Ch11 Fracturated Fm Ev

7/24/2019 Ch11 Fracturated Fm Ev

1/45

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

239


2/45

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

240


3/45

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

241


4/45

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).

242


5/45

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 :


6/45

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-

244


7/45

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.

245


8/45

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-

;.: ;,.


9/45

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


10/45

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;

248


11/45

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

249


12/45

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).

250


13/45

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)

251


14/45

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.


15/45

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

253


16/45

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.


17/45

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.

255


18/45

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.


19/45

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).

257


20/45

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

258


21/45

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.

259


22/45

, --.

I

260


23/45

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).

261


24/45

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

262


25/45

263


26/45

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-

264


27/45

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).


28/45

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).

266


29/45

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

267


30/45

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-

268


31/45

PAD AZIMUTH

t

0.2m

+-0.2m-+

DEPTH

CAUPERS

Pad

Direction

*

4-

3

1

Pad 3

Images

Pad 4

Images

Pad 4

TMCl?S

269


32/45

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.

270


33/45

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

271


34/45

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).

272


35/45

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).

273


36/45

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-

274


37/45

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).


38/45

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).

276


39/45

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)

277


40/45

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)

278


41/45

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


42/45

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 ..^


43/45

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

AGUILERA. R. (1980). Naturally Fractured Reser-

voirs. Petroleum Publishing Co., Tulsa, Okla-

homa.

ANDERSON, T., & WALKER, T. (1972). Log

derived rock properties for use in well stimula-

tion design. SPE of AME, paper SPE 4095.

ATKINSON, A. (1977). - Fracture pressure gra-

dients from acoustic and density logs : an

updated approach. SPWLA, 16th Ann. Log.

Symp. Trans.

BABCOCK, E.A. (1978). Measurements of subsur-

face fractures from dipmeter logs. Bull. Amer.

Assoc. Petroleum Geol.. 62, p. 1111-11.26.

BATES, R.L., &JACKSON, J.A. (1980). Glossan/

of Geology. 2nd ed. Amer. Geol. Inst. Falls

Church, Virginia.

BECK, J., SCHULTZ, A., & FITZGERALD, D. (1977).

- Reservoir evaluation of fractured cretaceous

carbonates in South Texas. SPWLA, 16th Ann.

Log. Symp. Trans.

BELL, J.S., & GOUGH, D.I. (1979). Northeast-

southwest compressive stress in Alberta : evi-

dence from oil wells. Earth Planet. Sci. Let., 45,

p. 475482.

BIGELOW, E.L. (1985). Making more intelligent

use of log derived dip information. 5 Parts. The

Log Analyst, 26, 1, 2, 3, 4, and 5.

BILLINGS, M.P. (1942). Structural Geology. 1st

ed. Prentice-Ha ll Inc. Englewood Cliffs, New

Jersey.

BILLINGS, M.P. (1972). Structural Geology. 3rd

ed. Prentice-Ha ll Inc., Englewood Cliffs, New

Sibbit & Faivre (1985) related the opening (in

pm) of vertical and horizontal fractures to the

conductivity measured by the Dual Laterolog (DLT)

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

conductivity. The Formation M icroScanner tool

enables to determine if the fractures are vertical or

horizontal.

BOND, L.O., ALGER, R.P., & SCHMIDT, A.W.

(1971). Well log Applications in Coal Mining

and Rock Mechanics. Trans. SME, 250.

BOYELDIEU, C., & MARTIN, C. (1984). Fracture

detection and evaluation. SAID-SPWL4, 9th

Europ. Intern. Form. Eval. Trans., Paris, paper 21.

BOYELDIEU. C., & WINCHESTER, A. (1982). -Use

of the Dual Laterolo gfor the Evaluation of the

Fracture Porosity in Hard Carbonate Forma-

tions. Offshore South East Asia 1962 Confe-

rence, 9-12 Feb., Singapore.

BURKE, J.A., CAMPBELL, Jr. R.L., & SCHMIDT,

A.W. 119691. The Litho-Porositv Cross Plot.

SPWL, l&h Ann. Log. Symp. Tr&., papery.

CHEUNG, Ph. (1984). Fracture detection using the

sonic tool .SAID-SPWLA, 9th Europ. Intern.

Form. Eval. Trans., Paris, paper 42.

CLAVIER, C.. & RUST, D.H. (1976). - MID-PLOT : a

new Lithology Technique. The Log Analyst, 17,

6.

COATES, G.R., & DENOO, S.A. (1980). Log

Derived Mechanical Properties and Rock Stress.

SPWLA, 2lst Ann. Log. Symp. Trans.

COATES, G.R., & DENOO, S.A. (1981). Mechani-

cal properties programs using borehole analysis

and Mohrs circle. SPWLA, 22st Ann. Log.

Symp. Trans., paper DD.

COX. J.W. (1970). The high resolution dipmeter

reveals dip-related borehole and formation

characteristics. SPWLA, 11th Ann. Log. Symp.

Trans.

COX. J.W. (1983). Long axis orientation in elonga-

ted boreholes and its correlation with rock

stress data. SPWLA, 24th Ann. Log. Symp.

Trans., paper J.

281


44/45

DEERE, D.U., & MILLER, R.P. (1969). - Engineering

Classification and Index Properties for Intact

Rock. U.S. Air Force Systems Command Air

Force Weapons Lab., Kirtland Air Force Base,

New Mexico, Tech. Rep.

DELFINER, P., PEYRET. 0.. & SERRA, 0. (1984).

-Automatic determination of Lithology from

Well Logs.

59th

Ann.

Tech. Conf.

SPE of A/ME,

Houston, Texas; paper no SPE 13296.

DICKEY, P.A. (1979). - Petroleum Development

Geology. Petroleum Publishing Co., Tulsa.

DONATH, F.A. (1970). - Some information squee-

zed out of rocks. Amer. Sci, 58, p. 54-72.

EDWARDS, D. (1985). - Mechanical Properties

Evaluation and its applications. Schlumberger

Technical Services Inc.. Europe Unit.

EKSTROM, M.P.. CHEN, M.Y., ROSSI, D.J., LOCKE,

S., & ARON, J. (1988). -High Resolution Microe-

lectrical Borehole Wall Imaging. SPWLA, 10th

Europ. Symp. Trans., Aberdeen.

EKSTROM, M.P., DAHAN, c.A., WHEN: M.Y.,

LLOYD, P.M., & ROSSI, D.J. (1988). - Formation

Imaging with Micro-Electrical Scanning Arrays.

SPWLA. 27th Ann. Log. Symp. Trans., Houston.

FAIVRE, 0.. & SIBBIT, A. (1984). Dual Laterolog

Response in Fractures.

FONS, L. Sr. (1969). - Geological application of

well logs. SPWLA, 10th Ann. Log. Symp. Trans.

GARY, M., McAFEE, R.Jr., & WOLF, C.L. (1972).

-Glossary of Geology. Amer. Geol. Institute,

Washington, D. C.

GAUER, P., RUHlAND, M., BEAUCOURT, F. de, &

JANOT. P. (1984). -0valisation des trous de

forage Bvalu6e b partir des pendagemkres

&patios; cara&res gCologiques de ces varia-

tions; leur utilisation pour la determination de

lorientation des contraintes horizontales actuel-

les dans le Foss6 RhBnan. SAID-SPWLA, 9th

Europ. Intern. Form. Eval. Trans., Paris, paper

31.

GOGUEL, J. (1952). Trait& de Tectonique. Mas-

son, Paris.

GRIFFITH, A.A. (1920). The phenomena of rupture

and flow in s

Date post:	21-Feb-2018
Category:	Documents
Upload:	mario-enrique-vadillo-saenz
View:	216 times
Download:	0 times

Ch11 Fracturated Fm Ev

Documents