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EGS Technology: hydraulic fraccing:il d d h l b t tioil and gas and shale gas best practice

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Content

rationale

Borehole Stress and failure

What applications for hydraulic fracturing (general)

How does it work (theory and operational)

Models vs reality

Fracture aperture and permeabilityFracture aperture and permeability

What did we learn from gas shales

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Useful books

E. Fjaer et alPetroleum Related Rock Mechanics2nd edition

J. Jaeger, N.G. Cook & R. ZimmermannFundamentals of Rock Mechanics

George E. King Thirty Years of Gas Shale Fracturing: What Have We Learned? SPE 133456

Kevin FisherSPE YP presentation : Hydraulic Fracturing: Modeling vs. Reality

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WHY HYDRAULIC FRACTURING IN GEOTHERMAL?

•EU research project > 20 years•3 wells > 5 Km deep

Enhanced Geothermal SystemsWHY HYDRAULIC FRACTURING IN GEOTHERMAL?

•3 wells > 5 Km deep•Comprehensive Fracturing programe•3MWel Power via ORC plant

(www.soultz.net)

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WHY HYDRAULIC FRACTURINGIN GEOTHERMAL?

Doublet performance

Fl t Q

E [MWth] = Q*ΔT * CP Tp=70 C Ti=25 C

Flow-rate QΔp

Permeability X thickness

Δp generated by pumpsWhich consume electricity

y

ΔkHQ 2π Which consume electricity

Δp is restricted by safetymeasures⎟⎟

⎠

⎞⎜⎜⎝

⎛+⎟⎟

⎠

⎞⎜⎜⎝

⎛Δ=

SrL

pQ

w

lnμ

Δp at surface does not linearly lead toHigher flow rates (friction in tubes)Viscosity distance

⎠⎝ ⎠⎝ w

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Hydraulic fracturing can be considered as reducing skin

Lf = 50mRw = 0.15mS = - ln(0.5*Lf/rw) = -5

⎟⎞

⎜⎛ ⎞⎛

Δ=LkHpQ 2π L=1500m

Improvement Q factor 2

⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟⎟

⎠

⎞⎜⎜⎝

⎛S

rLw

lnμ

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Effect of hydraulic fraccing

Doublet Power (MWth) / Costs of Energy (EUR/GJ)

0

0,5

0 5 10 15 20DoubletPower orig.

Costs of1

1,5

2(km)

Costs  ofEnergy orig.DoubletPower2

2,5

3Dep

th ( Power

Costs  ofEnergy

Transmissivit3,5

4

4,5

yTransmissivity

0 20 40 60 80 100

Tranmissivity (Dm)

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Hydraulic fracturing – Types of applications

Tip-Screen-Out fracturing / Frac & PackGoal: Bypass damageTypically in higher-permeability reservoirShort fractureTip-Screen-Out to increase fracture widthTip Screen Out to increase fracture width

Well layout (5-7 km depth) Tensile fraccing

80C240C

400 m

10October 10, 2011

Depth of top Dinantian Carbonates(Geluk, 2007)

Seismic interpretation

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[mDm]

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Hydraulic fracturing – Applications

Frac & Pack Weak, permeable formations

Stimulating naturally fractured reservoir

Bypass skinSand control

M i H d li F t i

Activate fracture networkE.g. unconventional shale gas

Water injectionMassive Hydraulic Fracturing(EGS, aquifers)

Low-permeability reservoir

Water injectionMaintain injectivityThermal fracturingo pe eab ty ese o

Usually first minifrac testFracture pressure

Leakoff tests, Extended leakofftests

Fracture gradientContainment

Leakoff behavior

Fracture gradientMininum in-situ stress

Waste disposalpDrill cuttingsProduced water

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Hydraulic Fracturing – Coupled Processes

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Hydraulic Fracturing

• Tensile failure, NOT shear failure• Orientation of the fracture: that direction

where pf > σ + T0 first, i.e. σ is minimal (T t il t th)(T0: tensile strength)

• The normal stress on the fracture wall “tries” to close the fracture

• Therefore the orientation isTherefore the orientation is• Perpendicular to the minimum in-

situ stress direction• Parallel to the medium and the

maximum in-situ stress direction• Vertical• Sometimes horizontal for very

shallow fractures

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Ph i lPhysical process

pf>closure stress(σc ==σh)

Hydraulic fraccinng

S Elastic closureShut-in

Elastic closure

LeakoffPf==σc

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Hydraulic fracturing

Water Injection under Fracturing Conditions

Plugging and Channelling in

Fracturing Conditions

Fluid flow in Reservoirg

Fracture

CrackingFluid flow in Fracture

Fracture

ReducedPermeability

BRGM GFZHydraulic fracturing – gas shale learning base

Barnett shaleVery low permeabilityNaturally fractured

Goal: interconnected fracture networkfracture networkWaterfracturingMonitoring

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Hydraulic fracturing – Basic concepts

σStress: maximum stress vertical; minimum and medium stresses

σ1

horizontal

Modes of fracturingσ3

Modes of fracturing

σ2

Hydraulic fracturing: Tensile (mode I) – Vertical fracture has least resistance

Mode I: Opening Mode III: TearingMode II: Sliding

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Sollicited Induced seismicityEGS operations relies on generating permeability through shear fractures.

Through massive fluid injection typically 50l/s over various daysThrough massive fluid injection typically 50l/s over various days

Basel injection rate (haering, 2008)

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H d li F t i th d fi tHydraulic Fracturing – growth and confinement

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Hydraulic Fracturing

18000 20 40 60 80

Stress [MPa]Lithography induces contrasts in minimum in-situ stress 1800

2000

2200

in minimum in-situ stressLithgraphic density: 2200 kg / m3

gzz ρσ =ν=2200

2400

2600th [m

]

Fluid density:1000 kg / m3

0.4

ν=2600

2800

3000D

ept

zh σνσ =1

0.3

ν=3000

3200

3400

gzfαρνν

ν

−−

+

−

121

10.4

3400

Poisson’s ratio [-]0 1

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Hydraulic Fracturing –Effect of layering confinementEffect of layering, confinement

Layeringσh

y gElasticityStress

hFracture vs timePermeability

Porosityde

pth

injection

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H d li f t i ti ( i l )Hydraulic fracturing operations (pinnacle)

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How BIG are hydraulic frac jobs

Fracture treatment volumes can be over 10,000 m3

Pump rates can be 100 l/s or more

Proppant placed up to 1 mln kg

Fracture length ranges from 3 to 1500 m

$ $Treatments cost ranges from $5,000 to $5,000,000 USD

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Experiments (Fisher, 2010)

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Experiments (Fisher, 2010)2010)

Horizontal wellHorizontal well

Planar fracture surface (vertical)

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Stress CONTROLS fracture propagation over modulus

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Stratigraphic layering (and overpressure) cause fractures to be abruptly blunted

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Confinement mechanism related to high poison ratio (low critical stress + maybe very weak decoupling

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Multiple fracs:

Store excess volumeStore excess volume• Reduced length

Additional leakoff• Additional fracture faces• May change significantly with time• May change significantly with time

Higher pressure dropg p p• Additional fracture faces

Ti t d ff tTip generated effects• additional stress with shear dilatency

different prop settling/transport

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Modelling versus measuring

Fracture growth models

Mapping diagnosticsincomplete physical

understanding

diagnosticsnot predictive

C lib t d d lCalibrated models more realistically predict how fractures will grow forfractures will grow for alternative designs

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An example of a model:Effect of Stress Gradient and Stress Contrast

900 400900

950

1000 300

350

400

(m)Shale layer Increased stress

in shale layer1050

1100

1150Dep

th (m

)

150

200

250

ctur

e le

ngth

Fracture height

Fracture length

Δσ = 0 MPaΔσ = 1 MPaΔσ = 2 MPaΔ 3 MP

in shale layer

1150

1200

1250 50

100

150

Frac Δσ = 3 MPa

13000 2000 4000 6000 8000

Time (days)

0

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Width and length contours (Δσ = 2 MPa)

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What can we measure/ESTIMATE

• Lithology (logs) • Gamma Ray (GR)

• dynamic modulus (E) and• dynamic modulus (E) and • poision ratio (v)

• Micro-seismicity (shear failure only)y ( y)• Stress (special measurements MRX)• Pressure

Tilt t• Tilt meters

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P f bl d i i f t tPreferably do a mini-frac test

M i t f d iMore input for design:In-situ stressesFracturing pressures Minifrac test}Fracturing pressures Minifrac testLeakoff behaviour

}ISIP = initial shut –inPressure

Shut-in time

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Use pressure to constrain fracs

BRGM GFZStress changes during fracturing

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Sometimes model predictions and measurements agree well

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But in other cases not

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MICROSEISMICITY

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MICROSEISMICITY

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Microseismic monitoring

• Numerous cases where fracture

grows at or close to microseismic

observation well

• Height can be accurately assessed

• Usually observe fractures following

lithologylithology

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Fracture containment AS a conseqence of strength of surrounding layers

Variable containment in shales• Containment (e.g., Barnett)• Bounded by carbonates• Upward growth• Continuous shale• Continuous shale

Faulting effects

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Microseismic data and model calibration-cotton valley sst

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Offset due to natural fractures and faults

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Hydraulic Fracturing in Shale Gas - Observations

No two shales alike. They vary aerially, vertically & along wellbore.

Shale “fabric” differences, in-situ stresses and geologic variances often require stimulation changesoften require stimulation changes.

First need - Identify critical data set y

Second need – never stop learning about the shale.

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Natural pathways. Open at 50 to 60% ofOpen at 50 to 60% of rock frac pressure. Open by low viscosity fl id i ifluid invasion. Difficult to prop. DominateDominate Permeability

Natural fracture systems

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Coupling betweenCoupling between geomechanics(friction; fault reactivation) and flow behaviour (dual porosity(dual porosity system)

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Effect of elastic / plastic behaviourBrittle shales are more easily fracturedSoft material: Healing of fractures

Dynamic E=sonicStatic E=mechanicalexperiment

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Re-Fracturing

Th W k B t Wh ?They Work – But Why?Old fractures with gel

Slick water fracturingSlick water fracturing connects to larger part of reservoir

Change of stress orientation

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Fracture Network Complexity

Complexity develops if natural fracture system is connected to induced fractureComplexity develops if natural fracture system is connected to induced fracture and openedObserved with microseismic monitoring

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P t l tProppant placement

Proppant settles due to low water viscosityyUnpropped fracture part still contributes to flow th h d tthrough propped partDistinction between brittle material (fractures stay)material (fractures stay) and ductile material (fractures heal)

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Water Management

Cleanup water produced back early

Use produced water for later fracture treatments

Economic and Ecologic advantages

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Interference concerns with groundwater?Not so likely due to excellent vertical confinementNot so likely due to excellent vertical confinement