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Fracture Analysis Using Borehole Image Logs

Petrom Technical Day

Bucharest, 26-27th 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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Fracture Detection & Conventional Openhole Logs I

� Dipmeter

– Fracture Anomalies – vugs, pyrite & shale clasts

– Borehole breakout – stress field, not fractures

– Resistivity anisotropy – stress field

� Sonic

– Cycle Skipping – Could be caused by Gas

– Waveforms – attenuation with excentralization.

– Variable Density log – chevrons at washouts

� Caliper

– Washouts & breakouts – stress field not fractures

� NGT

– High Uranium – cemented fractures, organic shales

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Fracture Detection & Conventional Openhole Logs II

� Resistivity

– Laterolog – invasion effects, borehole corrections

– MicroResistivity anomalies - washouts

– Anomalous high induction readings in resistive fractures – cemented fractures

didn’t produce

� Density

– Anomalous corrections – tool rotation, incipient

– breakout.

� PEF

– anomalies in barite mud – micro rugosity.

Most anomalies are associated with borehole 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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Available Borehole Image Logging Techniques

ARIHALS

Wireline Resistivity Tools

OBMIOBDTSHDT-OBM

Oil Based Mud

RABGVR

LWD

Requires clear fluid in the boreholeOptical image using down hole camera

Optical

ADNLWDDensity Changes of Borehole wall

Nuclear

UBIOBM/WBMAcoustic Impedance changes of Borehole wall

Acoustic

FMI (FMS)Slim FMISlim FMS

Water Based MudResistivity Changes of Borehole Wall

Electrical

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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 coverage: 360 degrees

� Main problem: opaque nature of borehole fluid, which prevents

common use in open hole for geological applications

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FMI* Measurement Principle

Current

Mass

insulated

sub

Upper

electrodes

Lower

electrodes

(SHDT pad)

� The FMI measurement principle use passive focussing

around the measurement electrode.

� FMI* = Fullbore Formation MicroImager

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� 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.

Borehole Image Logs

N E S W N

Core presentation

Unrolled image of a fault

S

E W

N

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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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OBMI2 for Increased Borehole CoverageOBM IOBM IOBM IOBM I

1 ft1 ft1 ft1 ft

OBMI2: Double Coverage => Double Borehole Geometry Data

32’

34’

0’

15’

17’OBM I2OBM I2OBM I2OBM I2

OBM IOBM IOBM IOBM I OBM I2OBM I2OBM I2OBM I2

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UBI* = Ultra-sonic Borehole Imager

� 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 subUBI subUBI subUBI sub

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Focused

Transducer

Borehole Wall

Measurements:

UBI Measurement Principle

Pulse EchoTransit Time

First echo amplitudeUBI signal

� 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)

BioturbatedNo fractures

No BioturbationFractured

FMI image

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Fracture Distribution

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Stress Perturbation in the Vicinity of a Fault

From V. Auzias

Depleted zone:

widely open fractures

Highly stressed zone

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Influence of Layering on Fracture Distribution

From V.Auzias et al 1998

5m

ThicknessM

ean

spac

ing (modified from Bouroz 1990)

5 10 15m

Fracture spacing Fracture spacing Fracture spacing Fracture spacing vsvsvsvs layer thicknesslayer thicknesslayer thicknesslayer thickness

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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.

Fracture Density

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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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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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W=W=W=W=c.c.c.c.AAAA....RmRmRmRm ....RxoRxoRxoRxobbbb 1111----bbbb

Tool current

Excess currentAAAA

Empirical formula from Luthi & Souhaité (1990):

W

Rxo

Rm

Fracture Aperture from Electrical Images

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)

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0 .1 .2 .3Aperture (mm)

Bbl/day

1000

100

10

1

Fracture Aperture can be estimated from conductive fractures on FMI/FMS resistivity images

Fracture Aperture Calculation from Electrical Images

Fracture Aperture can have a big impact on hydrocarbon production

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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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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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Fracture Porosity

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Scaling the Borehole Image using BorScale

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Scaling the Borehole Image using BorScale

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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).

Fracture Assessment using BorView

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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

� 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

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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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Types of Intersection Between Fracture Sets

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Dimensions of Fractures Parallel or

Perpendicular to Bedding?

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Fracture length in a layerDeduced from the ratio complete/interrupted sinusoids in a horizontal well

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In this particular example

6 truncated/223 complete

= 0.027 �

Fracture length= 120 times

the borehole diameter

Truncated

Truncated/complete fracture ratio0. 0.1 0.2 0.3 0.4

x borehole diameter

60

50

40

30

20

10

0

min

max

medium

Interval with 95% of confidence

70

Modified from JP . Delfiner(personnal communication)

Fracture Dimension

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Faultzone

500m

Cemented Fractures Schmidt Plot

Cemented Fractures Strike Stereogram

Anisotropy due to Cemented Fractures

in Horizontal Well

Av = spacing average

AvM

M/Av = .56

Spacing

Fre

quen

cy unsaturated bed

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Fracture Modeling – Vertical Cross-SectionJurassic Carbonate of Saudi Arabia

-6250

-6500

-6250

-6500

GR (gAPI)

150.0 0.0

FVDC (1/ft)

0.0 20.0

0 1000 2000 3000ft ft

ft ft

Conductive Fract. Density UTMN 1419 profileDensity of Conductive Fractures along the wellbore in

a sub-horizontal wellENE

CrossCrossCrossCross----sectionsectionsectionsection

Gamma-Ray

Density of Conductive fractures

Bedding (stick mode)

WSW

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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

Chevron

pattern

BHC Energy

attenuation

Open

fracture

Stoneley – Fracture Permeability

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Stoneley & FMI example: Fracture Permeability

FMI sees a conductive fracture: is it open or clay-filled?

X057.5m

Do

wn

go

ing

Do

wn

go

ing

Do

wn

go

ing

Do

wn

go

ing

refle

ctio

n

refle

ctio

n

refle

ctio

n

refle

ctio

n

TOP

BOTTOM

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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

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Conclusions:

� Fractures can provide an interface with the rock matrix which is many times greater than

that provided by the borehole. They can therefore play an important role in the

productivity of low permeability formations, yet some porosity in the rock is required.

� Fractures often behave as chaotic fractals and then they cannot be predicted with any

certainty. Direct measurement is the only way to confirm the presence of fractures and

mathematical extrapolation of fracture density through the reservoir is often wrong.

� Open fractures can also have a detrimental effect, e.g. when they extend into the water

zone. Healed fractures act as barriers to cross-flow in the reservoir.

� Electrical borehole images are currently the best available openhole logs for detailed

fracture detection, orientation and classification. To further evaluate fracture networks

sonic logging techniques and productivity tests can be applied.

� Enhanced interpretation techniques are now available to also extract textural information

from electrical borehole image logs. This textural information can be combined with other

logs to create a morphological facies classification.