Tuesday 21 May 2013 - Keynote - Charles Fairhurst

7/23/2019 Tuesday 21 May 2013 - Keynote - Charles Fairhurst

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Fractures and Fracturing

a presentation

by

Charles Fairhurst*

to

The International Conference for Effective and Sustainable

Hydraulic Fracturing

20-22 May 2013,

The Hilton Brisbane, Australia.

*Senior Consultant, Itasca Consulting Group, Inc. Minneapolis, USA.

Professor Emeritus, University of Minnesota. Minneapolis.


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Purposes of Hydraulic Fracturing

to enhance connectivity between existing fractures

for improved

fluid flow

conditioning

-where natural fracturing is not sufficient

(e.g. Caving of longwall goaf ;block caving in mines).


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Kirsch (1898)Circle

Inglis (1913)

Ellipse

Griffith (1921). Stress is highly concentrated at tip of

degenerate ellipse very thin crack or flaw.

Tensile Strength of Solid should be about

E/3. Actual strength is three orders lower.

Inter-atomic Force-Separation Relationship

Stress Concentrations around

holes in Isotropic Continuum

Fracture Mechanics

Linearly Elastic Fracture Mechanics (LEFM)


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Theorem of Minimum Potential Energy.

The stable equilibrium state of a system is that for whichthe potential energy

of the system is a minimum.

The equilibrium position, if equilibrium is possible,must be one in which

rupture of the solid has occurred, if the system can pass from the unbroken to

the brokencondition by a process involving acontinuous decrease of potential

energy.


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P = W +U + S +K ( + )

Changes in

P = Potential Energy of System.W = Potential Energy of Applied Forces

U = Strain Energy

S = Surface Energy

K = Kinetic Energy

F =Frictional Energy etc.

Crack accelerates, decelerates, goes

around or through grains seekingMinimum Potential Energy path.

Potential Energy Changes during

Crack Propagation throughHeterogeneous Solid under Tension Crack may stop, but rock strength may

decrease with time - and crack may extend.


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Crack Propagation from Internally Pressurized Hole in a Glass Plate

Porter and Fairhurst (1970) - after Mindlin (1939)


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Hydraulic Fracturing (1949)Unlike previous discussion, fracture extension is by fluid injection

In an impermeable medium, the viscous energy dissipation associated with driving fluid through the fracture

(Reynolds , Poiseuille laws) competes with the energy required to break the solid material.In a permeable medium, fluid leak-off further complicates the mechanics of crack propagation.

Fracturing of the rock at the crack tip is governed by LEFM i.e. the asymptotic form

w = K1s

1/2

,

w is crack aperture, and s is distance from the crack tip.

Where viscous flow dominates, the coupling between the fluid flow and solid deformation leads

to the asymptotic formw =K2s

2/3

K1, K2 are constants.


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Scaling

The discovery of scaling laws very often allows an increase,

sometimes even a drastic change, in the understanding of not only asingle phenomenon but a wide branch of science.

G.I.Barenblatt. Scaling [Cambridge Univ.Press (2003)]

Important attribute of classical analyses is that solution is given in

terms of dimensionless groups.

..

Previous analyses assumed that the medium was a

homogeneous continuum.

Will now consider rock mass.


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Rock in situ is probably the most complex material encountered in any

engineering discipline.

Pre-loaded by tectonic and gravitational loads for many millions of years,

transected with fractures and a variety of planar discontinuities, etc., the

mechanical behavior can only be determined directly in the field.

.

Mining evolved empirical rules over many, many years; well aware of

complexity

- technology going beyond limits of experience - into uncharted territory.

-direct 3D access to subsurface. (shared with Civil Engineering)

- little R&D investment in rock mechanics in past 2~ 3 decades.

Petroleum- (150 year history) - direct 1D access only; some rules ;

- intensive R&D - results proprietary.

Other subsurface engineering developments -Enhanced Geothermal,Carbon Sequestration; Drill Cuttings Injection; Tight Shales, etc.?

- Relatively little experience


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Importance of Pre-existing Fracture Networks

Enhanced Geothermal Systems - Fenton Hills, New Mexico, 1970

.idea that hydraulic pressure causes competent rock to rupture and

create a disc-shaped fracture was refuted by the seismic evidence.

Instead, it came to be understood that

hydraulic stimulation leads to the opening of existing

natural joints

that have been sealed by secondary mineralization.

Over the years additional evidence has been generated to show that the

joints oriented roughly orthogonal to the direction of the least principal

stress open first, but additional joints open as the hydraulic pressure is

increased

(Duchane and Brown GHC Bulletin Dec. 2002 p.15 GHC (Geo-Heat Center)
http://geoheat.oit.edu/


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Increasingfluidpressure

Hydraulic Fracturing may be preceded by Slip on JointsIn a Permeable Jointed Rock.

Hydro-Shearing

Hydraulic

Fracturing


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Fracture

representation

3D Discrete

Fracture Network

(DFN)

Intact rock

representation

(including brittlefracture)

Synthetic Rock Mass (SRM)

Bondedparticle

assemblyintersected

withfractures(SRM)


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13

The consequence is that each element appears to bephysically isolatedfrom its neighbors during one time step;

Thus

Explicit Finite Difference Method - The Calculation Cycle.(Results of each cycle can be superimposed to form a movie of evolution of deformation)

Forces are fixed

duringthis

calculation

Strain rates are

fixed duringthis

calculation

(forallmass-points)

(forallelements)

All Itasca codes use an explicitsolution method that marches on in time

(even for static problems). Cundall, 1970.


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Observed Path of Hydraulic Fracture as revealed by Mine-Back

Courtesy Jeffrey et al; CSIRO


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Response to Complexity and Lack of Understanding in Engineering.

1. Develop empirical rules based on experience.

2. Develop theory to refine and improve practice.

Industrial Revolution later development of Continuum Mechanics.Rock Mechanics in Mining and Civil Engineering. (3D access to rock mass)

Petroleum - 1D access (borehole) greater research emphasis (proprietary)

Limits to Empiricism

requires practical experience;

applicable only within bounds of available experience.

Proposed applications of Hydraulic Fracturing to situations where

Little prior experience.

No time to develop empirical rules.

Enhanced Numerical Modeling and Observational Tools are Available.


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Numerical Experiments.

Rock mechanics models fall into the class of data-limited problems;

one seldom knows enough about a rock mass to model it unambigously.

The purpose of modelling data-limited problems is to gain understanding

and to explore potential trade offs and alternatives, rather than to make

absolute predictions

A model is an aid to thought, rather than a substitute for thinking.

plan the modelling exercise in the same way as you would plan a laboratoryexperiment.

(Starfield and Cundall, 1988)


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Major Challenge

is to define

realistic

Discrete Fracture Networks (DFNs)

especially for borehole access situations.

Progress!

Do DFN characteristics depend significantly on rock

formations e.g. crystalline; sedimentary?


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Fracture Imaging

Schlumberger

P

P

P

P P

S

S

P

Imaging of reflected waves (PtPand StS) when the tool is above or

below the fracture.

Imaging with mode conversions(PtS and StP) when the fracture is

between the transmitter and

receiver.

Waves return to the tool based on

the geometry of the event relative

to the tool:

Up-dip reflector imaged when the

tool is above the fracture

Down-dip reflector imaged when

the tool is below the fracture


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Horizontal well with open fractures away from the wellbore

Schlumberger

24

00

-20

20

-40

F

eet

X000 X100 X200 X300 X400 X500


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Summary #

Reflections from monopole receivers have been

used to successfully image structure and fractures

Depth of investigation can vary greatly, but can

beas much as 140 ft on each side of theborehole.

Bridges the resolution gap between wireline and

seismic data.

Complementary data from Stoneley mobility,

Anisotropy, Rock properties and potentially

Radial Profiling can be acquired in the same

pass.

0.1 1.0 10 100

0.1

1.0

10

100

Range [m]

Resolution[m]

ultra-

sonic

sonic

2D seismic

3D seismicVSP

(3D)Deep

Acoustic

Imaging

The Acoustic Gap

0.1 1.0 10 100 1000

100

10

1

0.1

10

D.Grael et al*; Borehole Acoustic Reflection Survey (BARS) from Modern, Dipole Acoustic Logs for High-Resolution

Seismic-Based Fracture Illumination and Imaging SPWLA (Soc. Petrophysicists and Well Logging Analysts),

53rd

Annual Logging Symposium, June 16-20, 2012 Cartagena , Columbia .

*D. Grae, G. A. Ugueto C., J. A. Roberts, H. Yamamoto, T. Oliver and G. Martine


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(a)Micro

Seismic

Indication

of

Upward

Migration

of

Fluidalong Fault during HydraulicFracturing

(b)NumericalExplanationof

UpwardMigration.


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In Situ Stress and Critically Stressed Sub-Surface


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In-Situ Stress State

1. Magnitude and Orientation affected by undetected fractures/faults


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Observed variability of normal stress across a thrust fault at the URL, Pinawa,Canada.

Local In situ Stresses Affected by Local Variations on Faults

How do In situ Stresses Change with Rock Type;


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Limestone

H > V> h

Argillite(indurated clay)

H = V = h

Limestone

V > H > h

In

Situ

Stresses

Change

with

Rock

Type

(Underground

Research

Laboratory,

Bure.

France)

How do In situ Stresses Change with Rock Type;

Relevance to Tight Gas Shales ?


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Aseismic slip induced by forced fluid flow as detected by P-wave tomogrqphy

(Soultz-sous-Frets, France) Cornet et al; 2012

(a) The injection program (black curve is flow rate; blue curve is well-head pressure; horizontal axis is time in

days;

(b) 3D view of the seismic cloud with respect to the GPK2 borehole. Vertical axis is depth and horizontal axes

are distances respectively toward the north and toward the east; and

(c) Horizontal projections corresponding to the yellow horizontal plane. The vertical green plane is shown as

line AB in the plots of part c. P-wave velocity tomography for sets 2,3 and 4 are indicated respectively by

orange, yellow and green colors in the injection program. The vertical axis corresponds to North.

20%Drop

in

Vp.


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Vp 6.4km/s=1.00

Vp 5.1km/s=0.80

Aseismic Deformation

20%dropinPwavevelocityinregion

~500moutsideseismiccloud.

2.0E+07

d


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0

5

10

15

20

25

0 2 4 6 8 10

BoreholePressure(MPa)

Total

Time

(days)

0.0E+00

2.0E+06

4.0E+06

6.0E+06

8.0E+06

1.0E+07

1.2E+07

1.4E+07

1.6E+07

1.8E+07

0 250 500 750

Pressure(Pa)

Distance(m)

2days

6days

7days

8days

9days

10days

0.0E+00

2.0E+06

4.0E+06

6.0E+06

8.0E+06

1.0E+07

1.2E+07

1.4E+07

1.6E+07

1.8E+07

2.0E+07

0 250 500 750

Pressure(Pa)

Distance(m)

2days

6days

7days

8days

9days

10days

=104 mD

Fluid Flow into Far -Field

continues

after Borehole Depressurization.

=250mD


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9

Why Doesnt Microseismicity Correlate

With Production?

The Total Rock Volume

Affected by

Microseismicity

Accounts for Less

Than 1% of Gas

Production in First 6

Months

IngrainInc

Micro-permeabe gas shales.

Courtesy Prof.A.Nur

Stones

have

begun

to

speak,

because

an

ear

is

there

to

hear

them.

..


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g p ,Cloos,ConversationswiththeEarth(1954),4

Fracture

Network

Engineering.

Synthetic

Rock

Mass

and

Synthetic

Seismicity

Modelsarecomparedwithobservedmicroseismic signalsforrealtime controlof

fracturenetworkdevelopment. (EnhancedGeothermalSystems.)

Microseismicity predicted

and

observed.


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Rock Conditioning

In situ Rock Weakening /Size Reduction


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Pre-Splitting (Atlas Copco)

Plexiglas after Detonation of Explosive in Hole

(Persson et al; (1970) ISRM Congress, Betgrade (1970)

Rock Conditioning - Hydraulic (Gas) Fracturing


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C l i


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Conclusions.Hydraulic Fracturing applications increasingly important for extraction of resources and

injection /disposal of waste products.

Pre-existing fracture systems have dominant effect on hydra-frac development and related

stimulation procedures.

Rock Fracture Mechanics is potentially as broad as classical Fracture Mechanics.

Rock mass embraces wide variety of constitutive behaviors; rock is not critically

stressed everywhere in crust; concept can be very misleading.

Micro-seismic detection systems are essential but do not identify full response of system to

stimulation additional geophysical tools needed.

Datalimited nature of Earth Resource Engineering problems gives added importance

to analysis and modeling.

Numerical experiments (scalable) can be major aid to practical advance but will require

planning/co-ordination to allow sound Mechanics-Informed practical decisions.

Fracture Network Engineering is sound goal, but requires considerable additional

development.


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Thank you

- to the organizers of Hydraulic Fracturing 2013

for the invitation to participate in the Conference

and to present this Lecture;

- to the audience for your attention.


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Back - up Slides

D/R = 0.1 D/R = 0.2Potential for core damage during


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0

0

0.2

2 4

0.4

0.6

D/R = 1D/R = 2

t

t

tt

t

o

o

oo

o

/

/

/ /

/0.00

0.50

0.25

t o/

0.00

0.50

0.25

t o/

0.0

1.0

0.5

t o/

0.0

1.0

0.5

Maximum

Maximum Maximum

Maximum

= 0.05

= 0.5 = 1.0

= 0.1

Core Depth / Core Radius, D/R

t

o

/

Max.tensilestressincore

Far-fieldstress

,

D

R

g gcoring operation

Numericalresults

Best fit curve

Let

Let

n

m

=

=

=

=

t

ti

c

c

c

o

o

o

tensile strength

induced tension in core

compressive strength

in-situ horizontal stress

[n ~ 0.1]

[m ~ 0.5]

Then core damage occurs if

>

>

n/m

0.2i.e.,


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Tuesday 21 May 2013 - Keynote - Charles Fairhurst

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