Fracture Analysis Using Borehole Image Logs Petr Pe trom om Te Tech chni nica call Da Dayy Bucharest, 26-27th September, 2007
Jurry Jurry van van Doorn Doorn Geology Domain Champion Schlumberger
With some examples from E. Etchecopar, S. Luhti, Ph. Montaggio Montaggioni, ni, O. Serra Serra & E. Standen Standen
S c h l u m b e r g e r C o n f i d e n t i a l
Fracture Detection Detection & Conventiona Conventionall Openho Openhole le Logs I
Dipmeter – Fracture Anomalies Anomalies – vugs, pyrite & shale clasts clasts – Borehole breakout breakout – stress field, field, not fractures fractures – Resistivity anisotropy – stress field field
Sonic – Cycle Skipping Skipping – Could be caused by by Gas – Waveforms – attenuation attenuation with excentraliza excentralization. tion. – Variable Density Density log – chevrons at at washouts washouts
Caliper – Washouts & breakouts – stress field not fractures fractures
NGT – High Uranium – cemented fractures, organic organic shales shales
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S c h l u m b e r g e r C o n f i d e n t i a l
Fracture Detection Detection & Conventiona Conventionall Openho Openhole le Logs I
Dipmeter – Fracture Anomalies Anomalies – vugs, pyrite & shale clasts clasts – Borehole breakout breakout – stress field, field, not fractures fractures – Resistivity anisotropy – stress field field
Sonic – Cycle Skipping Skipping – Could be caused by by Gas – Waveforms – attenuation attenuation with excentraliza excentralization. tion. – Variable Density Density log – chevrons at at washouts washouts
Caliper – Washouts & breakouts – stress field not fractures fractures
NGT – High Uranium – cemented fractures, organic organic shales shales
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S c h l u m b e r g e r C o n f i d e n t i a l
Fracture Detection Detection & Conventiona Conventionall Openho Openhole le Logs II
Resistivity – Laterolog – invasion effects, borehole borehole corrections corrections – MicroResistivity MicroResistivity anomalies anomalies - washouts – Anomalous high induction induction readings readings in resistive resistive fractures – cemented fractures didn’t produce
Density – Anomalous corrections – tool rotation, rotation, incipient incipient – breakout.
PEF – anomalies in barite mud mud – micro rugosity. rugosity.
Most anomalies anomalies are associated with borehole borehole rugosity rugosity effects, problems with bad mud systems, correlation to fractures in core is often related to drilling induced / coring induced fractures, orientation cannot be determined…
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S c h l u m b e r g e r C o n f i d e n t i a l
Available Borehole Image Logging Techniques Electrical
Resistivity Resistivity Changes Changes of Borehole Wall
Water Based Mud
FMI (FMS) Slim FMI Slim FMS
Oil Based Mud
OBMI OBDT SHDT-OBM
Wireline Wireline Resistivit Resistivity y Tools Tools
ARI HALS
LWD
RAB GVR
Acoustic
Acoustic Impedance changes of Borehole wall
OBM/WBM
UBI
Nuclear
Density Changes of Borehole wall
LWD
ADN
Optical
Optical image using down hole camera
Requires clear fluid in the borehole
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S c h l u m b e r g e r C o n f i d e n t i a l
Optical Imaging
Optical imaging is the oldest borehole imaging technique.
Optical images allow for detection (orientation) and classification
The first devices were optical cameras lowered in the borehole.
Resolution: typically high
Depth of investigation: none
Azimuthal Azimuthal coverage: coverage: 360 360 degrees degrees
Main problem: opaque nature of borehole fluid, which prevents common use in open hole for geological applications
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S c h l u m b e r g e r C o n f i d e n t i a l
FMI* Measurement Principle Upper electrodes
Mass insulated sub
Current
(SHDT pad)
The FMI measurement principle use passive focussingLower electrodes around the measurement electrode.
FMI* = Fullbore Formation MicroImager
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S c h l u m b e r g e r C o n f i d e n t i a l
Borehole Image Logs S
Image logs correspond to virtual outcrops in which sedimentary and tectonic features can be observed.
They can be accurately oriented thus allowing for the measurement of bedding and fracture orientations
High-resolution resistivity measurements also allow for quantification of textures, fractured zones and facies over long intervals.
N
E
S
W
N
S c h l u m b e r g e r C o n f i d e n t i a l
Core presentation
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W
E
N
Unrolled image of a fault
OBMI*=Oil-Base Mud MicroImager Measurement
AC voltage applied between electrodes A and B
AC current I generated in formation
Resulting δV measured between paired buttons C and D
Ohm’s law, R=kδV/I, gives calibrated Rxo measurement
Five measurements per pad
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S c h l u m b e r g e r C o n f i d e n t i a l
OBMI2 for Increased Borehole Coverage OBM I
34’
OBM I
OBM I2
32’
17’ 15’
OBM I2
1 ft
0’ 10 JvD 26-SEP-2007
OBMI2: Double Coverage => Double Borehole Geometry Data
S c h l u m b e r g e r C o n f i d e n t i a l
UBI* = Ultra-sonic Borehole Imager UBI sub
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S c h l u m b e r g e r C o n f i d e n t i a l
Acoustic pulse-echo scan
Transducer frequency: 250 Khz (low res.) - 500 KHz (high res.).
Transducer rotates at 7.5 rps
180 azimuthal samples (2 deg. Interval)
Transit time image & amplitude image
Vertical Resolution 0.2-0.4 in. (5mm-1cm)
Logging speed: 850 ft/hr (low res.) – 425 ft/hr (high res.)
Can be used in water and oil-based mud
UBI Measurement Principle Focused Transducer Wall Borehole
Pulse Transit Time UBI signal
Echo
First echo amplitude
Measurements:
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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
S c h l u m b e r g e r C o n f i d e n t i a l
Naturally Fractured Reservoirs I
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.
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
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Fractures also frequently occur in corridors!
S c h l u m b e r g e r C o n f i d e n t i a l
Naturally Fractured Reservoirs II
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.
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S c h l u m b e r g e r C o n f i d e n t i a l
Natural Fracture Systems
S c h l u m b e r g e r C o n f i d e n t i a l
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Before/After Mini-Frac Job
S c h l u m b e r g e r C o n f i d e n t i a l
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En-echelon Induced Fractures
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.
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S c h l u m b e r g e r C o n f i d e n t i a l
Natural Fractures from Borehole Images Amplitude
UBI
Transit time
OBMI
FMI
OPEN FRACTURE
1. Tight non conductive cement (Calcite, Quartz…)
CEMENTED FRACTURE
2. Tight conductive cement (Pyrite…)
3. Soft conductive cement (Clay…) 18 JvD 26-SEP-2007
S c h l u m b e r g e r C o n f i d e n t i a l
Fractured Reservoir Characterisation
Parameters that can be extracted from electrical borehole images: – – – – – – – – – – –
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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
S c h l u m b e r g e r C o n f i d e n t i a l
Open Fracture Types: Carbonate Reservoir 1 m
m 1 F r a c tu r e C la ss
P l a n a r F r a c tu r e s
S o l u t io n - E n h a n c e d
F r a c tu r e s
B e d d i n g - C o n f in e d F r a c t u r e s
W id e C o n d u c ti v e Z o n e s
B r e c c ia t e d Z o n e s
I n d u c e d F r a c t u re s
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From S.Luthi
Im a g e E x p r e ssio n
S c h l u m b e r g e r C o n f i d e n t i a l
Electrically Resistive Fracture (Mineralised)
S c h l u m b e r g e r C o n f i d e n t i a l
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Electrically Resistive Fracture (Mineralised)
S c h l u m b e r g e r C o n f i d e n t i a l
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Halo effect around a mineralised fracture in a Canadian Shale
Mineralised fractures, Saudi Arabia
S c h l u m b e r g e r C o n f i d e n t i a l
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Role of cemented fractures ? 5 mm
S c h l u m b e r g e r C o n f i d e n t i a l
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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 25 JvD 26-SEP-2007
S c h l u m b e r g e r C o n f i d e n t i a l
Sub-vertical Conductive, Widely Open Fracture in a Horizontal Well, Saudi Arabia S c h l u m b e r g e r C o n f i d e n t i a l
6-8 inches
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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.
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 S c h interpreted as a high ul m permeability leached b e r g dolomite bed. The steep e r C conductive event seen at o n f X775.7 ft on the FMI d i e image possibly is eithern t a minor fault or more ai l 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 27 JvD 26-SEP-2007
Stylolite, Saudi Arabia
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 Note permeability barrier . below the stylolite the presence S c of two conductive (probably h l u open) fractures that enhance the m permeability in the direction b o e r g their strike (NE-SW). Jurassic e r C Carbonate of Saudi Arabia o n f i d e n t i a l
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Drilling-Induced Fractures, Saudi Arabia
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.
H
σ
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S c h l u m b e r g e r C o n f i d e n t i a l
Both Breakouts & Induced Fractures, Saudi Arabia
BR S c h l u m b e r g e r C o n IF f i d e n t i a l
induced Fracture
breakout
σh σH 30 JvD 26-SEP-2007
IF
Lower Permian cross-bedded sandstone – Saudi Arabia
BR
Relationship Litho-facies vs.Fracturing, Saudi Arabia
limestone
Tight fractured & bedded dolomite intercalated in a porous limestone. Note that the limestone beds are not affected by the fractures.
Dolomite
Jurassic Carbonate of Saudi Arabia
limeston e Dolomite 31 JvD 26-SEP-2007
S c h l u m b e r g e r C o n f i d e n t i a l
Influence of Formation Facies on Fracturing
S c h l u m b e r g e r C o n f i d e n t i a l
No Bioturbation Fractured
FMI image Bioturbated No fractures
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Daedalus bioturbation at the top of the Banquette Fm (unit III-2 of the Ordovician) (unit III-2 of the Ordovician)
Fracture Distribution
S c h l u m b e r g e r C o n f i d e n t i a l
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Stress Perturbation in the Vicinity of a Fault
Depleted zone: widely open fractures
Highly stressed zone
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From V. Auzias
S c h l u m b e r g e r C o n f i d e n t i a l
Influence of Layering on Fracture Distribution
Fracture spacing vs layer thickness g n i c a p s 5m n a e M
(modified from Bouroz 1990)
Thickness 5
From V.Auzias et al 1998 35 JvD 26-SEP-2007
10
15m
S c h l u m b e r g e r C o n f i d e n t i a l
Fracture Density
S c h l u m b e r g e r C o n f i d e n t i a l
The true length of fracture (the sum of visible segments) by surface unit is a much better indicator than the number of fractures by length of well.
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Fracture Density
There are two available fracture density calculations.
The raw fracture density is the number of fractures per foot or meter selected along the borehole. The corrected fracture density is the number of fractures per foot or meter selected along a line perpendicular to the fracture plane.
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S c h l u m b e r g e r C o n f i d e n t i a l
Fracture Aperture Calculation
Description:
As a button electrode approaches a fracture, which is filled with mud or other fluid of resistivity, Rm, an increased current will begin to flow because of the presence of this low resistivity anomaly. This increased current will continue to flow until the electrode is far enough away from the fracture that is no longer affected by the fracture.
For the above reason, a fracture, which is physically thinner than 0.1 mm, may have an electrical image, which appears to be an inch or more wide. Obviously it is impossible to resolve directly a fracture using a sensor button, which is many times the size of the fracture.
There is however, an indirect method, which provides the solution. From measurements and mathematical simulation, we know the response of the electrical image tool to fractures filled with fluids of different resistivities. Further, we know that the fracture aperture is proportional to the sum of the increased current flow.
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S c h l u m b e r g e r C o n f i d e n t i a l
Fracture Aperture from Electrical Images Empirical formula from Luthi & Souhaité (1990): Excess current
b
A
1- b
W=c. W= c.A c. A.Rm .Rxo
Tool current Button resistivity
Assumptions: – infinite fracture – completely open fracture – conductive material filling the fracture is drilling mud
Limitations: – same response if fracture sealed with conductive material such as pyrite or clay – aperture calculation affected by fluids (hydrocarbon bearing zones vs water bearing zones)
W Rxo Rm
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S c h l u m b e r g e r C o n f i d e n t i a l
Fracture Aperture Calculation from Electrical Images Fracture Aperture can have a big impact on hydrocarbon production Bbl/day 1000
S c h l u m b e r g e r C o n f i d e n t i a l
100
10
1 0
.1
.2 .3 Aperture (mm)
Fracture Aperture can be estimated from conductive fractures on FMI/FMS resistivity images 40 JvD 26-SEP-2007
Two Types of Fracture Aperture
Two calculations of fracture aperture are available. The first, mean aperture is simply the average width of the fracture along its length. The second, hydraulic aperture is the cubic mean of the fracture width. The term hydraulic is used since this method is proportional to fluid flow through the fracture. The mean aperture provides only information about the physical size of the fracture opening. A comparison of flow capacities of different fractures is possible with the hydraulic apertures but not with the mean apertures. Hence, hydraulic aperture values are displayed as aperture channels.
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S c h l u m b e r g e r C o n f i d e n t i a l
Fracture Porosity
Schlumberger Oilfield Glossary Definition: – A type of secondary porosity produced by the tectonic fracturing of rock. Fractures themselves typically do not have much volume, but by joining preexisting pores, they enhance permeability significantly. In exceedingly rare cases, non-reservoir rocks such as granite can become reservoir rocks if sufficient fracturing occurs. As discussed, in Schlumberger we measure fracture porosity from electrical images using a propriety algorithm developed by S. Luthi & Ph. Souhaité (1990). This algorithm is implemented in GeoFrame’s Borview module. It provides the fracture area (fracture trace length exposed to the borehole X aperture). To calculate the exact fracture volume, you would require fracture length (height and lateral extent). However, these parameters are based on modeling and can be obtained through constructing proper fracture models and by well testing.
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S c h l u m b e r g e r C o n f i d e n t i a l
Fracture Porosity
S c h l u m b e r g e r C o n f i d e n t i a l
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Scaling the Borehole Image using BorScale
S c h l u m b e r g e r C o n f i d e n t i a l
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Scaling the Borehole Image using BorScale
S c h l u m b e r g e r C o n f i d e n t i a l
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Fracture Assessment using BorView The aperture trace should show up as multi colored (or at least red and pink) and should not be a smooth line (should be somewhat wiggly). The trace should correspond with the fracture that it was computed for (in this case, Large Open Fracture. The Scale for the trace is a fixed logarithmic scale (10-5 – 101) giving values of: 0.1 - 1 microns = purple 1 - 10 microns = red 10 - 100 microns = yellow 100 - 1000 microns = green 1000 -10 000 microns = light blue 10 000 - 100 000 microns = dark blue (if the default of cm is used for your small length). 46 JvD 26-SEP-2007
S c h l u m b e r g e r C o n f i d e n t i a l
BorView Fracture Outputs
FVPA – Apparent fracture porosity: porosity of a given length of borehole due to fracture aperture(s)
FVAH – Average hydraulic electrical fracture aperture: Cube root of the mean of the cubes of the individual apertures along the fracture trace averaged over a given borehole length
FVA – Average fracture electrical aperture: Mean if the individual apertures along the fracture trace averaged over a given borehole length
FVDA – Apparent fracture density: umber of fractures in a given length of borehole (linear fracture density)
FVTL – Areal trace length: Cumulative fracture trace length seen in a given area of borehole wall (over a given borehole length)
FVDC – Corrected fracture density: Apparent fracture density corrected for orientation of borehole relative to the fractures
FCNB – Cumulative Number of Fracture: number of fractures in set counted from the bottom (1) to the top (1+n) of the well bore
FCAP – Cumulative mean aperture: sum of the mean apertures added from the bottom (0) to the top (0+n) of the well bore
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FCAH – Cumulative mean hydraulic aperture: sum of the mean hydraulic apertures added from the bottom (0) to the top (0+n) of the well bore
S c h l u m b e r g e r C o n f i d e n t i a l
Example Fracture Output Logs
PVPA – Apparent fracture porosity: porosity of a given length of borehole due to fracture aperture(s)
FVAH – Average hydraulic electrical fracture aperture: Cube root of the mean of the cubes of the individual apertures along the fracture trace averaged over a given borehole length
FVDC – Corrected fracture density: Apparent fracture density corrected for orientation of borehole relative to the fractures
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S c h l u m b e r g e r C o n f i d e n t i a l
Types of Intersection Between Fracture Sets
S c h l u m b e r g e r C o n f i d e n t i a l
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Dimensions of Fractures Parallel or Perpendicular to Bedding? S c h l u m b e r g e r C o n f i d e n t i a l
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Fracture length in a layer Deduced from the ratio complete/interrupted sinusoids in a horizontal well
S c h l u m b e r g e r C o n f i d e n t i a l
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Fracture Dimension x borehole diameter
In this particular example 6 truncated/223 complete = 0.027 Fracture length= 120 times the borehole diameter
70 60
max
50
medium min
40 30 20
Modified from JP . Delfiner (personnal communication)
10 Interval with 95% of confidence 0
0.
0.1
0.2
0.3
0.4
Truncated/complete fracture ratio
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Truncated
S c h l u m b e r g e r C o n f i d e n t i a l
Anisotropy due to Cemented Fractures in Horizontal Well
y c n e u q e r F
500m Fault
M Av
unsaturated bed
Cemented Fractures Schmidt Plot
Av = spacing average
zone M/ Av = .56
Spacing
Cemented Fractures Strike Stereogram
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S c h l u m b e r g e r C o n f i d e n t i a l
Fracture Modeling – Vertical Cross-Section Jurassic Carbonate of Saudi Arabia
UTMN profile Density 1419 of Conductive
Conductive Fract. Density
Fractures along the wellbore in a sub-horizontal well ft 0
WSW
ENE 1000
ft
2000
GR(gAPI)
150.0
ft
FVDC(1/ft)
0.0
0.0
20.0
Density of Conductive fractures
Bedding (stick mode) -6250
3000
ft
Gamma-Ray
-6250
CrossCross-section -6500
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-6500
S c h l u m b e r g e r C o n f i d e n t i a l
Stoneley – Fracture Permeability
BHC Energy attenuation
Open fracture
Chevron pattern
Preliminary results: • Recording affected by bad well conditions & LCM • Open fractures at interface shale/sand • More frac identified in shale than in sand (LCM effect?) • Further interpretation to be carried on along with DSI crossed dipole mode
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S c h l u m b e r g e r C o n f i d e n t i a l
Stoneley & FMI example: Fracture Permeability TOP
S c h l u m b e r g e r C o n f i d e n t i a l
X057.5m
D r e o w f l e n c g o t i o i n n g
BOTTOM
FMI sees a conductive fracture: is it open or clay-filled? 56 JvD 26-SEP-2007
Fracture Analysis using FMI & Stoneley
Identification of the location and the orientation of the fractures that most contribute to the reservoir permeability Selection of the intervals to test with MDT
Chevron pattern indicate energy losses of Stoneley waves in front large open fracture
S c h l u m b e r g e r C o n f i d e n t i a l
