Fractured Reservoirs Part 1

7/28/2019 Fractured Reservoirs Part 1

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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

[email protected]

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

[email protected]

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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1Part 1- What is a naturally fractured reservoir

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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1Part 1- What is a naturally fractured reservoirPart 1- What is a naturally fractured reservoir

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.


What is a naturally fractured reservoir ?


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Tectonic fractures




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2Part 1- What is a naturally fractured reservoir 2Part 1- What is a naturally fractured reservoir

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



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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



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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



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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



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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



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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



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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



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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



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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.)



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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.


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


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



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Fault related fractures

Diffuse fracture network

8 matricial reservoir

Lower 7 / 5c matricial reservoir

Cretaceous Lower Reservoir Interval


Basement Interval




Diffuse fracture network

8 matricial reservoir

Lower 7 / 5c matrix reservoir

Cretaceous Lower Reservoir Interval


Basement Interval



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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:



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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)


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.

Documents

Fractured Reservoirs Part 1