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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3 Hydraulic Fracturing
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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4 Hydraulic Fracturing
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
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Basic Principles in fracturing (2)
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)
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Basic Principles in fracturing (3)
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
sv max
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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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9 Hydraulic Fracturing
Basic Principles in fracturing (4b)
sh min
sv max
A fracture is characterised by :
itshalf length: Xf
itsconductivity: kf wf
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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10 Hydraulic Fracturing
Basic Principles in fracturing (4c)
sh min
sv max
A fracture is characterised by :
itshalf length: Xf
itsconductivity: kf wf
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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11 Hydraulic Fracturing
Basic Principles in fracturing (5)
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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12 Hydraulic Fracturing
Basic Principles in fracturing (6)
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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13 Hydraulic Fracturing
Basic Principles in fracturing (7)
Multi fractured horizontal wells
sh min
Transverse FracsLongitudinal Frac
sh min
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14 Hydraulic Fracturing
Basic Principles in fracturing (8)
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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15 Hydraulic Fracturing
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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17 Hydraulic Fracturing
1.- Frac height prediction (3)
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
1 F h i ht di ti (4)
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18 Hydraulic Fracturing
1.- Frac height prediction (4)
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
1 F h i ht di ti (5)
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1.- Frac height prediction (5)
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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21 Hydraulic Fracturing
-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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2.- Frac length design (2)
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
2 Frac length design (4)
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2.- Frac length design (4)
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
2 Frac length design (5)
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25 Hydraulic Fracturing
2.- Frac length design (5)
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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27 Hydraulic Fracturing
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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42 Hydraulic Fracturing
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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