Naturally Fractured Reservoirs pptx

Naturally Fractured Reservoirs
Roberto Aguilera
DEDICATION
Dedicated
to
my
wife, Maria Ester;
my
daughter, Maria Silvia,
and,
my
sons, Roberto Federico
and
Carlos Gustavo.
Acknowledgements
I wish
to
thank
the
Society
of
Petroleum Engineers,
the
Petrole
um
Society
of
CIM,
the
Society of Professional Well Log Analyst

s,
the
American
Ass
ociation oj' Petroleum Geolo-
gists, Western Atlas, Schlumberg"r, Halliburton, Gulf PubliShing Co.,
l'ennW
ell Publi
sh
ing
Co.,
and
Petm/ellln Engineer for permission to
draw
material from
th
eir
publication
s.
In
addition, I express
my
gratitude to
the
various
author
s
and
organ izations
that

have
pub-
li
s
hed
material on
the
subject of natllTally
fra
ct
ured reservoir
s.
Although I am
the
only person responsible for
the
final
fo
rm of this
bo
ok, I would like
to
thank
Dr.
H.K. van Poollen
and
Dr. G.R.
Pi
cket
t,

r.i.p., for their
help
and
encouragement
during
the
devdopment
of some
of
the
co
ncepts
and
techniqu
es presented here.
Me.
Alonso Marin,
r.i
.p
.,
provided valuable help
and
guidan
ce
during
my
early yea
rs
as
it

petro-
leum
engineer.
During
the
last 16 years I
have
been tea
ching
courses
on
the
subject of naturally frac-
tured
reservoirs a
ll
over
th
e world. 1
thank
my
st
ud
ents for their
qu
estions
and
suggestions
that
have

led
to
this second
edition
of
my
book.
Since 1984 I have
been
privileg
ed
to cooperate
with
AAPG
presenting
a co
ur
se
on
frac-
tured
reservoir analysis
with
Dr.
David S
teams,
Dr.
Melvin Friedman, and
Dr.
Ronald

Nelson.
Chapter
1
of
this
book
dealing
with
geologie aspects reflects
many
of
th
e geolog-
ical
techniques
I
hav
e learned with Dave, Melvin.
and
Ron. Howeve
r,
the
responsibility for
th
e way in
which
the
se tec
hniques
are pres

ent
ed is
only
min
e.
Last,
but
not
least, I wish to
thank
my
wife, Maria Este
r;
my
daught
er Maria Silvia,
and
my
son
s,
Roberto Federico
and
Ca
rl
os Gustavo, for their patience
and
under
s
tanding
dur-

ing
the
long evenings, wee
kend
s anti hOlidays
needed
for
the
pr
eparation
of
th
is
boo
k.
vii
Preface
.~.
Since
the
publication
of
the
first
edition
of
my
book on naturally
fra
ctured reservoirs in

1980,
the
science
and
art of evaluating these types of reservoirs has advanced
at
a rapid
pace. ,
The
first
edition
included
in
a single
bo
ok
many
subjects
that
are usually discussed
in
separate volumes. There were
nine
chapters of
the
first
edition
that
included geologic
aspects, drilling

and
completion
meth
ods, log
interpretation,
well testing,
fr
actured shale
s,
primary
and
secondary recovery,
num
erical s
imulation
, case
histo
ries
,
and
ec
ollOinic
('v
al·
uatlons.
Initially I
attempt
ed
to
include

the
same
nine
chapters
in
this
second
edition.
However,
I quickly realized
that
due
to
the
many
advances
in
the
field,
thi
s was
not
going
to
be fea -
sible. Consequently, this second edition includes
only
six Chapters dealing
with
geologic

aspects, drilling
and
completion
, formation evaluation by well log analysis,
tight
gas rcser-
voirs/ case histories, a
nd
economic
evaluations and reserves. Pres
en
tly I
an1
working
on
a
second
volume
covering
other
aspects
of
naturally fractured reservoirs.
Chapter 1 deals with geOlogic aspects of naturally fractured reservoirs, reasons for gen-
erations
of
fractures
including
tectonic, regional, contractional
and

surface-related
fra
c-
tures, migration
and
accumulation
of
petr61eum, direct
and
indirect
SOu
rc
es
of
informa-
tion,
fractured reservoirs
in
variolls lithologies,
how
to a
void
\valking a
'vvay
fr01u
a
commercial
fractured reservoir due to
an
lInpropcr

evaluatiOn,
and
tilt!
importance
of
in-
situ stresses
on
the
study
of
naturally fractured reservoirs.
Chapter 2 reviews
some
important
drilling
and
completion
concepts
for deviated
hole
s.
Since
most
fractures at
depths
of
interest arc vertical
to
subvcrticial, deviated

and
horizon-
tal
wells probably
stand
better
ch~nccs
of
l'il1ding
hydrocarbon
s
than
vertical wells.
The
advantages
and
disadvantages
of
open-hole
vs. perforated completions are reviewed. Key
.
elements
a~sociatcd
with
hydraulic fracturing
and
acidiz
ing
of
naturally

fractun
:
~d
reser-
voirs are also discussed. '
Chapter 3,
which
focuses
on
formation
evaluation by well log analysi
S,
examines
the
use
of
many
conventional
and
specialized well log curves
in
th
e
qualitativ
e
and
quantitative
evaluation
of
naturally

fractured reservoirs.
The
iinportance
of
electric
an
d sonic
imaging
tools
is
discussed
in
detail. Special
techniqu
es dealing
with
the
porosity
exponent
III
and
the
water
saturation
exponent
Ii
allow
quantitative
estimates of porosity
and

watcr satu-
ration
in
matriX, fractures
and
the
combined
matrix-fractures system.
The
effect of lithol-
ogy
variations and shalincss
is
rcvicwttl
a.s
well as
logging
of
hori
zo
ntal
well
s:
The uncer-
tainty
of
calculating
hydrocarbon
s-in-place in fracture
media

and
the
effect of
miscallibrated logs
is
also analyzed.
Chapter 4
is
concerned
with
tight
gas reservoi
IS
where
produ
ction
is
possible
in
many
cases
thanks
to
the
presence of
natural
fracture
s.
Many of these reservoirs are also multi-
layered

adding
a great deal
of
complexity to
the
evaluation. The c
hapter
covers
both
fra
c-
. tured shales
and
tight
gas sands. Geographical
distribution
is
discussed, Various
meth
ods
of well log
interpretation,
well test analysis
and
performance forecasts are reviewed in
detail.
Chapter 5
pres~nts
case histories
of

naturally fractured reservoirs
around
the
world.
Giant, modest,
and
non-commercial
oil
and
gas reservoirs are reviewed,
highlighting
the
most
important
features associale(l
with
each
one
of
them.
Fractured reservoirs in sand-
stones, ca'[bonates, cherts, shales,
basement
and
tight gas sandst
ones
are considered
ill
thjs
chapter.

The
case histories include
the
effect
on
recoveries of aqUifers,
var
io
us
injection
Ix
x
Preface
schemes
(w\\ter, gas, polym€rs,
C02,
steam), subsidence, rapid pressure decline,
strong
gravity
scgr~gation
with
counterflow, fold
and
fault-related fractures, vertical
communica-
tion
through
fractures,
slanted
and

horizontal
wells,
and
retrograde
condensation.
Chapter 6 discusses
some
of
the
most
important
aspects of
economic
analysis.
The
opti-
mum
equilibrium
between
well spacing,
maximum
efficient rate
and
economic
recovery is
reviewed. Emphasis is placed
in
the
economic
analysis

of
acceleration projects as recovery
from
most
naturally
fractured reservoirs
is
directly related to 'actual acceleration projects.
The
effect
of
directional
and
horizontal
wells
on
costs ,md recoveries is reviewed. The
chapte~
includes reserves
definitions
as provided
by
various
organizations
and
some
guide-
lines based
on
my

experience for
estimating
oil
and
gas reserves
in
naturally
fractured
reservoirs.
•
Contents
" ,.:


.
CHAPTER 1 GEOLOGIC ASPECTS
What
Is
a Natural Fracture?
What
Is
a Naturally Fractured
Re
servoir?
Requirements for Hydrocarbon Accumulation
Porosity
Ductility
and
Brittleness
Reasons for

Generation
of Fractures
Storage Capacities
Fracture
Morphology
.
Permeability
Mechanical Behavior
of
Ro
ck
Fractures ClaSSification
Migration
and
Accun~ulation
Sources
of
Information
Mapping
Fractured Trends
Fractured Reservoirs
Exploring
fo
r Naturally Fractured Reservoirs
Subsurface Fracture Spacing
Imp
~c
t
Fr
act

ures
Ef
fect
of
Fractures
on
Flow Behavior
Undi
scovered Naturally Fractured Reservoirs,
Wh
y
and
How?
Practical Applic
at
ions
References
CHAPTER 2 DRILLING
AND
COMPLETION
METHODS
Drilling Fluids
Drilling Mud
Air a
nd
Gas as Drilling Fluids
Directional Drilling
Horizontal Drilling
C
ompletion

Method
s
Hydraulic Fracturing
Acidizing
Explosive Frac
turing
References
xi
1
1
1
1
4
7
9
11
14
14
20
24
30
32
42
50
61
68
77
77
77
85

91
97
97
97
i
103
-I
104
I
124
137
I
148
173
I
177
I
177
I
xii
Contents
CHAPTER3
FORMATION
EVALUATION
BY
WELL
LOG
ANALYSIS
Sonic
Amplitude

Logs
Variable
Intensity
Logs
Long Spacing Sonic
Shear-Wavc Splitting.
Induction
Log!:
Dual
Induction
Laterolog 8
Sonic,
Neutron
and
Density Logs
Comparison
of
Porosity IOstimates from Different Sources
Borehole Televiewer
Diprneter
and
Fracture Identification Log (I'lL)
Formation Microscanner
and
FulJbore Formation Microimager
Spontaneous
Potential
Cnrrcction Curve
on
the CompcllSated Density Log

Comparison
of Shale Volume
to
Uranium
Index
Lithoporhsity Crossplot
Production Index
Log
Temperature Log
Sibilation Log
Kerogen Analysis
Gamma
Ray
Circllmferential Acollstical Log
Dual Laterolog-Microspherically Focused Log
P,
Curve
on
the
Litho-Density Log
Sonic Porosity Greater
than
Neutron-Density
Porosity
Mud
Log
Velocity Ratio Plot
Production
Logs
Fracture Plausibility

Fracture Evaluation
in
Cased Holes
Quantitative
Analysis
Lithology
Det~rmination
Estimate of Water
Saturation
Exponent
(n) from Logs
Flushed Fractures
Water
Saturation
of
the
Composite
System
Practical Application
Estimates of Water Saturation in Matrix
and
Frattures
Fracture
Completion
Log
Shaly Formations
Nuclear Magnetic-Resistivity
Comhination
Lithodensity-Resistivity
Combination

Electromagnetic Propagation (EPT) Resistlvity
Combination
Borehole
Gravimeter-Resistivity
Combination
Effect
of
Variations in Water Resistivities
Horizontal Wells
Measnre of
Uncertainty
in Log Calculations
Original Oll-In-Place
Measure of
Uncertainty
Hydrocarbon Recoveries from Logs
181
181
183
184
186
188
189
189
190
191
192
194
201
201

202
203
203
205
207
209
210
210
211
215
215
218
219
221
225
226
231
251
257
259
259
264
267
268
268
280
285
286
287
289

291
300
300
302
306
•
•
Geothermal
Reservoirs
Miscalibration
of
Logs
General Remarks
References "

CHAPTER
4 TIGHT
GAS
RESERVOIRS
Fractured Shales
Worldwide Distribution of Black Shale
Oil
an
d Gas Oc
cur
rence
in
Fractured Shales
Remote Sensing Surveys
GeophYSical Investigation

Physical Characterization
Geochemical
Cha
racteriza
tI
on
Stimulation
Int
eraction between H
ydrau
lically
Induc
ed ami Natural Fractures
Met
amorph
ism
Lo
g Analysis
Formation
Strength
Parameters
Estimates
of
Gas-in-Place
Recoverable Gas
We
ll
Testing
Tight Gas Reservoirs
References

CHAPTER
5
CASE
HISTORIES
Frac
tured
Sandstones·
Fractured
Carbona
tcs
Fractured Cherts
. Fractured Shales
Fra
ctu
red Basement
Tight Gas Reservoirs
Naturally Fractured Reservoirs StatiSlics
Reiereliccs
CHAPTER
6
ECONOMIC
EVALUATION
AND
RESERVES
Com
pound
Interest
Period, Effective, Nominal,
and
Co

ntinuous
Interest R
at
es
Discounted Cash Flows
Net
Present Worth
Future
Worth
Rate of Return
Payo
ut
Time
Profit to
Investment
Ratio
Infla
tion
Acceleration Projects
Comparing
Evaluation Methods
Depreciation, Depletion arid Amortization
Contents
xiii
306
306
309
309
315
315

315
316
321
322
322
323
323
327
327
328
342
343
345
365
369
379
383
383
400
443
447
447
44
8
450
457
461
461
464
464 .

465
466
466
46
8
469
470
470
482
482
xlv Contents
Tax
Treatment
Well Spacing
Risk Evahlatiort
Size of Helds
found
Monte
Carlo Simulation
Direct.ional
and
Horizontal Wells
Reserves
Other
Definitions
Guidelines for Estimating Oil
and
Gas Reserves
References
Appendix

Index
,-
t
487
489
490
~
492
494
494
496
499
5,62
504
507
515
•
Ii
•
CHAPTER
1
Geologic
Aspects
Many
of
present-day
producing
naturally
fractured
reservoir~

have
been accidentally dis-
covered
when
looking for
some
other
type
of
reservoir. Some years ago
McNaughton
and
Garb (1975)
estimated
that
ultimate
recovery from
producing
fractured reservoirs
would
surpass
40
billion stock
tank
barrels of oil (STllO). Today I firmly believe
that
this figure
was very conservative. I
am
con

vinced
that
there
are significant volumes
of
hydrocarbons
that
have
been
left
behind
pipe as undiscovered,
or
behind
plugged
and
abandoned
wells
or
because
of
vertical wells
that
have
not
intercepted
vertical fractures.
Figure
1-1
shows

the
location
of
some
important
naturally
fractured reservoirs.
They
are
found
all
over
the
world,
in
all types
of
lithologies
and
throughout
the
geologic strati-
graphic
column.
This
is
demonstrated
in
Chapter
5

dealing
with
Case Histories.
WHAT
IS
A NATURAL FRACTURE?
A
natural
fracture is a macroscopic
planar
discontinuity
that
results from stresses
that
exceed
the
rupture
strength
of
the
rock (Stearns,
1990).
Another
definition
proVided
by
Nelson (1985)
is
as follows:
itA

reservoir fracture is a
natu!ally
occurring Inacroscopic planar
discontinuity
in
rock
due
to
deformation
or
physical diagenesis."
WHAT
IS
A NATURALLY FRACTURED RESERVOIR?
A
naturally
fractured reservoir
is
a reservoir
which
contains
fractures created by
mother
nature.
These
natural
fractures
can
have
a positive

or
a negative effect
on
fluid flow.
Open
uncemented
or partially mineralized fractures
might
have, for
example,
a positive effect
on
oil flow
but
a
negative effect
on
water or gas flow
due
to
coning
effects. Totally mineral-
ized
natural
fractures
might
create
permeability
barriers
to

all types
of
!low. This
in
turn
might
generate small
compartments
within
the
reservoir
that
can
lead
to
uneconomic
or
marginal recoveries.
In
my
opinion
a1l
reservoirs cuntain a certain
alnount
of
natural
fracturing.
However,
froIn a
geologic

and
a reservoir
engineering
point
of
view, 1 regard as naturally fractured
reservoirs
only
those where
the
fractures have
an
effect,
either
positive
or
negative,
on
fluid tlow as suggested
by
Nelson (1985).
REQUIREMENTS FOR HYDROCARBON
ACCUMULATION
In
general,
a
petroleunl
reservoir
consists
of

source rock, reservoir rock, seal rock, trap,
and
fluid
content.
Source rock,
Or
source
environment,
is believed to be responsible for
the
origin of petro-
leum.
Most geologists believe
that
the
origin of
petroleum
is organic, related
mainly
to
veg-
etables
which
were altered by pressure,
temperature,
and
bacteria. Some geologist
(Hunt
et
aI,

1992),
however, believe
that
the
origin of
petroleum
is
igneous
and
indicate
that
oil
rises from
depth
in
granitic shield terrains
of
the
world.
1