Fractured Reservoir

What is a Fractured Reservoir ?

BEICIP-FRANLAB

NATURALLY FRACTURED RESERVOIRS

ADMA-OPCO - from 06 to 07 of January 2008

Copyright© 2008. Beicip-Franlab - All rights reserved

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232, Avenue Napoléon BonaparteP.O. BOX 21392502 Rueil-MalmaisonFrancePhone: +33 1 47 08 80 00Fax: +33 1 47 08 41 [email protected]

Naturally fractured reservoirsNaturally fractured reservoirs

ADMA-OPCO

06 - 07 January 2008

ADMA-OPCO

06 - 07 January 2008

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2Part 1- What is a naturally fractured reservoir

OBJECTIVES of the course:

Background, methodologies and tools to account for the presence of fractures in oil & gas reservoirs

Part 1: Introduction to fractured reservoir. What is a fractured reservoir? Types of fractures and fractured reservoir.

Part 2: How to characterize a fractured reservoir? How to detect fractures? How to model their distribution as well as their geological and flow properties?

Part 3: How to develop a fractured reservoir? How to identify the appropriate recovery mechanism?

Part 4: How to simulate a fractured reservoir? How to develop fractured reservoirs?

Naturally Fractured Reservoirs


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232, Avenue Napoléon BonaparteP.O. BOX 21392502 Rueil-MalmaisonFrancePhone: +33 1 47 08 80 00Fax: +33 1 47 08 41 [email protected]

Naturally fractured reservoirsNaturally fractured reservoirs

Part 1: Introduction to fractured reservoir

Part 1: Introduction to fractured reservoir

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4Part 1- Introduction to fractured reservoir

φ FK F

φ mKm

Fractured reservoir :

MatrixFractures (=matrix heterogeneity)

Impact recovery

What are Fractured Reservoirs ?


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MAIN OUTLINES

� 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 TYPES OF FRACTURED RESERVOIRS

Introduction to fractured reservoir

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Illustration of fractures / fracture sets



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Fracture density (biased / unbiased)

50 m

A fracture set is characterized by its avg. strike and dip,

length distribution, and density


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Illustration of the fracture connectivity

Local connection

Not connected network at

grid scale



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Illustration of the fracture connectivity

connected network at grid scale

If fractures are open, this connected fracture network will

have an impact on fluid flow


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Illustration of the fracture flow anisotropy

The connected fracture network will induce flow

anisotropy in the reservoir: ∆Px < ∆Py

X

YQ1

Q2

~ Q1

Q2



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Matrix block size definition

The block size is determined by length of matrix

blocks surrounded by connected fractures


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Length of homogenisation

The length of homogenization (REV) is a value of grid size

not impacting the fracture properties

REV = Representative elementary volume



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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


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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 structural discontinuities affecting flows

[ R.A. Nelson, in Geologic Analysis of Naturally Fractured Reservoirs,

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

increased reservoir permeability and/or porosity or increased permeability anisotropy” ]



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Mode I: Fractures are purely dilational

(extension)

Mode II: Fractures may exhibit shearing with

components parallel (mode II) to the direction

of propagation of the fracture front.

Mode III: Fractures may exhibit shearing

with 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 canbe formed by one or more mechanisms such as hydraulic fracturing, thermal contraction, etc.

Fracture propagation modes


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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 three

stresses called principal stresses and

oriented perpendicular to each other.

The principal stresses are defined:

σ 1 > σ 2 > σ 3

Fractures and stress state



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Joints (mode I) in green

Shear fractures/faults (mode II) in red

Stylolites in blue

Example fractures and stress state

Increased confining Stress and/or Temperature


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Natural Fracture Classification

� Tectonic Fractures

• Joints, Fracture swarms, Fault-related, Fold-related

� Diagenetic Fractures

• Bed-parallel stylolites, Stylolite-related features (tension gashes, etc..)

Main types of fractures


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Tectonic fracturesTectonic fractures


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JOINTS

Joints are fractures developed over large areas

of the earth’s crust with relatively little change

in orientation, with no evidence of offset along

the plane, and perpendicular to bedding.



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Homogeneous density

Joint sets in sandstones


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A

Homogeneous fracture density with constant fracture orientation

Joint sets in carbonates



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Joints are controlled by bed thickness


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Relation between bed thickness and fracture density

Density 3 > Density 2 > Density 1

h1> h2 > h3

h1

h2

h3

Well trajectory and fracture density log



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Joints are controlled by lithology

High Shale content


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LITHOLOGY

FRACTURE

DENSITY

0 10

FS

MFS

Fracture density

controlled by :

1 : Shalyness

2 : Porosity

3 : Bed thickness



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FRACTURE SWARMS

Fracture swarms are areas where fracture density

is high and fractures are preferentially oriented.

They are large-scale objects (several hundred

meters). Usually fractures cross layers

boundaries.


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Fracture swarms in sandstone reservoirs

2m



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Fracture swarms in carbonates

Fracture density log


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Faults and fault-related fractures


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FAULTS

Faults are discontinuities, with discrete

displacement. They are usually associated to

fractures. In the fault areas the fracture density is

high.

Fault planes are, by definition, planes of shear. The majority

of fractures associated are parallel to the fault (Ronald A.

Nelson)


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Hanging wall

block

Normal Fault

Reverse/Thrust Fault

Strike-slip Fault

Fault terms and fault types



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Faults and stress state


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Caine et al. 1996

Micarelli et al., 2003

Density of fault-related fractures progressively

decreases with increasing fault distanceCaine et al., 1996 - Geology

Micarelli et al., 2003 – Journal of Geodynamics



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Fractures

sub-verticales

Lozenge-shaped fractures

Fractures

sub-verticales

Background Background

Fault coreZone très

endommagéeZone faiblement endommagée

Zone très endommagée

Zone faiblement endommagée

Cataclasite

5 cm

5 cm

E W


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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 the

upper or lower tip)



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FOLD RELATED FRACTURES

The stress history during the initiation and

growth of fold in rock generate 3 fractures

families.

- 1. Parallel to sigma 1

- 2. Oblique to sigma 1

- 3. Perpendicular to sigma 1


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•Stylolitic joints and contractionalfaults are oriented orthogonal to the

maximum stress axis (σσσσ1)

•Fissure veins, extensional fracturesand extension faults are oriented

parallel to the maximum stress axis(σσσσ1)

•Conjugated shear joints are oriented

oblique to the maximum stress axis(σσσσ1)

Folds result from a compressive ductile deformation, in which themaximum stress axis (σσσσ1) is sub-horizontal

Fold-related fractures are:

σσσσ1



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CompressionExtension

Fractures produced by extension are pure

extensional and open fractures

Fractures produced by compression are

closed, stylolithic and/or partially open.

Strain partitioning in a fold


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Stylolites related fractures


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Stylolite

Wave-like or tooth-like, serrated, interlocking

surfaces. Stylolites are thought to form by pressure

solution, a dissolution process that reduces pore

space under pressure during diagenesis.


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Fractures / tension gashes related stylolites

Origin of stylolites is overburden plus tectonic stressesThey form tight intervals that may be preferentially fractured



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Fracture

Stylolite

Stylolite related fractures observed on cores

Paleo-minimum stress direction

overburden

Stylolite peaks

Tight zone related to pressure-solution

Tension gashes

Tectonic fractures


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MAIN CHARACTERISTICS OF THE FRACTURES

• Various structural objects

Faults - swarms - Joints - stylolites related fractures

• Important scaling factor

Observation: from reservoir (10 km) - Grid block Cell (250 m) – to wellbore (1-10 m) scale

we need to integrate various data (logs, cores, seismic data, dynamic data)

Fracture properties: (phi, K, compressibility) cannot be measured at fracture scale

we need to upscale the fracture network = equivalent

parameters



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Main types of fractured reservoirs


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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)

φ FK F

φm = 0

Km = 0



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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)

φF

K F

φm

Km ~ 0



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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)

φ FK F

φ mKm




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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)

…

φm

Km

φF

K F



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• Haft Kel (Iran): 35% OIP recoveredPrimary recovery: depletion and imbibition 26%Secondary recovery: gas injection 35%

• Ekofisk: 35 - 40 % OIP (Water injection + Subsidence)- low-permeability chalk- small block size

• Qarn Al Alam: 1.5% OIP (17 years production, water breakthrough due to fractures)• viscous oil (16ºAPI, 220 cP)• low-permeability oil-wet matrix

• Emeraude: 3 - 6 % OIP (Water/Oil)- viscous oil (# 100 cP)- no spontaneous imbibition - 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

Field recovery examples (1)


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Field recovery examples (2)

0

2

4

6

8

10

12

14

16

18

20F

req

uen

cy

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 84590

Figures obtained from 56 fractured oil reservoirs and 8 fractured gas reservoirs.

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Main geological evidences of fractured reservoirs:

Drilling information:• High rates of penetration (in the fractured intervals)• Low core recovery (in highly fractured intervals)

Structural information• High structural dips, folding• Foothills environnments• 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 informations have to be integrated with dynamic data !!

Checklist of fractured reservoir evidences


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Main dynamic evidences of fractured reservoirs:

Drilling information:• mud losses• Rate Of Penetration• core sample recovery

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

• earlier breakthroughs than predicted by models ignoring fractures


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Caution:

• Some fractured reservoirs do not yield typical dual-porosity well test results: transition between fracture and matrix regimes may be hidden or delayed.

• A typical dual-medium 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 Reservoir