fractured reservoirs part 1
TRANSCRIPT
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232, Avenue Napolon Bonaparte
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Phone: +33 1 47 08 80 00
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Fractured reservoir characterization
Modelling and simulation
April 2009
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Part 1- What is a naturally fractured reservoir
Background, methodologies and tools to account for the presenceof fractures in oil & gas reservoirs
Part 1: What is a fractured reservoir? What is the impact of fractures on fieldbehavior? When do we consider a reservoir is fractured?
Part 2: How to characterize a fractured reservoir? How to detect fractures? Howto model their distribution as well as their geological and flow properties?
Part 3: Which parameters control the fluid flow in fractures ? How to upscalethese parameters into a flow simulator ?
Part 4: How to identify the appropriate recovery mechanism?
Part 5: How to simulate a fractured reservoir? How to develop a fracturedreservoir?
Objectives of the course
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232, Avenue Napolon Bonaparte
P.O. BOX 213
92502 Rueil-Malmaison
France
Phone: +33 1 47 08 80 00
Fax: +33 1 47 08 41 85
Naturally fractured reservoirs
Part 1: What is a Fractured
Reservoir?
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Fractures = matrix heterogeneity
Impact recovery
Naturally Fractured Reservoirs
f
m
m
K
Matrix
fF
KF
Fractures
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Definitions
- What is a fracture, a fracture set, a fracture network ?- Definition of the fracture properties
- What is a fractured reservoir ?
The main types of fractures
- Joints, swarms
- Faults
- Fold related fractures
- Stylolites related fractures
The main type of fracture reservoirs
Main outlines
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North
A fracture is characterized by its strike, dip, length,morphology, origin, aperture
Illustration of fractures / fracture sets
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50 m
A fracture set is characterized by its avg. strike and dip,
length distribution, and density
Fracture density (biased / unbiased)
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Local connection
Not connected network at
grid scale
Illustration of the fracture connectivity 1/2
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connected network at grid scale
If fractures are open, this connected fracture network will
have an impact on fluid flow
Illustration of the fracture connectivity 2/2
Ill i f h f fl i
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The connected fracture network will induce flow
anisotropy in the reservoir: Px < Py
X
YP1
P2
~ P1
P2
Illustration of the fracture flow anisotropy
M t i bl k i d fi iti
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The block size is determined by length of matrixblocks surrounded by connected fractures
Matrix block size definition
L th f h i ti
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The length of homogenization (REV) is a value of grid size
not impacting the fracture properties
REV = Representative elementary volume
Length of homogenisation
S
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What is a fracture, a fracture set, a fracture network?
A fracture is a surface of discontinuity of mechanical origin. The fracture is
the failure of a rock (= deformation) resulting from applied forces (= stress)
a fracture is characterised by its attributes (dip, strike,
length, aperture, morphology and origin)
A fracture set (or fracture family) is a set of fractures with similar attributes
The fracture network involves the description of the fracture attributes and
investigates the relationship between the different fracture sets
the fracture network is characterised by the spatial
properties of fractures, such as the number of fracture sets,their relative fracture density, the fracture connectivity, the
length of homogenization
Summary
Summary
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What is a fractured reservoir?
For geologists:
A fractured reservoir is first and foremost a reservoir with structural
discontinuities resulting from a given paleostress history
For reservoir engineers:
A fractured reservoir is first and foremost a reservoir with structuraldiscontinuities affecting flows
[ R.A. Nelson, in Geologic Analysis of Natural ly Fractured Reservo irs,
quotes: A fractured reservoir is defined as a reservoir in which naturally occurring fractures either have, or
are predicted to have, a significant effect on reservoir fluid flow either in the form of increasedreservoir permeability and/or porosity or increased permeability anisotropy ]
Summary
Fracture propagation modes
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Mode I: Fractures are purely dilational(extension)
Mode II: Fractures may exhibit shearing withcomponents parallel (mode II) to the direction
of propagation of the fracture front.
Mode III: Fractures may exhibit shearingwith components perpendicular to the
direction of propagation of the fracture front.
Shear fractures are also known as faults
Fracture mode nomenclature is purely descriptive, not genetic. For example, a mode I fracture can
be formed by one or more mechanisms such as hydraulic fracturing, thermal contraction, etc.
Fracture propagation modes
Fractures and stress state
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Stress is defined as the force per unit area
acting on a given plane.
Any stress state at a point in a solid body
can be described completely by the
orientations and magnitudes of threestresses called principal stresses and
oriented perpendicular to each other.
The principal stresses are defined:
s 1 > s 2 > s 3
Fractures and stress state
Example fractures and stress state
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Joints (mode I) in green
Shear fractures/faults (mode II) in red
Stylolites in blue
Increased confining Stress and/or Temperature
Example fractures and stress state
Fracture classification (genet ics)
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1. Tectonic fractures
Small-scale fractures (diffuse/sistematic fractures)
Joints, fold-related fractures
Large-scale fracturesFracture swarms, fault-related fractures
2. Diagenetic fractures
Bed-parallel stylolites, stylolite-related fractures, diagenetic cracks, etc.
Fracture classification (genet ics)
What is a naturally fractured reservoir ?
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Tectonic fractures
What is a naturally fractured reservoir ?
Fracture classification (genet ics)
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Fracture swarms (fracture corridors, large-scale fractures) They consist of several sub-parallel and aligned fractures clustered in a well-defined
zone (Bahat 1988; Becker and Gross 1996; Rijken and Cooke 2001).
Large lateral extents
Not controlled by lithology, porosity, etc.
They go through the different reservoir units thus connecting / disconnecting them.
Diffuse fractures (small-scale fractures) They consist of smaller objects in a vertical scale, as they are often restricted to the
bed boundaries (Gross, 1993).
More diffuse over a large area
Bed-confined
Controlled by lithology, porosity amongst others
Fracture swarms
Diffuse fractures
Fracture classification (genet ics)
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Tectonic fractures
Joints
Definition of joint
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Joints (small-scale fractures)
Joints are fractures developed over large areas of the
earths crust with relatively little change in orientation,
with no evidence of offset along the plane, and
perpendicular to bedding.
Common features:Extension fractures
Vertical maximum stress
2 directions 90 deg to bedding & 90 deg to one another
Systematic set 1st & Non-systematic 2ndUnrelated to local structure
Definition of joint
Joint sets in sandstones
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Homogeneous density
Joint sets in carbonates
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A
Homogeneous fracture density with constant fracture orientation
Joints are controlled by bed thickness
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y
Factors controlling the fracture density
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Density 3 > Density 2 > Density 1
h1> h2 > h3
h1
h2
h3
acto s co t o g t e actu e de s ty
2. Bedding
Relation between lithology and fracture density
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Joints are controlled by lithology
High Shale content
Factors controlling the fracture density
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3. Grain size and porosity
g y
Relation between lithology, bed thickness and fracture density
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LITHOLOGY
FRACTURE
DENSITY
0 10
FS
MFS
Fracture density
controlled by :
1 : Shalyness
2 : Bed thickness
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Tectonic fractures
Fold-related fractures
Definition of fold
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Folds result from a compressive ductile deformation, in which the
maximum stress axis (s1) is sub-horizontal
Anticline
Oldest YoungestYoungest
YoungestOldest Oldest
Syncline
Fold geometry
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The Menchaca anticline is a
kink-style box fold, cored by
evaporites (Olvido Fm, Middle
Jurassic)
Study-case in Northern Mexico: the Menchaca Anticline
Courtesy of PEMEX
Fracturing and folding
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1 and 2 form during the early phases of folding(e.g. layer parallel shortening)
3 and 4 form in the latest phases of folding.
Fold and fractures relationship 1/3
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Tectonic Fractures on the fold flank
(K/T Ss, Rogers Mnt., WY)
20 ft
80 deg
Fold and fractures relationship 1/3
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Tectonic Fractures on the fold flank
(Arroyo Lapa, Argentina)
80
Strain partitioning in a fold
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CompressionExtension
Fractures produced by extension are pure
extensional and open fractures
Fractures produced by compression areclosed, stylolithic and/or partially open.
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Tectonic fractures
Fracture swarms
Definition of fracture swarms
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FRACTURE SWARMS (Large-scale fractures)
Fracture swarms are areas where fracture density is high
and fractures are preferentially oriented. They are large-
scale objects (several hundred meters). Usually fractures
cut through layers boundaries.
They consist of several sub-parallel and aligned fractures
clustered in a well-defined zone (Bahat 1988; Becker and
Gross 1996; Rijken and Cooke 2001).
Common features:
Large lateral extents
Not controlled by lithology, porosity, etc.
They go through the different reservoir units thus connecting /
disconnecting them.
Fracture swarms in carbonate reservoirs 3/5
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Fracture density log
Fracture swarms in sandstone reservoirs
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2m
Fracture swarms in carbonate reservoirs 1/5
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Fracture swarms in carbonate reservoirs 2/5
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Fracture swarms in carbonate reservoirs 4/5
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100m200m0
Fracture
density
15
Fractures per meter
Fracture swarms in sandstone reservoirs 5a/5
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Calvisson quarry (France - Gard)Limestone
Fracture swarms in sandstone reservoirs 5b/5
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Calvisson quarry (France - Gard)Limestone
Minerals
+ Cristals
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Tectonic fractures
Fault-related fractures
Definition of fault
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FAULTS (Large-scale fractures)
Faults are defined as structures across which appreciable
shear displacement discontinuities occur. Fault blocks
predominantly move along the plane or zone of the
discontinuity.
The term fault zone is used when referring to the zone of
complex deformation (fracturing) that is associated with
the fault plane.
Fault terms and fault types
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Hanging wall
block
Normal Fault
Reverse/Thrust Fault
Strike-slip Fault
Faults and stress state
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Faults in outcrop
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Faults and fracture density 1/2
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Caine et al. 1996
Micarelli et al., 2003
Density of fault-related fractures progressivelydecreases with increasing fault distance
Caine et al., 1996 - Geology
Micarelli et al., 2003 Journal of Geodynamics
Faults and fracture density 2/2
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Subvertical
Fractures
Lozenge-shaped fractures
Background Background
Poorly damaged
area
5 cm
5 cm
E W
Poorly damaged
area
Damagedarea
Damagedarea
Faultcore
Subvertical
Fractures
Fault
Core
Faults and wide damaged zone 1/3
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Example of a fault with an intensely deformed and wide damaged zone
Geological factors controlling damage zone formation in normal faults
Faults and wide damaged zone 2/3
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Syn-sedimentary faults often have
not a damage zone associated
(because sediments are not yet
compacted during faulting)
The absence of a damage zone can
also depend from the mechanical
behaviour of rocks affected by faults
Displacement 1m
Displacement 1m
Porous sandstones
Shales
Tight sandstones
Faults and wide damaged zone 1/3
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The widest damage zone for normal faults forms either
in the hanging wall near the upper tip of the fault or in
the footwall near the lower tip.
Modified from Knott et al. (1996)
The width/location of the
damage zone observed at wells
may depend on where the well
intersects a fault (near either theupper or lower tip)
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Diagenetic fractures
Bed-parallel stylolites
Definition of stylolites
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A bed-parallel stylolite is an irregular discontinuity
commonly found in limestones and other sedimentaryrocks. They result from compaction and pressure solution
during diagenesis and may be enlarged by subsequent
groundwater flow.
Stylolites appear as jagged discontinuities in outcrops and
are often filled with insoluble clays, opaques (such as iron
oxide), or dark organic matter.
Bed-parallel stylolites in outcrop
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Bed-parallel stylolites in pink Ordovician
limestone (Tennessee, US)
Bed-parallel stylolites in limestones
(Southern France)
Bed-parallel stylolites in limestones
(Southern France)
Stylolite related fractures observed on cores
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Fracture
Stylolite
Paleo-minimum
stress direction
overburden
Stylolite peaks
Tight zone relatedto pressure-
solution
Tension
gashes
Tectonic fractures
Fractures / tension gashes related stylolites
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Origin of stylolites is overburden plus tectonic stresses
They form tight intervals that may be preferentially fractured
Various structural objects
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Swarm
Fault
Large-scale fractures
Stylolites related fractures
Joint = Systematic set
Fold related fractures
Small-scale fractures
What is a naturally fractured reservoir ?
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Main types of fractured
reservoirs
Type 1 Fractures provide both porosity and permeability in the
Main types of fractured reservoirs 1/5
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Type 1 Fractures provide both porosity and permeability in the
reservoir (no hydrocarbon in the matrix)
Examples:
LA PAZ (Venezuela)
WHITE TIGER (Vietnam)
MONTE ALPI (Italy)
ROSPO MARE (Italy)
Fracture
Kf
Matrix
Hydrocarbon
Km
~ 0
fm
~ 0
ff
Type 2 Fractures provide permeability in the reservoir (the
Main types of fractured reservoirs 2/5
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Type 2 Fractures provide permeability in the reservoir (the
hydrocarbon is mainly in the matrix)
Examples:
QUARTZITE SANDSTONE (Algeria)
HUSSUM SCHNEEREN (Germany)
OROCUAL (Venezuela)
AGHA JARI (Iran)
HAFT KEL (Iran)
VILLAFORTUNA (Italy)
Fracture
Kf
Matrixfm
Hydrocarbon
Km
~ 0
ff
< 1%
Type 3 Fractures enhance permeability in the reservoir (matrix is
Main types of fractured reservoirs 3/5
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Type 3 Fractures enhance permeability in the reservoir (matrix is
porous and permeable)
Examples:
KIRKUK (Iraq)
GACHSARAN (Iran)
CANTAREL (Mexico)
LACQ (France)
EKOFISK (Norway)
Fracture
Kfff
MatrixfmKm
Hydrocarbon
M i t f f t d i
Main types of fractured reservoirs 4/5
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Type 4 Fractures generate a high flow anisotropy in the reservoir
Examples:
HASSI MESSAOUD (Algeria)
GHAWAR (Saudi Arabia)SHAH (Abu Dhabi)
Main types of fractured reservoirs
Fracture
Kf
ff
MatrixfmKm
Control of production from naturally fractured reservoirs (from Nelson R.A.)
Main types of fractured reservoirs 5/5
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Type 1 : Fractures provide both porosity and permeabilityIn crystalline and metamorphic rocks and in shales where matrix porosity and
permeability are negligible.Ex. : Big Sandy Field (fractured shale gas reservoir: fracture planes (opening =300)
often coated with crystalline dolomite)
Type 2 : Fractures provide the permeability
Typical fractured reservoirs, with matrix providing the essential porosity and fractures theessential permeability
Ex. : Sprawberry Field (Kmatrix=0.3-0.5 md while overall permeability is 16 md, fmatrix~8%
ff~0.1%)
Type 3 : Fractures provide a permeability assistFractured reservoirs where both matrix and fractures contribute significantly to productionat field scale
Ex. : Kirkuk Field (highly-productive fractured limestone)
Influence of fractures on field behavior
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High productivity/injectivity
Anisotropy of flows
Early breakthrough
Communication between different reservoirs
Specific recovery mechanisms (dual medium)
If sealed fractures : compartmentalization into several
reservoir units
Duality between Matrix and Fractures
Specificity of fractured porous reservoirs
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Duality between Matrix and Fractures
a Fractured reservoir
=
a Matrix reservoir+
a Network of useful fractures
Matrix = High fluid capacity / low permeability
Fractures = Low fluid capacity / high permeability
Ratio of capacity (F/M): 10-3 to 10-2
Ratio of permeability (F/M): 10 to 1000
(Well test interpretation methods are based on this dual-porosity flow behavior)
Fractures bypass the matrix spontaneous (= non-forced)displacement mechanisms (expansion, capillarity, gravity, diffusion)control oil recovery from matrix blocks
Absolute criteria of fracturation 1/2
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Absolute criteria of fracturation: well testing versus core analysis
(K.H) test >> (K.H) matrix (at least 10 tim es)
Required information:
an interpreted pressure build-up
matrix permeability from representative core measurements (orf-K lawsand porosity log)
- continuous sampling through the reservoir- K measurements under stress (or corrected for stress effects)
Ex. Meillon field : K test /K core = 100-10000 (SPE 22915)
Difficulties:
Which H has been tested ? Reservoir / Perforated /Producing height ?
(KH) calculation from plug measurements? Which average?
100000
Absolute criteria of fracturation 2/2
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1
10
100
1000
10000
100000
1 10 100 1000 10000 100000
K.H Core
K.
HT
est
Fractures reduce K.H
No evidence
of fracture
Fractures enhance K.H(Ratios from 5 to 1000)
Carbonate field with conductive faults 1/2
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Lagalaye (Total), Grard (Beicip-Franlab) Conductive Fault Modelling and History Match Improvements on a Fractured Carbonate Field - GEO2002 - Bahrein 14-17th of april 2002
Type 4 example Conductive faults
2 fractures sets
- NS
- N120
Anisotropy N120
Carbonate field with conductive faults 2/2
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Lagalaye (Total), Grard (Beicip-Franlab) Conductive Fault Modelling and History Match Improvements on a Fractured Carbonate Field - GEO2002 - Bahrein 14-17th of april 2002
Well E
Well C
Well B
Conductive faults
Sealing faults
INJ 2
INJ 1
Well D
Well A
Early water breakthrough due to conductive faults
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A
B
C
DE
A
B
C
D
E
Lagalaye (Total), Grard (Beicip-Franlab) Conductive Fault Modelling and History Match Improvements on a Fractured Carbonate Field - GEO2002 - Bahrein 14-17th of
april 2002
Sweep efficiency
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Water saturation after 20 years in a carbonate field with fracture swarms
Recovery mechanisms in fractured reservoirs
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Flow mechanisms in fractured reservoirs- In the fractures
- In the matrix
- Between matrix and fractures
Main drive mechanisms in fractured reservoirs
- Convection segregation
- Imbibition
- Gravity drainage
Flow mechanisms in fractured reservoirs 1/3
Principle: fractures enable the large-scale transport but most of the oil is
contained in the matrix blocks: matrix-fracture transfer is essential
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- Forced displacement in fractures only, not
significant in matrix (except if low permeability
constrast between fracture and matrix) ;
- Only spontaneous mechanisms are efficient for
recovering matrix oil: expansion, capillarity,
gravity drainage, diffusion;
- Fractures act as a saturation (or pressure orcomposition) boundary condition for matrix
blocks: they impose on the limits of the blocks a
fixed potential different from that of matrix; large
exchange surfaces are offered.
- Determinant parameters for exchanges: block
size and shape (height); matrix properties;
wettability, permeability, boundary conditions
(rate of fracture invasion), fluid properties.
Flow mechanisms in fractured reservoirs 2/3
Viscous flow (forced displacement) is in most of the cases
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negligible in a fractured reservoir
P1
P2
P2 ~ P1 and Kf>> Km
Flow mechanisms in fractured reservoirs 3/3
AA AWater
InjectionDepletion Gas Injection
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Gravity Drainage
Reimbibition
Diffusion
Imbibition
WOC
GOC
Water drive
Gas drive
AA AGOC
WOC
Segregat ion
+Convect ion
within fractures
Convection phenomenon
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High vert ical permeabil i tyGravity segregat ion
Thermal gradients
GOC
Gas liberated
Heavier oil
Lighter oil
Field observation: bubble point pressures
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GOC, 1900 m2000 m
3000 m
1950 m2255 m
2620 m
84 bar
124 bar
112 bar
(initially 150 bar everywhere in 1977)
Reduction of bubble
point pressure withtime
In 1998
Field example of Matrix-Fracture transfer
Cretaceous Upper Reservoir Interval
Cretaceous Upper Reservoir Interval
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Cretaceous Upper Reservoir Interval
Fault related fractures
Diffuse fracture network
8 matricial reservoir
Lower 7 / 5c matricial reservoir
Cretaceous Lower Reservoir Interval
Fault related fractures
Basement Interval
Fault related fractures
Cretaceous Upper Reservoir Interval
Fault related fractures
Diffuse fracture network
8 matricial reservoir
Lower 7 / 5c matrix reservoir
Cretaceous Lower Reservoir Interval
Fault related fractures
Basement Interval
Fault related fractures
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Start
Gas-cap evolution in the matrixField example of Matrix-Fracture transfer
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10 years
20 years
55 years
Gas Drainage = small effectIn the matrix blocksReason: Block reduced size
Start
Water evolution in the fracturesField example of Matrix-Fracture transfer
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10 Years
20 Years
55 Years
Water InjectionStarted 40 years
after production
Injected waterNo natural waterencroachment
Start
Water evolution in the matrixField example of Matrix-Fracture transfer
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10 Years
20 Years
55 Years
important Effect ofmatrix blocks imbibitionDue to water injectionReason: blocks wettability
Haft kel (Iran): 35% OIP recovered
Primary recovery: depletion and imbibition 26%
Field recovery examples 1/3
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Secondary recovery: gas injection 35%
Ekofisk: 35 - 40 % OIP (Water injection + Subsidence)- low-permeability chalk, water wet rock
- small block size
Qarn al alam: 1.5% OIP (17 years production, water breakthrough due to fractures)
viscous oil (16API, 220 cP)
low-permeability oil-wet matrix
Emeraude: 3 - 6 % OIP (Water/Oil)
- viscous oil (~ 100 cP)
- Oil wet - solution gas drive recovery mechanism
Idd el shargi north dome: 1.6% OIP (28 years production, 1991)
Thick water-oil transition, conductive faults, low productivity (Km= 1 to 5 mD)Secondary recovery : ring pattern waterflood, crestal gas injection
Gas fields examples:
Field recovery examples 2/3
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- Meillon: 60% GIP (Gas / Water)
Fractures provide enhanced productivity BUT early water breakthrough
- Lacq Profond: > 95% GIP (single-phase depletion)
Fractures enhance productivity (Km # 10-3 mD)
Conclusion:
- Fractures can either enhance recovery (Lacq, Haft Kel) or stop it
prematurely (breakthroughs)
- Recovery may be very low in fractured reservoirs with poor oil and matrix
properties (high o, low K, oil wettability) and/or an unsuited productionmethod (early breakthroughs)
Field recovery examples 3/3
20
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0
2
4
6
8
10
12
14
16
18
Frequency
0 - 10% 10 - 20% 20 - 30% 30 - 40% 40 - 50% 50 - 60% 60 - 70% 70 - 80% 80 - 90% 90 - 100%
Ultimate recovery
Gas reservoirs
Oil reservoirs
Ref: SPE 84590Figures obtained from 56 fractured oil reservoirs and 8 fractured gas reservoirs.
Main geological evidences of fractured reservoirs:
Drilling information:
Checklist of fractured reservoir evidences 1/2
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Drilling information: High rates of penetration (in the fractured intervals)
Low core recovery (in highly fractured intervals)
Structural information High structural dips, folding
Field located close to regional faults
Core description Presence of numerous continuous open (or partly open) fractures
Seismic data analysis Presence of numerous faults
These information have to be integrated with dynamic data !!
Main dynamic evidences of fractured reservoirs:
Drilling information:
Checklist of fractured reservoir evidences 2/2
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g mud losses
Well testing: Kh test >> Kh core
dual porosity signature
presence of no flow boundaries or constant pressure boundaries
dispersion of skin data
Production logs Low temperature gradient in the oil column (convection in fractures)
Flowmeters with sudden changes
Production data/history
high productivity/injectivity earlierbreakthroughsthan predicted by models ignoring fractures
Caution
Cautions and conclusions
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Some fractured reservoirs do not yield typical dual-porosity well test
results: transition betweenfracture and matrixregimes may be hidden or delayed.
A typical dual-media well test behaviour may also result- from communication between layers (cross-flow);
- from a high-permeability heterogeneity of the matrix (permeable streaks).
Mud losses or well productivity are not sufficient indicators.
Conclusion
Evidence of fractures and of matrix-fracture flow-property contrast results
from the cross-checking of several sources of information.