04e Frac Design Variables (Fluid Viscosity) v3 SPE
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Transcript of 04e Frac Design Variables (Fluid Viscosity) v3 SPE
FRACTURE DESIGN VARIABLESFLUID VISCOSITY
H , E , C , KIc m , Q
Fluid Viscosity
•Why is it important ?
•What is it ?
•How do we measure it ?
•How much do we need ?
•How is it affected by time,
•temperature, proppant, … ?
Fracturing - Fluid Viscosity
• Net Pressure/
Geometry
• Proppant Transport
(Prop Settling to m)
• Fluid Loss Control
Why we WANT Viscosity
2 51020 50100
50
100
200
500
1,000
Pump Time (min)"Time 0" When Gel On Perfs
Net Pressure (psi)
500 cp
100 cp
30 cp
1 cp
H = 150' E = 6e6 psi Q = 30 bpm
4/1
'
E
xQP
f
Net
m
ViscosityStrongly Changed
By Conditions
Must know viscosity
as a function of time
& temperature !
Distance Along Frac
Fluid Temperature Wellbore
Temperature
FormationTemperature
Distance Along Frac
ViscosityTemperatureDegradation
Time/ShearDegradation
How Do We Measure It ?
d
F,velocity
t , Shear Stress = F / A (psi)
(pressure drop or drag)
g , Shear Rate = vel / d (1/sec)
(for fracture = vel / (w/2)
A
v (x)
Ideal TestRotating Cup
& Bob
w (RPM)
Torque
What Do We Measure ?
Shear Rate (1/sec)
Shear Stress (psi)Newtonian
t= m g
mis viscosity
Slope = m
Shear Rate (1/sec)
Shear Stress (psi)
Bingham Plastic
Slope = Plastic Viscosity
t= Y + m gPP
log g
log t
Power Law
Slope = n'
t= K' gn'
Most Common
Rheological Model
for Fracturing
Fluids
“Apparent” Viscosity
t
g
Slope = m app
)(sec
),/sec('),(
/'48000
)(
1
2'
'1
g
m
gm
gg
tm
ftlbKcp
K
ondepends
nf
napp
app
Example
Power Law Fluid
n’=0.6, ma=100 cp (at 170 sec-1)
Find: K’ and ma at 50 sec -1
cp
ftlb
K
a
n
163
10050
170
)/sec(0163.0
48000/170100'
)6.01(
)50(
2'
)6.01(
m
Slurry Viscosity
24681012141
2
3
5
7
10
lb Sand / Liquid Gallon
Viscosity Multiplier
Fracturing - Fluid Viscosity
• COSTS
• Net Pressure/Geometry
• Proppant Pack Damage
(10 to 70% KFW Loss)
Why we DO NOT WANT Viscosity
Photo Courtesy of StimLabEverything that increases viscosity
costs money & does damage!
How Much Viscosity Is Needed
How Much Viscosity
Fracture Penetration (ft)500 1000 1500 2000 2500 3000
444.40 minTVDft
7800
7900
8000
8100
Stress (psi)5000 6000 7000
Sh
ale
Ga
s
Sh
ale
0.000
0.060
0.120
0.180
0.240
0.300
0.360
0.420
0.480
0.540
0.600Pro
pp
an
t V
olu
me
Fra
ctio
n P
VO
L
0.760 m/sec
300_to_40_over_4_hours
Vis
cosity (
cp)
45
55
70
90
150
200
Wellbore Temp 1 2 3 4
Time at Formation Temperature (hrs)
N'
0.6
0.7
0.8
0.9
(@ 170 1/sec)
100’s of cp NOT required
for near perfect proppant
transport !
Where Do We Get Data ?
• Routine data acceptable for preliminary designs, scoping studies, etc.
• SPECIFIC data required for final design, mini-frac analysis, etc.
Lab Tests
The End
How Much Viscosity Is Needed
• If n’=0.6 and g=50 sec-1, the final reference
apparent viscosity is 81 cp
• 1 PPG --> 10 PPG gives an average concentration
of 5 PPG, viscosity multiplier of 2 --> 162 cp
Assume a fluid with 50 cp viscosity (at
170 sec-1) at the end of the job, just as
prop laden fluid is reaching the frac tip,
after being in the fracture for 4 hours.
How Much Viscosity Is Needed
• Fluid enters fracture with 500 cp and degrades to
50, average of about 225 cp or a multiple of 4.5 -->
729 cp
• For many fluids (cross link gels, foams) settling is
much slower than predicted by Stoke’s Law,
assume a factor of 2
--> 1,458 cp
How Much Viscosity Is Needed
•Use 1,450 cp in Stoke’s Law gives
a predicted proppant settling of
only 15 feet during the four hour
period
Near perfect transport using
a fluid with a final lab viscosity
of only 50 cp !