feet
(60
cm)
Open fault

66. Structure 2/66
Sealing Open
Partially sealing
Juxtaposition
Non-cataclastic
Cataclastic
Cemented
Clay smear
Vuggy/open
Seal effectiveness

67. Structure 2/67
Fault
orientation
Fault density & clustering
relationship to bedding
Fault seal type
Fault rock type
Host rock lithology
In-situ stress
Damage zone
- dimension
- fault clustering
- fault offset populations
Deformation history
Fault seal influences

68. Structure 2/68
Fault
damage
zone
Look at all scales!
Core to seismic
Borehole images allow conceptual models to be made which may
be refined and extended using other available data: up-scaling to
seismic and field analogues, down-scaling and ground-truthing to
core and quantifying flow with production data

69. Structure 2/69
Orientation
Density & clustering
relationship to bedding
Fracture size
(length)
Abutting
relationships
Sealing or open?
Aperture
In-situ stress
FRACTURE MODEL
Fracture modelling input data

70. Structure 2/70
Images & core
Fracture model
Reservoir simulation
mapped seismic
faults
Cemented
Partially cemented;
vuggy
Closed
Mudstone
shear
Open
Unlithified
breccia
Cement
fault seals
Phyllosilicate
fault seals
Cataclastic
fault seals
Cataclastic
Fracture types and properties
The impact of images

71. Structure 2/71
Why use borehole images for fracture
characterisation?
• Data is gathered over a long interval, allowing large-scale
structure to be inferred
• Borehole images give fracture orientations
• Borehole images are cost effective when compared to coring
and production tests
• Scale bridge between core and seismic
• The effect of well operations on fractures may be inferred

72. Structure 2/72
• Well design to maximise intersections with flowing
fractures and enter fault compartments by crossing
sealing faults
• Inputs to developing discrete fracture model
• Inputs to geological models and reservoir simulations
• To plan wells where reservoir units are offset by faults
• As an input in multi-attribute analysis of 3D seismic
data and derived acoustic impedance and coherence
volumes, allowing fractured reservoir units to be imaged
Applications