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 !