Hydraulic Fracturing Avec Commentaires

8/11/2019 Hydraulic Fracturing Avec Commentaires

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Reservoir Stimulation

Hydraulic fracturing


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2 Hydraulic Fracturing

Hydraulic Fracturing

Basic principles and design steps

Operational realisation


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Let s Remember Darcy law

How to improve productivity?

Bypass the damage:

But what to do:

If the formation is tight (K


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Design steps

Basic principles in fracturing

1.- Frac height prediction

2.- Frac length design

3.- Frac pressure prediction

4.- Completion design

5.- Perforation strategy

6.- Fluid selection

7.- Proppantselection


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Basic Principles in fracturing (1)

A fracture is a rupture in traction mode.

The frac plan is thus perpendicular to the minimum stress

Below 500-600 m, the maximum stress is vertical(overburden)

=> Most of the time the fracture plan is verticalsmax

sintermediate

smin smin




sh min

sv max

The goal of a hydraulic fracture is to

increase the well PRODUCTIVITYby creating an artificial permeable channel.

PI multiplied by 2 to 4

A frac can alsoincrease the reserves

by connecting thin isolated layers

Architecture : cased & perforated

(for selectivityand to

help the frac initiation)




sh min

The target reservoirs :

Hydraulic propped fracture treatment

are mainly recommended in

sandstone reservoirs,

Best case: low permeabilityand laminated

It is pumped in two steps :

Frac initiation & propagation with gel

Proppant placement

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Basic Principles in fracturing (4a)

sh min

sv max

A fracture is characterised by :

itshalf length: Xf

itsconductivity: kf wf

Xf

wf

kf


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Basic Principles in fracturing (4b)

sh min

sv max


itshalf length: Xf


If the fracture propagates in radial mode, what is the

amount of proppant required to fill up the following

fracture:

Xf = 15 meters, Kfwf = 2500 md.ft (eq to 2 lb.ft)

Xf = 45 meters, kfwf = 2500 md.ft (eq to 2 lb/ft)

Xf

wf

kf


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Basic Principles in fracturing (4c)

sh min

sv max


itshalf length: Xf


What fracture length is required for a Fcd =2 =

K formation = 10 md

Kf.wf = 2500 md.ft

How much proppant is required to create such a frac ?

What is the fracture width if the proppant permeability is

Kf = 250 D (in mm)

Xf

wf

kf

kf wf

k Xf


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sh min

Acid fracture treatment

are recommended incarbonated reservoirs.

It is pumped in two steps.

Frac initiation & propagation with gel

The acid then fingers through the gel and

etches the frac faces to create an artificial

fissure.

pro : infinite conductivity

con : small penetration length


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Chalk reservoirs :

Chalk flows and can plug a proppant pack.

Acid frac treatments are more suited.

They may have to be regularly repeated.

To be economically interesting, North Sea developments

include Multi fractured horizontal wells.


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Multi fractured horizontal wells

sh min

Transverse FracsLongitudinal Frac

sh min


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Last application case :

in unconsolidated reservoir : FRAC-PACK.Fracture treatment to prevent sand production

Assuming a radial shape of the

fracture,

Targeted Fcd = 1

Perforated height = 25 mK formation = 500 mD

What should be the fracture

conductivity ?

What should be the fracture

width ?

kf = 250 D


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1.- Frac height prediction (1)

The fracture height is not controlled, it is imposed.

The frac will vertically grow until reachingGEOLOGICAL barriers

of higher stress than the reservoir.

Most oftenshale's or shaly layers

sometimescompact & indurated layers

orhigh pressure layers


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1 F h i ht di ti (3)


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thegamma-raylog enables to locate the shaly layers

anArray-sonic log indicates the relative mechanical propertiescontrast between layers

(Young Modulus E (Stress / strain)andPoissons Ratio n )

The exact E and nvalues must bemeasured on cores

to calibrate the Array sonic.

From the mechanical properties, some hypothesis are to be made for :

stress in the reservoir

stresses in the barriers

The frac height will be function of the stress profile best guess

Sensitivity runs will show various propagation scenarii

Only the minifrac will assess the best scenario



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Radial mode

Confined mode

Height & length

growth

unconfined

height growth

Frac height is also a function of target frac length

target 1

target 2

0.7 psi/ft

0.9 psi/ft

0.8 psi/ft

Shaly sandstone

pay-zone

shale



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If several pay-zones or if a thick reservoir :

the treatment is done in several fractures, withisolationin between

target 1

Shaly sandstone

pay-zone 1

shale

pay-zone 2

Sand plug

Bridge plug

2 F l th d i (1)


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2.- Frac length design (1)

Frac length is a function of the target productivity.

Some important parameters:

the dimensionless conductivity: Fcd =

represents the ratio between the channel

conductive potential and the matrix potential.

The goal is at least : Fcd > 2

the fracture skin

kf wf

k Xf

Sr

X Ffracture

w

f cd

=

ln 22

2

between -4 et -6

Xf

wf

kf

Effective Skin from Fracturing


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-8

-6

-4

-2

0

2

4

0.1 1 10 100 1000

Dimensionless Fracture Conductivity

Fracture

Skin

1

10

50

100

250500

1000

Xf/Rw

KfWf/XfKi---> After SPE 1017

Effective Skin from Fracturing

2 Frac length design (2)


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theFold of Increase: FOI = =represents the production increase due to the frac.

IP =

Usual range : ln(re/rw) between 7 et 9

re = 500 to 1000 m, rw = 0.1 to 0.05 m (OD= 81/2 to 41/2)

FOI usually between 2 to 4

IP with frac

IP without frac (Skin = 0)

kh

141.2 mB [ ln (re/rw) + S]

ln (re/rw)

ln (re/rw) + Sf



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A technico - economicoptimumhas to be found between :

the cumulative production for a given frac geometry

its technical operational feasibility

the treatment cost

Xf

Net Present Value

NPV



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Whats about the fracture width ?

Depends on

- Rock mechanics (Young modulus, Poison ratio)

- Fluid used to create the fracture

- The pumping rate


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3 - Frac pressure prediction (2)


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3.- Frac pressure prediction (2)

The stress in the reservoir is estimated

as a function of the rock type

and of the reservoir pressure

( )sn

na ah OB SP SP - - +1

Poissons ratio

measured on core

overburden

~ 1 psi/ft

Static pressure

Biot coefficient

from 0.8 to 1

The initial stress gradient can range from 0.50 to 0.9 psi/ft

and . Tectonic history

+ T


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4.- Completion design


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WHP

=

+ gel friction in tubing

- hydrostatic

+

Co p et o des g

= 6700 psi

1400

5400

Well Head Isolation Tool

Pressure in the annulus

to monitor leaks and

decrease tubing movements.

4000

psi

Perforation strategy

8500 + 2000 + 200 = 10 700 psi

Stress +Net Pressure+ BH friction =BHTP

Completion design(weight on packer,

locator stroke)

HHP =Q (BPM) * WHP (psi)

40.8

4 - Completion design strategy


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p g gy

Determine Maximum WH pressure When initiating the fracture

When pumping the minifrac If a screen out occurred.

Example: Depth = 10 000 ft

Stress = 0.85 psi/ft

P breakdown = 1,07 psi/ft

Fluid density = 1,02 SG

Friction = 1500 psi @ 20 bpm

Expected net P = 500 psi when initiating the fracture= 1500 psi when propagating the fracture= 3000 psi during the screen out

Determine max differential pressure and pressure to beapplied in the annulus

On casing On tubing

On packer

Perform a Triax - analysis


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Mixing the fluid


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LA1 LA2 LA3 LA4

LA5

LA

6

LA7LA12 LA11 LA10 LA8

Gel

MixingTank

Sand silos

Water tanks

Acid tank

containing

KCl

LGC tanks

Brine

tanks

containing

KCl

Blender

tubDry

additives

To

intensifiers

8

Batch

Tank

To intensifiers

LA tanks

88

Conveyor belt

Auger

= flow meter

Dry

additi

ve

Hoppe

r


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6.- Fluid selection (6)


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The selected service company proposes a fluid formulation,

as a function of the pumping program.

Some lab quality control testsare conducted to check the formulation

- compatibility with the reservoir (Clays)

- cross link time (Should be less than pipe time)

- stability while pumping

- good break with the least residues possible

BUT STILL AROUND50% PERMEABILITY REDUCTION

ON THE PROPPANT PACK

7.- Proppant selection (1)


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- size: as a function of the target conductivity : 20/40 or 16/20

careful to the perforation entry hole diameter and the frac width

- substrate type: Could be high quality sand or man made (Aluminate derivative

They are chosen as a function of the effective stress

ceramic, bauxite etc....

- resin coating:

pro : decreases the risk of proppant flowback

con : more expensive, less frac conductivity,

Temperature limited

The proppant flowback probability is still poorly predicted.


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6.- Proppant selection


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- size: as a function of

the target conductivity :20/40 (0.5 - 0.7 mm) or

16/20 (1 mm)

careful to the perforation

entry hole diameter andthe frac width

- Type: As a function of

the in-situ formation

stress


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Hydraulic Fracturing Avec Commentaires