ch11 fracturated fm ev
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Chapter 11
FRACTURED FORMATION EVALUATION
In view of the influence of fractures on tool
responses, and of their contribution to the produc-
tivity of formations, it is appropriate to devote a
whole chapter to the study of fractured forma-
tions.
11 .l. INTRODUCTION
Fracture is a general term that indicates all
breaks or ruptures in a rock, whether accompanied
by a displacement or not. It corresponds to a
surface along which there is a loss of cohesion.
These ruptures are caused by tectonic forces
(tension, compression or torsion), or by changes of
temperatu re, by drying out, or by leaching in the
plane of stratification or schistosity.
Generally grouped in the category of fractures
are :
crack is a partial or incomplete fracture;
fissure is a surface of fracture or a crack
along which there is a distinct separation, often
filled with crystals;
- joint is a surface of fracture without displa-
cement; the surface is usually plane and occurs
with parallel joints to form part of a joint set
(Glossary of Geology, 1980);
- gash is a small-scale tension fissure of several
centimetres to a few decimetres in length, and
several millimetres to a few centimetres in width.
It may be gaped or, most often, filled with crystals.
Several gashes are most frequently arranged in en
dchelon (Fig. 11-1). They are produced by simple
shear;
fault is a fracture or a zone of fractures along
which there has been displacement of the sides
relative to one another parallel to the fracture
(Glossary of Geology, 1980).
Calling a joint or fault a fracture depends on the
scale of observation.
The fractures may be cemented (filled with
crystalline material) or open. C learly it is the open
-I
Undeformed
Deformed by simple shear @
Fig. 11-l. En &helm tension gashes produced by simple
shear. (a) : Theory. (b) : Phatograph of an actual case (from
Ramsay.
1967).
fractures which are of interest for production,
because they create substantial permeability, and
a preferred flow path for the fluids. The latter are
largely caused by tension or torsion, while c losed
fractures are generally associated with compres-
sion.
Fractures are usually perpendicu lar to the plane
of stratification, and are usually more or less
planar. Moreover, the occurrence of fractures is
not random (Fig. 11-2). In a constrained formation,
the fractures appear as interconnected systems,
each system consisting of a group of more or less
parallel fractures. They result in the rock being
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broken up into small volumes or parallelepipeds
which can be broken off by the drill-bit or the
rotating drill-pipe.
The average gap of a fracture, or fracture
aperture, is often less than 0.1 mm, and so the
porosity of fractures is generally negligible [less
than 2%). Boyeldieu et al. (1982) have estimated
that, if the fracture system breaks the rock into
cubes with 10 cm edges, a gap of 1 mm would be
necessary to create a porosity of 3 %.
Fractures appear predominantly in brittle rocks,
hence in consolidated formations. Very often they
disappear on entering formations which are more
plastic (clays or halite), or friable (sands ).
11 l .1 Fracture Orientation
It has frequently been observed that the frac-
ture system, or network, in a given region tends to
have the same orientation as the fault system.
However, although the orientation may be statisti-
cally significant, it must be remembered that there
can be considerable dispersion.
11 .1.2. Importance of Fractwes
In formations of low porosity and permeability,
the production potential relies on an extensive
system of open fractures. The productivity will vary
greatly according to the number. extent and
opening of the fractures and to the porosity and
permeability of the matrix.
As already mentioned, the porosity of fractures
is insignificant in all but a few exceptional cases
(highly com pacted rocks), and makes no signifi-
cant contribution to the reserves. How ever, the
presence of fractures may significantly enhance
the drainage surface, and thereby the contribution
of the matrix porosity to the production. Open
fractures considerably increase the permeability
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but may cut the potential output of a reservoir if
they are not taken in to account during the secon-
dary recoven/ phase.
A subvertical fracture system may be fed by an
underlying reservoir. Finally, in the case of injec-
tion to maintain pressure, they act as preferred
paths for the injected fluids with the risk of
isolating formation blocks which are still hydro-
carbon-satu rated, and of having early production
of injected fluids.
11.2. REVIEW OF GENERAL CONCEPTS
Fracture creation and propagation being de-
pendent on mechanica l behaviour of rocks, it is
useful to review the general concepts involved I_
11.2.1. concepts of stress
Every element of a rock is subject to a series of
forces. These forces are of two types :
The first type corresponds to the forces that
are applied to the whole body of the rock. These
are called body forces, and are proportional to the
mass of the substances, e.g. gravity, centrifugal
forces, magnetic forces. They are measured in
force unit per unit volume (dimension : mLTe2) 2.
- The second type are known as surface forces.
They act on the surface of a body and, because of
this, are measured in force units pe r unit of
surface area (dimension : mLT-IL2 = mL-Te2).
In a solid, the force per unit area, acting on any
surface within it, is termed stress (Glossary of
Geology, 1980). Stress is equivalent to a pressure,
in which the SI unit is pas&. Taking into conside-
ration all the elements of a rock or bed (Fig. 11.3).
Fig. 11-3. surface forces acting on a body.
the surface forces acting on any imaginary surface
are represented by :
the weight of the above sedimen ts, or the
geostatic pressure, S, and the reaction of the
material below;
the fluid pressure pP; if the fluid is in equili-
brium (no movement) the fluid pressure is equal to
the hydrostatic pressure;
the tectonic forces, T.
One must distinguish between the external
forces that act on a body, and the resulting
internal actions and reactions that constitute the
stress. If the forces acting on a body are equal on
all sides, the body is in equilibrium. The all-sided
pressure is called the confining pressure, C.
In many cases the forces acting on a body are
not equal in all sides. This will cause deformation.
If the external forces tend to pull a body apart, the
body is said to be under tension. If it is subjected
to external forces that tend to compress it, it is
said to be under compress ion. If two equal forces
act in opposite directions in the same plane, but
not along the sam e line. we have a couple, and the
body is said to be under distortion (Fig. 1 -4).
Torsion is the state of stress produced by two
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force couples of opposite momen t acting in diffe-
rent but parallel planes about a common axis (Fig.
11.4d).
Let us take A as a point in a rock (Fig. 1 -5). and
X as a small plane surface, defined by the intersec-
tion o a plane P passing through A. A pressure.
?;= A /AZ will act on X. We can break theadown
into two components : (cr) normal to Z, called the
normal stress, and (r), parallel to Z, called shear
stress.
Generally, the pressure$ as well as o and t,
varv in maanitude and dire&n depending on the
-
orientation of the surface on which they are
applied. The set of all the pressures exerted on
point A on all planes that pass through this point
is called the state of stress.
The state of stress at any point may be descri-
bed in terms of nine stress components of which
only six are independen t if the body is in equili-
brium. The stresses on each face of a cube (Fig.
11-6) can be resolved into three parts, one normal
stress, and a shearing stress which itself can be
resolved into two components parallel to the
direction of two of the coordinates.
There is no direct way to measure the stresses
in a body, but they may be calculated if the
external forces are known.
But it is possible to calculate all the stresses at
any point of the body if the applied stresses at this
point on three mutually perpendicular planes are
known. It is also possible to demonstra te that at
each point A, there exist three orthogonal planes,
called principal planes of stress, for which r = 0,
and therefore the stress is perpendicular to them.
They constitute symmetry planes for the state of
stress.
The three normal vectors to these planes are
called the principal stress axes . On these three
mutually perpendicular axes, the three principal
stresses are ai follows (Fig. 11-7) :
- greatest or maximum principal stress, 0,;
intermediate principal stress, CB;
least or minimum principal stress, 03;
with cr, > 0~ > 03.
When the normal stresses are equal no shea-
ring stresses exist in the material. This state of
stress is known as hydrostatic stress. When they
are different, shearing stresses appear. The geo-
metric representation of the state of stress at a
point is known as the stress ellipsoid (Fig. 11-7).
One can demonstrate that six planes of maximum
shearing stresses exist associated in pairs each
pair countaining one of the principal axis, and
forming between them an angle of 900 (Fig. 11-8).
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1
The greatest shearing stress always occurs on the
planes which contain o2 axis (z is maximum the
stress difference, crl - oz being maximum ), and
make an angle of 450 to the principal stresses o1
and 0~ irrespective of the signs or values of the
principal stresses (ruptu res and slippages are
produced more or less along these planes, Figs.
11-7 and 11-9). In fact, fractures form an angle 0
less than 450 and close to 300 with the principal
axis. By reference to Coulombs work, this can be
related to the concept of internal friction which
suggests that, at failure, the relationship between
the magnitude of shear stress ITI and normal stress
0 is :
where G is the cohesive strength (sometimes
expressed as c for cohesive);
w being the coefficient of internal friction of the
material which is related to the angle of internal
friction $ by :
P = t d
4 being related to I3 by the following equation :
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t
t
(b)
(4
Fig. 11-11. Marble cylinder deformed in a laboratory by
compression. (a) : undeformed; (b) : 20% main. 270 am.
confining pressure; (c) : 20 % strain, 445 am. confining pres-
sure. 0, indicates the direction of maximum principal *tre**
(adapted from P ress & Sever, 1978).
The
relation
between
stress
and
rupture may
be
determined graphically by the Mohr stress circle
(Fig. 1 -10) which is a graphic representation of
the state of stress. To determine the cohesive
strength and angle of internal friction, a series of
experimen ts with different values of the confining
pressure must be run on cylinders submitted to
compression tests (Fig. 11.11). and the results
reproduced as a Mohr stress circle (Fig. 11.12).
The lines drawn tangent to the successive circles
define the Mohrstress envelope. Their intersection
with the vertical axis define the cohesive , or shear,
strength of the rock r,, which corresponds to the
inherent strength of a material when normal
stress across the prospective surface of failure is
zenf (Glossary of Geology, 1980). The slope of
each of these tangents defines the angle of inter-
nal friction q5, or each state of stress.
Strain is the deformation caused by stress. This
deformation may correspond to a change in vo-
lume which is called dilation or compression. It
may also result in a change in shape : dis tortion.
Fig. 11-12. (a) : Mohr stress envelope (adapted from Billings,
1972). (b) : Different types o f Mahr tiress envelopes in relation
with the rock type : (A) : wet clay: (6) : dry sand; (C) : rock
materials (adapted from Ramsay, 1967).
11.2.2. Mechanical Behaviour of Rocks
Every stress field imposes a strain field, but the
resulting deformation also depends on the nature
and the mechanical behaviour of the deformed
medium.
There are three mechanical behaviours :
- Elastic behav icur :
This is characterized by a possible return to the
initial state. Deformation appears immediately
after the force is applied and strain does not build
up. The deformation obeys Hookes law, which
states that strain is proportional to stress. The
solid regains its dimensions and its shape when
the stress is removed (Fig. 11-13). However, this
return to the initial shape is not necessarily imme-
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St**
r.pl.r*
(T
k -
(I =E*
PM.
e
Fig. 11-13. Linear elastic stressstrain law (adapted from
Ramsay, 1957).
Fig. II-Ma. Stress-strain diagrams for different rock beha-
viours. A : elastic; S : elastic-planic; C : elastic-plastic with
strength hardening; D : actual e&tic-plastic (from Billings.
1972).
Fig. ll-Mb. Spectrum of behaviour illustrating the transition
from perfectly brink (A) to perfectly ductile(E) behaviour. The
shape of the specimen is indicated along with the manner in
which it deforms under compression or extension, and the
shape of the stress-strain curve (adapted from Griggs &
Handin. 1960).
Fig. 11-14~. Differential stress (a? 6%) versus strain diagrams
explaining the transition from brirde to dunile bahaviourwhen
the confining pressure increases (c,).
diate, and may indeed take some time. An elastic
solid stands up until a certain limiting stress, ca lled
the elastic limit. If this is exceeded , the solid does
not return to its original shape. W hen the stress
exceeds the elastic limit, the deformation is plas-
tic. It means that the solid only partially returns to
its original shape. When the stress increases, at a
certain value the solid fractures. We reach the
rupture point. The relation existing between stress
and strain is expressed by a stress-strain diagram
(Fig. 11.14).
The resistance of a material to elastic deforma-
tion is defined as the srress-stra in ratio. This ratio
is the Youngs modulus E :
E=O
E
with :
0 = stress
E = strain. E is equal to the ratio of the change
in length, Al, to the original length, I,,
Rigidity measures the resistance to change in
shape.
G=1
Y
where G is the rigidity modulus, T the shear
stress, and y the shear strain.
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Fig. 11.16. Rheologic model of ela*tic strain : elastic spring
(from Ramsay, 1967).
Fig. 11.18. In a vi*cous material its strain is a function of time
(a). and the rapidity of its strain is a function of its viscosity (b).
-Bz2-
;.: ;,.
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Fig. 11-20. Rhedogic model Of viscous behavio : a damper.
Viscosity, 11. s the property that has a substance
to offer internal resistance to flow. It is equal to
the ratio of the shearing stress, r, to the rate of
shear strain, y. per unit of time, or dy/dt. The rate
of shear strain, y. is measured by the change in
angle I+I per unit of time t (Fig. 11-19) :
The viscosity unit is called poise. Viscosity is
very high for rocks but decreases when tempera-
ture increases (Table 11-l). Viscosity is an impor-
tant property in geological processes. It determi-
nes, for example, the flow of magma or lava during
intrusive or volcanic activity, and the velocity of
displacement in plate tectonics.
The rheologic model for viscous behaviour is a
damper, a perforated piston moving without fric-
tion in a-fluid (Fig. 11.20).
11.2.3. Factors Controlling Rock Behaviour
In addition to their inherent properties (minera-
logy, texture), the mechanical behaviour of rocks is
controlled by several factors such as confining
pressure. temperature and time.
11.2.3.1. Confining pressure
The strength of a rock increases with the
confining pressure . Figure 11-21 illustrates the
effect of confining pressure on the breaking
strength of several standard rocks. At low confi-
ning pressure, all the rocks deform only a few
percent before fracturing. Under a high confining
pressure. we observe a different behaviour for the
rocks.
When fractures appear at less than 3-51 plastic
deformation, the rocks are said to be britth?. When
rocks are able to sustain. under a given set of
conditions, 5.10 96 plastic deformation before frac-
turing. they are ductile. Ductility is a measu re of
the degree to which a rock exhibits ductile beha-
viour under given conditions, commonly expressed
by the strain at which fracture commences (Glos-
sary of Geology, 1980). As a consequence, when
the confining pressure increases a brittle rock
becomes ductile (i.e. limestone).
Fig. 11-22 Effect of temperature on deformation of marble
(from Griggs. 1939).
11.2.3.2. Temperature
The elastic limit decreases when the tempera-
ture increases. Moreover, less stress is necessary
to produce a given strain when the temperature
increases (Fig. 1 -22).
11.2.3.3. Time
Time plays a very important part in the beha-
viour of the rocks. R ocks may exhibit elastic
behaviour if they are subjected to very short
duration stresses, becoming plastic if these stres-
ses are applied over a long time. This effect is
observed in creep experiments, where a small load
applied for a sufficiently long time produces a
strain that may continue and eventually cause
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Fig. 11.23. - Ideal creep cwve. A : instantane-xs deformation.
8 : primary creep. C : secondaw creep. 0 : tertiary creep (from
9ioings. 1972).
rupture. The same stress in instantaneous tests
would not cause any measurab le strain. Figure
11-23 illustrates an ideal creep curve.
11.2.4. The Actual Behaviour
of Rocks
In nature, rocks have a complex behaviour of all
three types of response visco-elasto-plastic. One
of these components may dominate according to
physical conditions (temperature and pressure)
and the way the stress is applied.
At low temperature the elastic deformation of
the crystal of quartz shows an almost perfect
reversibility.
Rocks which show a good reversibility and
admit the greatest elastic deformation are :
quartzite, plutonic rocks;
- slates.
Such rocks are brittle.
Some other rocks are more or less ductile, or
show an elasticoplastic behaviour. Few rocks, such
as halite and undercompacted shales, may have a
plastic to viscous behaviour.
According to the previous factors, it is possible
to determine the different kinds of strain following
the depth :
an upper zone, where most of the rocks have
an elastic (brittle) behaviour;
an intermediate or middle zone. where the
rocks have an elasticoplastic to elasticoviscous
behaviour (ductile):
- a deep zone, where rocks w ill show a plastic
behaviour. This zone is characterized by the ap-
pearance of schistosity, and then of foliation. It
corresponds to anchimetamorphism and to meta-
morphism. This type of rock has no interest in oil
exploration, since porosity and permeability disap-
pear.
Fig. 11-24. Change in shape without change in volume under
shear stress. (adapted from Lee, et a/., 1978).
Table 11-2
Compressive, tensile, and shearing s trengths of
some rocks
(from Billings, 1942).
Sandstone
..........
Granite..
..............
...................
Gabbro..
.............. 1 Wo to 1900 ...................
8aSalt.. ...................................
Fekite..
...................................
11.2.5. Types of State of Stress
These are three types of state of stress :
tension or traction : stretches the material and
may increase its volume;
compressional: leads to a decrease in the
volume of the material;
pure shear stress: produces a change in
shape, but not in volume (Fig. 11-24).
11.2.6. Rock Strength
Rocks are more or less resistant to stresses. The
strength of a rock corresponds to the stress at
which the rock starts a permanent deformation.
Rocks show different types of strength, be-
cause they respond differently to various s tresses.
Hence. there is, for each rock, a compress ive,
tensile and shear strength.
The compressive strength for a brittle rock is
sometimes 10 to 30 times more than its tensile
strength (Table 11-2).
11.2.7. The Results of Stresses :
Strains
The reaction of rocks to stress falls into two
categories :
- continuous strains which are folds and flows.
They will be studied in the chapter : Information on
Tectonics;
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discontinuous strains which are fractures
(studied here after), faults (s tudied in the chapter :
Information on Tectonics), and pressure-solution
(stylolites) studied in the chapter : Information on
Diagenesis.
11.3. MECHANICAL PROPERTIES
EVALUATION FROM LOGS
Knowledge of the mechanical properties of a
rock is required in several domains.
11.3.1. Mechanical Behsviour of the Reservoirs -
Stress Computations
To know if reservoirs require tubing or gravel
packing, or if they can be produced in open-hole
conditions, or if they will collapse, it is necessav
to estimate the critical wellbore pressure P.. It can
be demonstrated that PC is expressed by the
following relation using the Mohr-Cou lomb failure
criterion :
1.50. - 0.5ci - 0.5 a P, - 1.732~,
P. =
where a = 1 Cd&. C, and Cb being respect-
vely the rock compress ibility (at zero porosity) and
the bulk compressibility (with porosity), P, is the
pore pressure. q is the initial shear strength (= b),
and v the Poissons ratio. ox is the minimum
horizontal stress. It can be obtained assuming a
horizontally constrained elastic model and is ex-
pressed, following the Griffith and Mohr-Coulomb
-failure criteria, by :
ox = & (PO, -UP,) + CCP,
( 1
where Pob is the overburden pressure, assumed
to be equal to crz. In the simplified Terzaghi and
hard rock options a is assumed equal to unity.
Only elastic constrains determine oz = Pob. The
laws of elasticity associate to this vertical stress a
minimum horizontal stress ox, and the tectonic
stresses are estimated through the value of o,
which can vary between ox (in a non tectonic
regime), and 0;.
11.3.2. Fracture-Pressure Computations
The fracture initiation pressure P, is a function
of several parameters. It is expressed by the
following relation :
Pb = 3ox - 0 - UP, + To
Fig.
11.25.
Example of
borehale damage due to breakout
effect along the borehole wall. On these images, obtained by
the Formation MicroScanner tool. compare the right figure to
the left one which shows a series of natural fractures in a
cemented sandstone (courtesy of Schlumberger).
where ox and ciV are the minimum and maximum
horizontal stresses respectively. I+ is usually defi-
ned in terms of the tectonic imbalance factor
oY/op Existence of tectonic imbalance can be
inferred from borehole deformation tests, or from
break-out identification with the aid of multiple-
diameter caliper logs or, better, from Formation
MicroScanner images (Fig. 11-25). Pore pressure is
obtained from measurements with the RFT tool in
new wells, or from pressure build-up tests in
producing wells. T., is the tensile strength. In
Terzaghi or hard rock options, a is assumed to
be equal to unity.
To compute the fracture re-opening pressure Pt,
the tensile strength is set equal to zero. So we
obtain :
Pf, = 31sx D - Pp
These parameters are computed and displayed
in the MECHPRO program (Fig. 11-26).
11.3.3. Dynamic Elastic Prope rties
Computation of some of the previous factors
require the knowledge of the dynamic elastic
properties. If a sonic waveform recording has been
made using a Long Spacing Sonic tool (LSS) or
the Array Sonic Service, Ato and A& can be
obtained from the waveform analysis. By combi-
ning these two data with the corrected bulk
density, it is possible to compu te the dynamic
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MECHANICAL PROPERTIES
WELL B-10
COlllpUtCd Test
Initiation 1 oo psi/A 0.96 pa
Re-opening 0.95 pat 0.96 psi/A
139UOps.i 14050 psi
ClCISUR 0.80 psi/A 0.81 psi/A
11710 psi 1185Opsi
-4
r
.Y
Fig. 11.29. Example of a display of the mechanical properties of rocks computed with the MECHPRO program (from E dwards.
1985).
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Pa
L
1
-
-
-
Yh%E
CB
CiiOO
Fig. 11-27. Example of a display of the elastic properties and formation strength computed with the MECHPRO program (from
Edwards. 1985)
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Table 11-3
Dynamic elastic parameters and how they can be
computed from wireline log data.
Table 11-4
Uniaxial compressional and tensile strengths for
rocks.
elastic parameters at each sampling level (Table
11-3). This is achieved by the MECHPRO program.
An example of the display of the results is given in
Figure 11-27.
11.3.4. Inherent Strength Computa tions
The inherent rock strengths are computed by
the MECHPRO program. They are related to one
another by simple functions expressed below.
Initial shear strength %
This parameter is derived by an empirical model
based on Deere & Millers work (1969) and elabo-
rated by Coates & Denoo (1981).
z, = =$[O.OOSV,,,, + 0.0046(1 - /,I.,)]
Uniaxial compressive strength C,
q5 is the angle of friction in the Mohr-Coulomb
failure model. It is set at 300.
Tensile strength r.
The tensile strength is set at one-twelfth of C,
as the average value (Table 11-4).
In addition to these applications mechan ical
properties evaluation can be used for :
_ mud weight control to avoid hydraulic fractu-
ring and loss of circulation;
- drillability of the formation : adaptation of
drilling parameters, choice of rock bit. of the
rotation speed, weight on the rock bit .
_ dipmeter interpretation by enabling a choice
between the faulting or folding of rocks.
Uniaxial Compressional and Tensile Strength+ fw Rocks
G
3,
c.
MPa 70
Quart&z, Cheshire
461 28 16.5
Granite, Westerly 229 21 10.9
Diabase, Frederick
466 40 12.2
Sansdtone, Gosord
50 3.6 13.9
Marble, Carrara
90 6.9 13.0
coulombs p and e for Rclcks
P
ML
Granite
0.64 0.31
sandstone
0.51 0.29
Marbre
0.75 1.1
11.4. EFFECTS OF FRACTURES
ON THE RESPONSES
OF THE LOGGING TOOLS
With the exception of the Borehole Televiewer
and the Formation MicroScanner tools, which can,
in favourable circumstances, see fractures directly,
the responses of the logging tools are affected
only indirectly by the presence of fractures. It is
only by these indirect effects that the fractures
can be detected.
With this in mind, we will now examine, tool-by
-tool, the effects of fractures on their responses,
and so get an idea of the capacity of each tool for
detecting them.
11.4.1. Natural Gamma Radioactivity
To the extent that the circulation of fluids may
have contributed to the precipitation of uranium in
the fracture system, the standard gamma ray tool,
or the spectrometn/ of the natural gamma ray, will
show increased activity levels or increased ura-
nium content in front of fractured zones (Fig.
11.28).
Similarly, a comparison between two succes-
sive gamma ray measurements. the first with a
non-radioactive mud, and the second over the
same section after a radioactive tracer has been
circulated briefly in the mud, may show up fractu-
red zones. The tracers invade the permeable zones
and cause the open fractures to exhibit increased
radioactivity. A further measurement made some
time later, or after the start of production, should
show decreased radioactivity over the fractured
ZClC?S.
NOTE: In cases of deep invasion, the start of
production may cause a temporary increase in
activity by bringing the radioactive mud closer to
the borehole wall.
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GAMMA RAY
SPECTROMETRY
I
/
Fig. 11.%a. Fractured zones in this Ordovician formation of
Algeria identified by uranium peaks (from Schlumber9er. Well
Evaluation Conference. Algeria, 1979).
11.42. Caliper
Fractured zones may appear on the caliper
log(s) as :
a reduction in hole diameter in compacted
zones which are in gauge, most probably due to a
deposit of mud cake, especially if lost-circulation
material has been used (Fig. 11-29);
Fig. 11.29b. Natural Gamma Ray Spectrometry log over a
fractured section (from Schlumberger. Well Evaluation Confe-
rence. Egypt, 1984).
an increase in hole diameter due to crumbling
of the fractured zone during drilling resulting in
chunks of various sizes falling away.
These phenomena can best be seen by a four-
arm caliper tool. such as the BG T, or dipmeters
rather than the standard two-arm calipers (Fig.
11.30).
An increase on only one of the diameters is due
to the presence of fractures and follows their
orientation (Fig. 1 -31). The orientation can be
obtained from the inclinometry measurement. The
direction of elongation is often that of a major
system of faults and fissures, as has been shown
by various researchers (Babcock, 1978 (Fig. 11-32);
Bell 8, Gough. 1979; Cox, 1983).
11.4.3. Thermometer Log
The temperature gradient in the mud is affected
by the presence of open fractures due to the
invasion of the fracture system by the drilling mud
which has the effect of cooling the formations.
This phenomenon must not be confused with gas
production which also causes a drop in tempera-
ture.
The circulation of mud disrupts the normal
distribution of heat which depends partly on the
difference in temperature between the mud and
the formations, and partly on the thermal conduc-
tivity of the rocks. The latter varies conside rably as
each type of rock has its own thermal conductivity
(Table 11-5 and Fig. 11-33). For this reason, a
thermometer log recorded immediately after dril-
ling and measured on the run-in can be a good
indicator of the types of rock encountered.
The mud at the bottom of the well is usually
cooler than the formations, while near the surface
it is hotter. When circulation has been stopped for
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1
I
Fig. II-SO. Hole ovali~ation in fractured zones [from Babcock.
1978).
some time, the mud temperature tends to homo-
genize by thermal exchange , horizontally by
conduction , and sometimes vertically too, by
convection. Thus. temperature changes at all
depths are slow, and some time is required before
the tempera tures revert to their original values.
Fig. 11-31. Three possible reasons for the barehale ovalisa-
tion. (A, : single steeply di,,,,ing fracture: (B) : closely spaced
Thus. the mud becomes heated in the deeper part
steeply dipping fractures; (C) : intersecting fractures.
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r
I I
Fig.
11-32. [a) : Relationship between the hole avalisation and the direction of pnts on outcrops (Cretaceous to Devonian
sandstones in Canada); (b) : Remarkable consistency in direction of hole ovalisatian over a large region (from Babcock. 1978).
of the well. This means that the temperature
gradient of the mud intersects the geotherma l
gradient at a certain depth (Fig. 11-34). Above this
point of intersection the mud is hotter than the
formations , while below it is cooler. Consequently,
mud invasion in the upper zone increases the
formation temperatu res, while in the lower zones
they are decreased. Clearly, the interpretation of
temperature logs must take account of the posi-
tion of this point of intersection.
When a cool fluid such as the drilling mud
penetrates the formation it displaces the forma-
tion fluid. The time taken fo r the formation to
revert to its normal temperatu re will depend on the
duration of circulation and on the degree of inva-
sion (Fig. 11.35).
Zones which have been more deeply invaded
will thus appear as cooler zones on the tempera-
ture log. This will be particularly noticeable in
zones with open fractures where there has been a
partial or total loss of circulation.
Fig. 11.33. Theoretical temperature profile as a function of
lithology and depth.
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11.4.4. Formation Density
In the case of the compensated formation
density tool, two measurements may be conside-
red : the density measurement itself, and the
density correction.
Being a pad-mounted device, the density tool
may face in different directions on two successive
runs over a fractured interval. One would then
expect a drop in density if on one of these runs the
pad was facing an open fracture. However, the
dense, compact formations in which fractures
usually occur will produce low count rates on the
detectors, and hence a high level of statistical
variations. The resulting poor repeatability bet-
ween successive runs, which is a feature of
high-density formations, whether they are fractu-
red or not, makes it impractical to look for a
variation in density as an indication of the pre-
sence of fractures across one axis of the hole.
The fact that the tool is unidirectional and not
free to rotate does not simplify matters. However,
it may be assumed that, if the hole is eccentric, the
long axis will have the same orientation as the
vertical fractures. as long as these are more or less
unidirectional.
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The readings of pad-mounted tools will be
affected by small depressions in the borehole wall
which are the result of small pieces of rock falling
away. The short-spacing detector is more influen-
ced by the mud filling these small cavities than is
the long-spacing detector.
In zones where the caliper indicates a smooth
borehole wall, the Ap curve will show a higher
correction than normal in the case of baryte m uds
(Fig. 11-36). This is often accompanied by a very
low density reading, but may be localised. blurred
or even hidden by the time constant of the
measurement circuit.
- The caliper may indicate sudden changes of
hole diameter. When these changes are due to
scaling of the formation wall, they can be seen
by the short-spacing detector.
11.4.5. Photoelectric Capture
Cross-Section
This measurement, which is made with the
Schlumberger Litho-Density tool (LDT], is more or
less independent of porosity. Consequently it is of
no use for detecting fractures in normal muds.
However, the measurement is very sensitive to
baryte, and so can detect fractures which have
been invaded by baryte muds. Wh en the pad of
the tool passes a fractured zone, the photoelectric
capture cross-section will show very high values
(Fig. 11-37). This is due to the high atomic number
of barium compared to those of the elements
making up the majority of sedimentary rocks. This
property can be useful for estimating the porosity
of the fractures (see below).
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11.46. Neutron-Hydrogen Index
This measurement responds essentially to for-
mation fluids, and so it is a measu rement of total
porosity. Since the porosity of fractures is usually
small compared to that of the matrix (e. g. in chalk
or compac ted clays), it is difficult to identify
fractures because the small variation in porosity is
masked by statistical variations. In any case,
because it is not a directional measurement, the
CNL tool will give a more stable measurement.
This is especially true in dense. compact forma-
tions because of higher count rates and lower
statistical variations.
11.4.7. Sonic Travel Time
In theory, the travel time of the compressiona l
wave is unaffected by fractures which do not cross
the shortest time path. This is the case with
subvertical fractures, or more correctly fractures
which are parallel to the tool axis, and these are
generally not detected by the sonic tool.
Whenever the fracture system is more complex,
diffraction and reflection will attenuate the com-
pressional wave to such a degree that detection
may not occur until the second or third peak in the
wave train, resulting in erratic increases in the
apparent travel time (so-called cycle -skips, Fig.
11.38). This phenomenon is detected more easily
with the older, uncompensated tools. Newer tools
are capable of detecting cycle-skip conditions and
may automatically take steps necessary to avoid
cycle skipping that may be due to presence of
fracture.
The shear wave velocity, on the other hand, is
more affected by fractures than that of the com-
pressional wave. It is seen to decrease while the
compressional velocity remains constant. Thus, by
comparing A& with AL possible fractured zones
can be identified when A& increases while Att,
remains constant. These measurements can be
made with the Schlumberger Array Sonic Service.
11.4.8. Attenuation of Acoustic Waves
In general, the amplitude of an acoustic wave is
decreased when it crc~sses a fracture. This is the
result of a transfer of energy. The coefficient of
transmiss ion is a function of the apparent dip of
the fracture relative to the direction of propaga-
tion. Energy transmission across a fracture de-
pends to a large extent on the efficiency of mode
conversions at the fracture interface. For acoustic
b
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energy to cross a fracture, a propagating com-
pressional or shear wave must be converted to a
fluid wave at the first fracture interface and then
converted back again at the second. Obviously, the
inclination of the fracture is crucial here. Figure
11.39, from Morris et. a/. (1963). is based on
experimental results and shows that compressio-
nal waves suffer little attenuation on crossing
fractures which are parallel or perpendicular to the
tool axis. The attenuation is high when the angle is
between 350 and 800. Shear waves on the other
hand, are strongly attenuated by fractures at low
angles. According to J. Gartner (in a personal
memorandum) this contrasting behaviour could
suggest a conversion from one mode to the other
(compress ional to shear) for certain values of
inclination of the fractures. The attenuation de-
creases with increasing dip. It becomes very small
when the dip of the fracture is above 650 (250 to
the axis of the tool or borehole).
A technique for measuring the attenuation is
the acoustic Variable Density Log (VDL). It invol-
ves presenting the shape of the wave train in a
continuous manner. The values of amplitude are
represented by varying shades of grey.
In this measurement, zones with fractures at an
angle to the tool axis will be characterized by
distortion and interference due to reflection and
refraction at the fracture planes. This disrupts the
normally parallel appearance of the waves on the
VDL, and causes a reduction in the density of the
grey band. This is accompanied by blurring and
loss of vertical coherence in the wave train (Fig.
11-40).
In addition, the appearance of chevrons, asso-
ciated with a reduction of amolitude without any
change in At may indicate the existence of fractul
res at a high angle (Fig. 11-41).
The interpretation of these measurements is not
always straightforward, because other phenomena
can produce the same effects.
11.4.9. Stoneley Wave
The Stoneley wave, and especially its low fre-
quency component known as the Tube wave, is a
borehole fluid mode that propagates as a pressure
wave along the borehole.
The way fractures affect the Stoneley wave is
quite different compared to the way they affect
compressional and shear waves. Acoustic energy
is not lost through inefficient mode conversions.
but more as a result of moving the fluid in the
fracture system, resulting in a pressure drop in the
borehole. As a result, the direct Stoneley wave is
attenuated. and a reflected Stoneley is generated.
Three advantages of the Stoneley wave analysis
can be considered.
In fast formations, where we generally look
for fractures, Stoneley wave amplitude is much
higher than the other two arrivals (compressional
and shear Fig. 11-40). so it is more straightforward.
The Stoneley wave, being mainly influenced
by borehole fluid, does not react much to changes
in lithology. Thus, a strong Stoneley reflection
most likely indica tes an open fracture, not a bed
boundary.
- The roughly cons tant Stoneley velocity eases
the signal processing task of measuring the reflec-
ted signal.
Stoneley wave attenuation may correspond to
fractures if other possibilities such as caves,
change in rigidity, and crossing a bed boundary
can be eliminated by analysing the other open-
hole logs.
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, --.
I
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r
11.4.10. Resistivities
The electrical system consisting of the forma-
tion, the borehole and the fracture network is
represented by the diagram in Figure 11-42. The
fractures are assumed to be subparallel to the
borehole axis and invaded by a conductive fluid.
Taking into account the current distribution for
each type of device, it will be observed that, in the
case of fractures which are subparallel to the
borehole axis :
- the induction is unaffected by the fractures
which only constitute a negligible part of the
whole circuit since they are in series for the
Foucault currents;
the electrode tools will be strongly affected
because the fracture network presents paths of
lowered resistance which act as shunt resistances
to the current.
In the case of fractures which are subperpendi-
cular to the borehole axis :
- the induction will be strongly influenced
because now the fractures are in parallel rather
than in series, and their conductivity is very high
compared with that of the surrounding formations;
- for the other tools, these fractures continue to
offer paths of lowered resistance.
Thus, a comparison of resistivity values from
induction and electrode tools in zones containing
subparallel open fractures will show substantially
lower resistivities on the laterologs than on the
induction (Fig. 11-43). However, we must bear in
mind that the induction measurement is not re-
commended in resistive, compact formations
because of low signal level. The analysis will
therefore rely on the relative behaviou r of the two
laterologs (deep and shallow) and of the microde-
vices.
When the fratures are subparallel to the bore-
hole axis, the apparent drop in resistivity becomes
more pronounced with decreasing depth of inves-
tigation although it remains constan t w ithin a
Fig. 11-43. Comparison between the reswn~es of the induc-
don and laferolog in a fractured zone (courtesy of Schlumber-
9-I.
b
Fig. 1144. Current distribution in the case of a fracture which
is subparallel to the borehole axis. a) : Claviers model; b) :
&aus model (courtesy of Schlumberger).
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Fig. 114. Example showing the responses of the latemlogs
and the MSFL in a fractured zone.
fracture. Consequently the deeper-reading device
is less affected by the fracture than the shallow-
reading device. A ratio of 1.5 to 2 is commonly
observed between RLLo and RLLS. Moreover, if the
drilling mud is more conductive than the original
formation fluid (gas, oil or fresh water), the resisti-
vity of the LLS will be substantially less than that
of the LLD (Fig . 1 -44).
If the mud is less conductive than the original
fluids in the fractures, the separation of LLS and
LLD is much less and may even be inverted.
In compact zones of low porosity which are not
fractured, and therefore with little invasion, the
two measurements will read about the same
resistivity (Fig. 11.45, top interval).
Because they are pad-mounted, the microdevi-
ces only respond to fractures in front of the pad.
But because the borehole wall tends to crumble
near the fractures, it becomes ovalised, and the
pad tends to ride the low side of the major axis.
Hence, the probability of following the fracture
network is increased. Clearly the presence of
fractures will strongly influence these devices
because of their sma ll volume of investigation.
Moreover, this part of the fracture system will be
invaded by mud or mud filtrate, and so the resisti-
vities will be much lower (Fig. 11-45, bottom
interval). In addition, crumbling of the borehole
wall will create zones of current leakage. All this
enhances the difference in the resistivity readings
of the micro- and macrodevices.
11.4.11. Dipmeter
Several parameters must be analysed with this
tool :
11.4.11.1. Resistivity curves
As with all the pad-mounted microdevices, only
the pads which are in front of the fractures will be
affected and show a drop in resistivity (Fig. 11.46).
If the hole is ovalised because of fractures, the
usual orientation of the tool will be with two of the
four arms across the major axis, the other two
being perpendicular. Thus in compact, fractured
formations. the two opposite pads which see the
fractures will show a drop in resistivity, while the
other pair, which does not see them, show a high
resistivity value with little or no curve activity (Fig.
11.47). assuming that a low EMEX value has been
used.
Superimposing the resistivity curves of two
adjacent (i.e. 900 apart) pads will reveal fractured
zones whenever there is a separation between the
two curves. A visual representation of the pre-
sence of fractures is obtained by shading between
the two pairs of adjacent curves (Fig. 11.48).
This technique is known as Fracture Identifica-
tion Log (FIL), and this presentation can be obtai-
ned at the wellsite using the CSU system.
Unfortunately, the FIL is often confused by
sedimentary features such as laminations, flasers
or pebbles, and the majority of the shaded areas
correspond to beds with an apparent dip rather
than to fractures.
This problem has bean eliminated with the
introduction by Schlumberger of a new program
known as DCA (Detection of Conductive Anoma-
lies). Conductive events which cannot be correla-
ted are searched for, and only these can be
interpreted as possible fractures. The events are
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defined during GEODIP processing. The conduc-
tive anomaly is then reproduced only if the follo-
wing conditions are satisfied :
the conductivity exceeds a certain value;
- there is a sufficient difference between the
conductivity values;
the anomaly is detected on a minimum num-
ber of successive intervals.
The three thresholds can be set by the log
analyst and so adapted to local conditions. The
results are presented in the form of a log. The
azimuths of pads 1 and 2 are displayed against
depth in the leftrhand track (Fig. 11-49 & 11-50).
The shaded areas indicate a difference between
the nominal hole diameter and the readings of the
two calipers.
The azimuths of pads 1,2,3 and 4 are displayed
against depth in the right-hand track. The conduc-
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j
I
i
-c
I
55
II
F
m
-
Fig. 11-X. Further DCA example with the SHDT tool
(Schlumberger. Well Evaluation Conference, Egypt, 1994).
tive anomalies are then indicated along the COT-
responding azimuth curve. The available fracture
indicators with this presentation include :
the conductive anomalies revealed by the
DCA program;
borehole rugosity and the axis of ovalisation;
- changes in the speed of rotation of the tool.
A polar frequency plot of the conductive anoma-
lies is also provided (Fig. 11-51). It is used to
determine the direction of the fracture network or
networks. This direction is related to the axis of
maximum constraint and to the general orientation
of the faults in the region.
When the hole is not very ovalised, the tool will
rotate because of the torque in the logging cable.
The fractures are then seen successively by the
different pads (Fig. 11-52).
The SHDT tool gives even better detection of
fractures by comparing the measurements of two
buttons on the same pad (Fig. 11-53). In certain
favourable cases, the dip of the fracture can even
be determined (Fig. 11-54).
11.4.11.2. Azimuth Curve of Pad 1
As we have seen, the tool normally rotates as it
travels uphole. Any slowing, stopping or change of
Fig. 11-51. Example of a polar frequency plot which provides
a means of orienting the fracture network (courtesy of
Schlumberger).
direction in the rotation usually indicates the
presence of fractures. This phenomenon is the
result of the pad following a sort of subvertica l or
oblique pathway created by crumbling of the
fractured zone for a certain distance (Fig. 11-55).
The tool then resumes its normal rotation, usually
after a brief period of more rapid rotation to
release the torsion which has built up in the cable.
11.4.11.3. Caliper
Since the dipmeter tool has two measurements
of diameter 900 apart, comparison between them
will reveal any hole ovalisation, sudden variations
in diameter, or restrictions due to deposits of mud
cake or lost circulation material in the fractured
zones (Fig. 11-29).
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b
Fig. 11-W (a) : Examples of conductive anomalies which can
be detected by the SHDT fool. (b) : They can be correlated to
determine the dip and the azimuth of the fractures (courtesy
of Schlumberger).
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11.4.11.4. Dips
In compact fractured formations, the fractured
zones can be identified from the CLUSTER pro-
gram for the HDT tool, or the MSD program for
the SHDT tool by examining the values of erratic
dips or dips of poor quality. Correlations which are
due to conductivity peaks have no reason to
produce dips which are consistent in either dip
angle or azimuth.
When the GEODIP program is used for the HDT
tool, or the LOCDIP program for the SHDT tool,
there is a noticeable absence of four-pad dips.
There may, however, be some dips which are
erratic in dip angle and azimuth which are due to
three-pad correlations. In certain favourable cases
(e. g. a single fracture), the conductive peaks can
be correlated to give the dip of the fracture (Fig.
11-54 & 11-56).
11.4.12. Formation MicroScanner Tool
When one of the 54 button electrodes (two pads
of 27 electrodes each) on these pads of this tool
passes an open fracture in the formation, the
current it emits will take the least resistive path.
This will be reflected on the corresponding
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conductivity curve as a sharp increase, while the
images will represent fractures as one or several
dark irregular lines (Fig. 11-57).
One of the major advantages of this tool is the
continuous lateral coverage it provides across
twice a 7 cm wide strip, due to the large number
of electrodes with overlap of each raw over the
surrounding raws. As Figure 11-58 illustrates indi-
vidual fractures can be identified. If borehole
coverage is built up through several passes.
between which the pad rotation has changed, their
direction and average dip can also be obtained
(Fig. 11.59).
Healed cemented fractures can also be detec-
ted, if the resistivity contrast with the surrounding
rock is sufficient. These appear as white irregular
lines on the images (Fig. 11-60).
In most cases the Formation MicroScanner tool
enables distinction between natural fractures and
those induced during the drilling of the well (Fig.
11.25).
11.4.13. Spontaneous Potential
Negative anomalies are sometimes observed
on the spontaneous potential in fractured zones.
This is often explained by the developm ent of an
electrofiltration potential w hen they have been
drilled with a fresh mud (salinity of less than 5,000
wm).
11.4.14. Borehole T&viewer
This tool (Zem anek et al., 1989) provides an
acoustic image of the borehole wall (Fig. 11-61). It
is obtained by measuring part of the acoustic
energy reflected from the borehole wall. The same
transducer acts as both transmitter and receiver.
The formation is more reflective when the rock
is smooth and compact. When it is rugose, fractu-
red or vuggy. the acoustic energy is more disper-
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PAD AZIMUTH
t
0.2m
+-0.2m-+
DEPTH
CAUPERS
Pad
Direction
*
4-
3
1
Pad 3
Images
Pad 4
Images
Pad 4
TMCl?S
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J
sed and these irregularities then appear as darke-
ned areas on the film.
This tool provides, then, not only a detection of
all the open fractures, but also their orientation
and dip. The only requirement is to minimize the
amount of material in suspension in the mud to
avoid having a speckled image due to dispersion
of the energy. Other adve rse conditions to be
avoided are excessive mud-cake, excessive hole
ovalisation or gas-cut mud.
Fig. 11-62. Fractures can be detected by bofh tha amplitude
and the filtered transit time recorded by the borehole fele-
viewer (courtesy of Schlumberger).
11.5. DETECTION OF FRACTURES
FROM WELL LOGS
As we have just seen. only two logging tools are
capable of detecting fractures themselves, that is
breaks in rocks. These are the borehole televiewer
(BHTV) and the Formation MicroScanner (FMS)
tools.
In the BHTV tool two parameters can be used
for fracture detection, the amplitude of the recei-
ved signal and its transit time. The amplitude of
the signal is reduced due to the dispersion of
energy at the edges of the fracture, while the
transit time will be increased (Fig. 1 -62).
When several passes are made in the same well
with a Formation MicroScanner tool, taking care to
ensure that the tool has rotated (azimuth of pad 1
has changed), it is generally possible to detect
each fracture (Fig. 11.58). Thus their number,
distribution, form, orientation and average dip can
be determined. It is also possible to verify if they
are ordered or consist of several networks.
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Other logging tools are not capable of detecting
fractures themse lves, but by the effect that the
fractures have on the log measurements. They
rarely allow the detection of individual fractures,
only indicating the presence of fractured zones.
But the variations in tool response caused by
fractures could also be caused by other pheno-
mena. The following procedure is recommended
to be sure of the origin of these variations :
it is necessa ry first of all to look for these
variations in intervals which are likely to be fractu-
red. These may be zones in which there has been
a loss of circulation or an inflow of fluids, or
consolidated formations such as chalks, iimesto-
nes or compact dolomites, quartzites, anhydrites,
or metamorphic rocks. In general terms, it is zones
of high resistivity which are of interest, and not
porous, unconsolidated sands or plastic clays. A
preliminary pass with the LITHO and MECHPRO
programs. which have already been described, will
identify facies which are favourable to fracturing.
The next step is to note all possible occurren-
ces by identifying on each available log all the
phenomena which could be attributed to fractures.
The probability of fractures is in fact much
greater than the phenomena observed on the logs
may indicate. Thus, if several of the phenomena
already described are detected, it is reasonab le to
conclude that fractures are present.
Schlumberger have recently made a new pro-
gram for the detection and evaluation of fractures
commercially available under the name of DET-
FRA. This program (Boyeldieu & Martin, 1964)
groups all the known fracture indicators into five
categories : electrical, acoustic, radioactive, elec-
tromagnetic and multi-pad.
Each log is analysed, and a fracture probability
is estimated using certain c riteria (threshold,
median and maximum probability (Fig. 11-63). The
probabilities are then combined using bayesian
logic. Thus, two criteria with individual probabili-
ties PI and P, will have a combined probability
which is given by :
P = 1 - (1 - P,)(l - P,)
This rule is associative, and can be extended to
an unlimited number of probabilities. The results
are presented in the form of a log (Fig. 11-64).
However, other techniques are also available.
11.5.1. crossplots
Combinations of various log measurements in
the form of crossplots are also useful in detecting
fractures.
11.5.1.1. Formation Factor - PorosiQf
If the porosity is plotted on a logarithmic scale
as a function of formation factor (FR = RJR,),
fractured zones will appear as zones having the
lowest values of Fn for a given value of porosity in
a low-porosity zone (Fig. 11.65). This is due to the
drop in resistivity associated with fractures.
Similar plots can also be made by replacing Fn
by R, or RLLo (Fig. 11.66).
11.5.1.2. M - N
Plot
This technique, introduced by Burke et al. (1969)
for the study of complex lithologies, combines the
responses of density, neutron and sonic tools. The
two computed parameters, M and N, are indepen-
dent of porosity, at least if we can assume that all
three tools respond linearly to porosity (Fig. 11-67).
M = Ati - At x 0.1
Pb - Prnf
N _ (Iti), - IH
Pb - Prnf
In this case, each pure mineral is represented
by a single point, regardless of porosiVy, when M
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1
._. . _ ..~- ../G_
L0G.F
%_ ._._....,_f..,I.tf....**..........~.*....,.~...~
_Li i_:c
?_LC l.1.J
P_Y%
...:,-,.l
.._.
t....._.*.
*
/.... G.SO
i.0:
.S,
-i
CORIGANO)
Fig. 11-65. Example of crossplots of formation factor vs. porosih/ (sonic, or derived from the neutron-density combination) (from
SW et a,.. 1978).
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SONIC-DENSITY
CROSS-PLOT FOR MINERAL A
DENSITY pb gmlcc
NEUTRON- DENSITY
CROSS- PLOT FOR MINERAL A
Fig. 11-W. Determination of the M and N factors (from Burke
e* al, 1969).
LITHO-POROSITY PLOT
(FRESH MUD)
I
J
3
.4 .5 6 .N. I .8 -9 1.0
Fig. 11-W. M v* N crossplot and its interpretation
(from Burke et a ., 19W).
EXAMPL
G*
uTHo40,0stn PLOT
(1)
,m~.........:.........:.......-.:.........:... :
*;.........)
)i&fT~ ..-I
;:i:, in :
ii:
:........-i: , a.::
.a z . .
.___._~____.._..:._.._.._.:
W ;
a:......
.I.........:.........i.........:.........i
.a 54 A4
II. O an
90
Fig. 11.69. Example of a M vs N cro~plot showing the
exktence of secondary porosity which can panly be related to
is plotted against N (Fig. 11-66). When there is
some secondary porosity (due to fractures, for
example), the sonic measurement is unaffected by
it. This is because the measurement is based on
the travel time of the fastest compressional wave,
which bypasses vugs and fractures, at least when
the fractures are subparallel to the borehole axis.
Consequently, At is reduced and M is increased.
The representative points are therefore displaced
towards the top of the diagram (Fig. 11-69).
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10 - 2.2
50
40
b
CNL NETRON WEX (AWare,,+ Limestone Porosity~
Fig. 11-70. Ghan for the determination of : (a) : p+,. and (b) :
At,,+. (from Clavier et a\.. 1976).
Fig. 11-72. Example of a MID-pEot indicating the presence of
secondary porosity w hich can pardy be related to fractures
(courtesy of Schlumberger).
11.5.1.3. MID Hot
This technique, very similar to the preceding
one, was introduced by Clavier et al. (1976). and
combines the measurements of the same three
tools. An apparent matrix density (p,,), and an
apparent matrix travel time (At& are defined
from charts (Fig. 11-70). These two parameters are
then plotted against each other (F ig. 11-71). In this
case also, each pure mineral, or fixed mixture of
minerals, is represented by a unique point regar-
dless of porosity, so long as each tool responds in
the same way to the porosity.
Again, secondary porosity reduces At and so
(At,,,&. The points representing fractured or vuggy
zones are then displaced towards the left-hand
side of the plot (Fig. 1 -72).
1
Fig. 11-71. Example of a MID-plot and its interpretation far
the determination of mineralogy (from Clavier era ., 19 76).
-.
. ..,. I .> . . .r , . ;; .,,-,;.
..mj ~ .f ? ? :: _: (If : : ..
f-
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11.5.2. Tortuos~Ey Factor m
This factor, also known as the cementation
factor, is defined by the following equation :
Since the open fractures are more or less
rectilinear planes, one would expect the tortuosity
factor to be close to 1, at least when the poros ity
is due to the fractures, and the current lines are
parallel to the plane of the fractures. In fact, even
if the fractures have not been healed, there will be
crystals in the fractures which are not evenly
distributed, and these will increase the tortuosity.
In addition, the fractures are not always planar o r
indeed open, and they are frequently at an angle to
the borehole axis. Finally, there are often several
crisscrossing fracture systems. As a result, the
tortuosity factor, m, is always greater than 1, but
usually well below 2 or 2.3, the values observed in
compact formations, and more usually around 1.4.
If the m factor is plotted against depth, the
fractured zones will show the lowest values,
usually between 1.3 and 1.6 (Figs. 11-73).
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11.5.3. Calculation of Secondary Porosity
Given that the sonic measurement does not
see the porosity of fractures or vugs, a secon-
dary porosity index can be defined by combining
the porosity from the sonic tool with that deduced
from the density-neutron combination :
SPI=&w-I&
On a plot of this index against depth, the
fractured zones will show the highest values. The
example of Figure 11-74 shows a good correlation
between this index and a drop in temperature in a
zone where density, sonic, gamma ray and caliper
are constant. These two phenomena can be taken
to indicate the presence of fractures.
This is only true of fractures which are subparal-
lel to the borehole axis. If the fractures are
subperpendicular, the sound wave must cross
them, and the sonic then sees the fractures.
RECAP
We can conclude that fractured zones are
present if examination of the Formation Micro-
Scanner and BHlV images indicate their presence.
In the absence of these m easurements, the exis-
tence of fractures can be concluded if several of
the following phenomena are observed simulta-
neously at about the same depth :
a change in temperature gradient;
- a change in hole diameter;
a localised decrease in density, accompanied
by a variation in Ap while Pe, At and & remain
steady, but not if there is a cave, or the mud
contains bary-te;
a very slight increase in porosity;
secondan/ porosity;
a reduction in the value of the m factor;
a change in the ratio LLD/LLS;
sudden drops in resistivity on the microdevi-
-Se*;
- high Pe values when the mud contains baryte;
conductivity peaks on the FIL;
- DCA showing conductive anomalies;
a pause in tool rotation;
- strong attenuation of acoustic waves;
a blurred zone on the VDL, or a lack of vertical
coherence on the wave train;
radioactivity peaks or uranium peaks;
strong negative SP deflections.
11.6. EVALUATION OF FRACTURES
The evaluation of fractured zones requires the
following information :
- depths of the fractured zones;
types of fractures : open or cemented;
- orientation (dip and azimuth) of fractures;
vertical and lateral extent of fractures;
- fracture density: number of fractures and
total fracture length per unit volume;
fracture porosity.
Well logs do not provide all of this information,
only the following being obtainable.
11.6.1. Depths of Fractured Zones
This is the simplest information to obtain from
the logs, especially from the Formation Micro-
Scanner or the BHTV. So there is no need to
elaborate.
11.62. Type of Fracture
The Formation MicroScanner tool can usually
differentiate between open fractures, fractures
induced by the drilling process. and healed cemen-
ted fractures. For the other tools, only open fractu-
res will affect the log responses and be detected.
In any case, it is only open fractures which are of
interest for production. Hence every fracture
which is detected as a conductive anomaly is by
definition open. However, not every conductivity
oeak is a fracture.
116.3. Orientation of Fractures
There are two parameters to be determined :
dip and azimuth. The borehole televiewer and the
Formation MicroScanner are the only tools which
allow us to determine both the orientation and the
dip of fractures (Fig. 11-75 & 11.59). The dip
cannot be determined with any certainty from the
other logs, because even if a correlation is made
between conductivity peaks, there is no guarantee
that they all belong to the same fracture.
If we now consider the size of an event detec-
ted by a pad (Fig. ll-76), we can attempt to define
two possible dips and select the ones which show
the most constant values. These data must also be
plotted as a function of pad azimuth.
The azimuth can be determined if fracturing is
accompanied by hole ovalisation, or from a polar
frequency plot of conductive anomalies detected
by the DCA program. Figure 11-32 shows the
consistency of results, and their correlation with
the predominant fracture or fault directions.
The two buttons on each pad of the SHDT
provide a means of determining the apparent dip
of the planes of the fractures picked up by each
pad. The dip and azimuth of the fractures can then
be defined if we assume that the two anomalies
correspond to the same fracture, or at least to the
same system of parallel fractures (Figs. 11-54 &
11.56).
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N E s w N
11.6.4. Fracture Density
11.6.5. Fracture Porosity from Photoelectric
Capture
Cross-Section (LDT
tool)
As previously illustrated (Fig. 11-58) individual
fractures can be identified with the Formation
MicroScanner tool if a borehole coverage is built
up through several passes, between which the pad
orientation has changed. This allows the determi-
nation of the number of fractures in a given
window, and of the length between fractures.
With the other tools this can be evaluated from
We have already seen that the photoelectric
capture cross-section is strongly influenced by
ban/te muds, and this feature can be used to
evaluate fracture porosity.
The following equation introduces the electro-
nic density :
Pe pe = B V, Pei p.,
(11-l)
the frequency at which the fracture indicators
occur, notably on the dipmeter and on the FIL (Fig.
11-46) and DCA (Fig. 11-49) presentations, and
from the porosity of the fractures. This can be
evaluated by various m eans.
Or, for the case of fractured rocks invaded by
baryte muds :
Pe pe = & Pet (p.)t + A, Pee. (P&8
+ (1 - &. - 6,) Pe,. (p.),,
(11-2)
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The first term is always very small and can be
ignored. The matrix porosity of compact fractured
rocks is also low (usually less than 10 96) while Pe
is also very sma ll (0.358 for water, 0.48 for oil and
0.807 for salt water). We can therefore write as a
first approxima tion :
Pe pS = & Peb (P&
+ (1 - &P) Pe,, (p.),, (-)
The porosity AP is derived from the density-
neutron combination, and includes both matrix
porosity and fracture porosity. This gives :
&I=
Pe pe - (1 - AP) Per, (P&n* (11-4)
Peea (P&a
Now, we can show that :
Pee, (p.).. = 1070
(11-5)
and further, as a first approximation, we can
take :
P. = pb et (P.),, = (P ,,),
which gives :
C#
Pe Pb - (1 - AP) Pern, (PC,),
1070
(1 -6)
Note: The last equation only holds if the
borehole wall is smooth, so that the pad fits
closely to the formation. Otherwise there may be
a cave doe to crumbling of the borehole wall filled
with baryte m ud. It is necessary, therefore to
examine the caliper and the density correction
before applying this formula. We must also bear in
mind that, being a unidirectional tool, it will only
analyse the part of the formation in front of the
pad, and so it will not necessarily measu re the
total fracture porosity. In any case, if the hole is
ovalised due to the presence of fractures, the pad
will usually ride the major axis of the hole, and so
face the fractures. The measurement will thus be
representa tive of the fracture porosity since it is
unlikely that there is another fracture network at
900 to the first when the hole is ovalised.
11.6.6. Fracture Porosity from DLL
Boyeldieu et a/. (1982) proposed the following
equation for fracture porosity after studying the
effects of fractures on the deep and shallow
laterologs, and making certain assumptions :
(#A = 7 Rrn~ C LLS Cm,) -c & (11-7)
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where I&, is the fracture porosity, I&.). is the
computed fracture porosity and CLLS and CLLDare
the conductivities in mhos of the LLS and LLD; m
is between 1.3 and 1.5.
The assumptions made by the authors are as
follows :
The fracture system is seen by both laterologs
as a system of resistivities in parallel with the
compact, non-fractured, formation (a perfectly
reasonable assumption).
There is no invasion of the non-fractured part
of the formation (the blocks contained within the
fracture system), but only of the fracture system.
This assumption is justified by the very high
permeability of the fracture system compared with
that of the rock itself, so that the overpressure of
the mud column will act preferentially on the
fracture network.
The invasion of the fracture system is not too
deep, but sufficient to ensure that the LLD reads
the virgin forma tion while the LLS reads the
flushed zone. The validity of this assumption will
depend on the type of mud and on the degree of
opening of the fractures. If the losses observed
during the drilling are low, it can be assumed that
the openings are small and that a mud-cake was
able to develop and limit the invasion. In this case
the assumption is valid. If the losses were conside-
rable, the invasion will be deep, and we can no
longer assume that the LLD reads the virgin zone.
The water saturation of the uninvaded frac-
ture system is almost zero. This is a reasonable
assumption given the permeability of the fractu-
res.
The filtrate saturation of the invaded fracture
system is 100 %. Again, due to the high permeabi-
lity of the fractures, we can assume that a ll the
hydrocarbons have been Rushed.
The authors then derived the following inequali-
ties :
(1 l-8)
and
1 < 4&p + #y%
(11-9)
RLLS w
where &, is the matrix porosity, A. is the
fracture porosity, S,. is the water saturation of
the non-fractured, uncontaminated formation.
Subtracting equ. 11-9 from equ. 11-9 gives :
The above hypotheses assume that Sxm = 1
and S,. = 0. This then gives equ. 11-7 by substitu-
ting conductivities for resistivities.
However, as the authors themselves pointed
out, the best results are obtained when the mud
resistivity is about equal to that of the formation
water, and when the formation contains hydrocar-
bons.
In water-bearing sequences. on the other hand,
the two salinities (mud and formation water)
should be very different. In this case the authors
proposed the following equation :
(Ad. =
m cus CUD
1J
(11-11)
cm-cc,
Figure 11-77 shows an example of results from
an interpretation of very compact, fractured for-
mations.
279
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a
I
tool centred on fracture
1-1 - *- I-..-_:_-
block reslstlvity = 10000 ohm.m
mud resistivity = 0.1 ohm.m
1000
1 10 100 1000
DISTANCE OF FRACTURE FROM AXIS in metros
INVASION RADIUS in inches
5-j-j
Infinite invasionnfinite invasion
- block reslstlvlty = 10000 ohm.mlock reslstlvlty = 10000 ohm.m
mud resistivity = 0.1 ohm.mud resistivity = 0.1 ohm.m
I
0.005
,/
I I
I
1 10 100 1000
FRACTURE APERTURE in microns
0.5 1
10
100 200
FRACTURE APERTURE in microns
Fig. 1 -78. Relationship between the fracture aperture E n pm for (a) : for vertical fractures and the conductivity: (b) : for horizontal
fractures and the resistivity (from Sibbit & Fsivre, 1985).
. . -. I ..^
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11.6.7. Liihology Determina tion
In compact,
non-fractured formations, the
mineralogy of the formation is easily determined
from the various log measurements using cross-
plots or if necessary the Schlumberger LITHO or
GLOBAL programs described in Chapters 2 and 9.
In fractured zones, the readings of the density tool
are frequently affected by caves or borehole
rugosity and are often unusable. It is then neces-
sary to use the neutron-sonic-gamma ray combi-
nation, and sometimes Pe to obtain a satisfactory
lithology determination.
11.6.8. Determina tion of Fracture Pe rmeability
In a recent publication, Mathieu et al. (1984)
have estimated that fracture permeability can be
determined from an analysis of Stoneley wave
detected by a tool which records the complete
acoustic wave train. The results they obtained in a
solid crystalline formation seem encourag ing.
11.6.9. Opening and Depth of Fractures
11.7. REFERENCES
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ATKINSON, A. (1977). - Fracture pressure gra-
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BABCOCK, E.A. (1978). Measurements of subsur-
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BATES, R.L., &JACKSON, J.A. (1980). Glossan/
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BECK, J., SCHULTZ, A., & FITZGERALD, D. (1977).
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pm) of vertical and horizontal fractures to the
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tool and the difference between the deep (LLD)
and shallow (LLS) resistivities. They also showed a
relation between their lateral extent (depth into
the formation) and the same Dual Later&g
measurements. In the case of vertical fractures
(parallel to the tool axis) the two measurement
curves separate (LLD > LLS) and their difference
is proportional to the product of the fracture
opening, E, and the conductivity of the invading
fluid, C,. For horizontal fractures (perpendicular to
the tool axis) the two curves show a resistivity
decrease over approximately 0.8 m (Fig. 11-78).
Again the separation is proportional to the product
of the fracture opening and the invading fluid
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CLAVIER, C.. & RUST, D.H. (1976). - MID-PLOT : a
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COATES, G.R., & DENOO, S.A. (1980). Log
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