Resolving fractures

1. Structure 2/1
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

2. Structure 2/2
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

3. Structure 2/3
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

4. Structure 2/4
Which fractures might be imaged?

5. Structure 2/5
Fracture sampling by well bore
Map view
Plan view

6. Structure 2/6
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)

7. Structure 2/7
Direct measurements and observations
from borehole images

8. Structure 2/8
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

9. Structure 2/9
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

10. Structure 2/10
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

11. Structure 2/11
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

12. Structure 2/12
Fracture morphologies in a carbonate reservoir
Luthi, 2000
Planar
Variable
width ‘blebs’
Bedding-
confined
Wide
conductive Breccia

13. Structure 2/13
• 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

14. Structure 2/14
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

15. Structure 2/15
The importance of ground-truthing
Resistive fracture swarm
Braided – may relate to shear?
Strain hardening?
Closed? Cemented?
Microfaults?
Granulation seams
Core or outcrop

16. Structure 2/16
Flow zone 2
Flow zone 1
Flowmeter data Acoustic images Manual dips
Vuggy fracture

17. Structure 2/17
2m 2m
Propped?
Open and part-open fractures

18. Structure 2/18
Conductive,
tension gashes
Resistive
cemented
limestone
Dissolution
seam
Stylolite-associated fractures

19. Structure 2/19
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

20. Structure 2/20
• 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

21. Structure 2/21
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