Schlumberger Confidential Fracture Analysis Using Borehole Image Logs Petrom Technical Day Bucharest, 26-27 th September, 2007 Jurry van Doorn Geology Domain Champion Schlumberger With some examples from E. Etchecopar, S. Luhti, Ph. Montaggioni, O. Serra & E. Standen
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� Transit time of first echo: distance = speed in mud x Transit time / 2
=> Transit Time image (borehole radii)
�First echo amplitude => amplitude image
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� Fractures form an interface with the rock matrix which is many times greater than
provided by the borehole.
� As most fractures are tensional in nature, they are perpendicular to bedding and
terminate on shales and porous layers which are more ductile.
Naturally Fractured Reservoirs I
� Note two orthogonal
directions of fracturing
� Absence of fracturing in
porous sands underlying
carbonates
� Spacing is more or less
constant
� Fracture density increases
towards edge of outcrop
� Fractures also frequently
occur in corridors!
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� Fracture length horizontally (strike) is far greater than height vertically (dip).
� Fractures are the result of deformation of the rocks and therefore, deformation
and folding precedes fracturing.
� Due to release of stress, fractures are far more abundant and extensive at the
surface (outcrop and unconformities) than at depth and some fracture
orientations in outcrop will seldom be seen open at reservoir depth (watch out
for geological studies that relate outcrop fracture density to the subsurface.).
� Tensional fractures will group onto two orthogonal directions of strike and the
open set will be sub-parallel to the principal far-field stress direction.
Naturally Fractured Reservoirs II
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Natural Fracture Systems
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Before/After Mini-Frac Job
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� A carbonate section with stylolites and
drilling induced fractures.
� Often these drilling-induced fractures
are classified as drilling enhanced
natural fractures because they appear to
have an apparent dip relative to the
borehole. This may in fact be due to a
tilted stress field orientation rather than
due to micro joints in the rock that have
been partially opened by the drilling
process.
En-echelon Induced Fractures
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Natural Fractures from Borehole ImagesAmplitude UBI
1. Tight non conductive cement (Calcite, Quartz…)
2. Tight conductive cement (Pyrite…)
3. Soft conductive cement (Clay…)
Transit time OBMI FMI
OPEN FRACTURE
CEMENTEDFRACTURE
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Fractured Reservoir Characterisation
� Parameters that can be extracted from electrical borehole images:
– Fracture depth
– Fracture typology (natural open or cemented, or induced)
– Fracture orientation (dip and azimuth)
– Information about type and degree of cementation
– Fracture net distribution, fracture length per unit volume
– Fracture density
– Mutual relationship
– Relationship to structures
– Fracture relationship to bed thickness
– Fracture aperture, porosity, permeability
– Present day stresses
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Open Fracture Types: Carbonate Reservoir
F r a c t u r e C l a s s I m a g e E x p r e s s i o n
P l a n a r F r a c t u r e s
S o l u t i o n - E n h a n c e d F r a c t u r e s
B e d d i n g - C o n f i n e d F r a c t u r e s
W i d e C o n d u c t i v e Z o n e s
B r e c c i a t e d Z o n e s
I n d u c e d F r a c t u r e s
1m 1mFrom S.Luthi
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Electrically Resistive Fracture (Mineralised)
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Electrically Resistive Fracture (Mineralised)
Halo effect around a mineralised fracture in a Canadian Shale
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Mineralised fractures, Saudi Arabia
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Role of cemented fractures ?
5 mm
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Open Fractures in Vertical well, Saudi Arabia
Electrically conductive
fractures are the expression of
open fractures in a predominantly vuggy dolomite
interval.
Jurassic Carbonate of Saudi Arabia
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Total loss of mud circulation was observed at this depth. This Sub-vertical 6-8 inches wide conductive feature most probably corresponds to a large open fracture and less likely to a fault. Jurassic Limestone of Saudi Arabia.
Sub-vertical Conductive, Widely Open Fracture in a
Horizontal Well, Saudi Arabia
6-8 inches
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Leached Dolomite in High-K Reservoir, Saudi Arabia
Total loss circulation was observed at X775 ft.
Note the large washout in interval X774-X775 ft
interpreted as a high permeability leached
dolomite bed. The steep conductive event seen at
X775.7 ft on the FMI image possibly is either
a minor fault or more likely a large open
fracture that probably favored fluids circulation
and is probably accountable for the
leaching of this dolomite bed. Jurassic Carbonate
of Saudi Arabia
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Highly conductive and uneven surface surrounded by a high resistivity zone on each side @ X487.5 ft. This feature is best interpreted as a stylolite caused by pressure dissolution and cementation due to the vertical overburden stress. This plane acts as a horizontal permeability barrier. Note below the stylolite the presence of two conductive (probably open) fractures that enhance the permeability in the direction o their strike (NE-SW). Jurassic Carbonate of Saudi Arabia
Stylolite, Saudi Arabia
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Mud density and overpressure
are the probable causes of these induced fractures. Note that they are preferably located in the tight
beds. The fracture strike corresponds to the direction of the maximum in situ horizontal
stress (ENE-WSW).
Jurassic Carbonate of Saudi Arabia.
Drilling-Induced Fractures, Saudi Arabia
σH
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BRBRBRBR
Both Breakouts & Induced Fractures, Saudi Arabia
IFIFIFIF
σσσσHHHH
BRBRBRBR
IFIFIFIF
σσσσhhhh
breakout
induced Fracture
Lower Permian cross-bedded sandstone – Saudi Arabia
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Tight fractured & bedded dolomite
intercalated in a porous limestone.
Note that the limestone beds are not affected by the
fractures.
Jurassic Carbonate of Saudi Arabia
Relationship Litho-facies vs.Fracturing, Saudi Arabia
limestone
limestone
Dolomite
Dolomite
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Influence of Formation Facies on Fracturing
Daedalus bioturbation at the top of the Banquette Fm (unit III-2 of the Ordovician)(unit III-2 of the Ordovician)