Polymer Transport in Porous Media

Jonathan W. Driver and Dr. Gary Pope

Research Showcase in Petroleum and Geosystems Engineering • September 6, 2016

Introduction

The basic premise of polymer flooding is simple: viscosify the water used to

displace formation oil so that the displacement is stable and efficient. The

viscosity of a polymer solution derives from the resistance of a sub-micron

sized polymer macromolecule to shearing flows within the aqueous solvent.

The larger the macromolecule, the more resistance imparted, so the trend in

polymers for EOR applications has been towards higher and higher molecular

weight. There are, however, some fundamental limits to this approach. Among

other things, large polymers become prone to plugging the pore throats of

tight rocks for the simple reason that their diameter in solution approaches the

diameter of the throats. This presents an optimization design challenge: given

some knowledge of formation properties, how does one create a polymer of

the highest permissible molecular weight?

The result of the chemical processes used to synthesize EOR polymers like

partially-hydrolyzed polyacrylamide (HPAM) is a distribution of polymer

molecular weights. The shape of this distribution can be subsequently

modified by filtration and irreversible mechanical degradation.

Preliminary Experiments with Polymer Core Flooding Experiment

Blender shearing is sufficient to reduce the viscosity (and molecular weight) of

HPAM solutions. The timecourse can be characterized for reproducibility.

Experimental Design

Cores of varying permeability are to be flooded serially with polymer solutions

of increasing viscosity until a near-complete loss of permeability is observed.

The same polymer solution will be mechanically degraded to varying extents

and then subsequently filtered using filters of varying stringency to produce

solutions that vary only in polymer molecular weight.

Conclusions

Core Flooding Experiment

Acknowledgements

The Pope Lab (Pathma Liyanage, Dr. Raphael Longoria, et al.)Chevron Energy Technology CompanySNF Holding Company

Pre-flood Brine (Seawater) Permeability Measurement (CQ1 Sandstone)

mechanical shearingreduces MW of all polymermass conservative

filtrationdoes not change MW of individual moleculesremoves mass from solution

##

The approach to be employed in this project is to use shearing and filtration in

combination to prepare polymer for flooding of tight rocks, and to evaluate

plugging behavior to create the most viscous solution possible without

plugging the pore throats.

10-50 md

1. 8 min

2. 4 min

3. 2 min

4. 1 min

5. ½ min

6. 0 min

1. 0.2 µm

2. 0.4 µm

3. 0.6 µm

1. 11

2. 12

6. 16

7. 2X

26

……

µ2X > µ16

…

Q

ΔP

(measured)

Unstable

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

00.20.40.60.811.21.4

β

Mesh Size (μm)

0.2% 3330S in 1% NaCl

20 sec blended

2 min blended

0.2 micron filtered

End

1

10

100

0.1 1 10 100 1000

Vis

co

sit

y (

cP

)

shear rate (1/s)

0.2% 3330S Blending Timecourse

0 min

1 min

2 min

4 min

8 min

16 min

0

200

400

600

800

1000

1200

1400

-30 20 70 120 170 220

Tim

e (

sec

)

Vol (mL)

Filtration Test at 15 psi

κ ≈ κ0/(1+βV)

t ≈ aV2 + bV + c

β = 2a/b

Filtration can be used to characterize polymer molecular weight distribution.

0.01 0.1 1 10 100

Sig

nal In

ten

sit

y

Pore Size (µm)

NMR Pore Body Size Distribution

Mn 1-18 MDa

HPAM

+

+

+

+

+

+

Polymer solutions plug a filter

membrane, causing a loss of

permeability.

The rate of loss of

permeability can be quantified

by measuring the curvature of

the time versus filtered

volume curve for a constant-

pressure filtration.

This parameter (β) is a

function of polymer molecular

weight distribution and filter

pore size.

By using a range of filter

pore sizes, a full

characterization of

polymer plugging

behavior can be achieved.

Plugging quickly

escalates as pore size

decreases.

Pre-filtration can be more

effective at reducing

plugging than mechanical

shearing.

19.4 cP

14.7 cP

23.1 cP

NMR can be used to estimate pore body size distribution. This is not the same

as pore throat size, but it may still correlate with plugging behavior. Below is a

pore body size distribution from a low permeability sandstone.

y = 8.2219x + 0.4353R² = 0.9992

0

1

2

3

4

5

6

7

8

9

10

0 0.2 0.4 0.6 0.8 1 1.2

ΔP

(p

si)

Q (mL/min)

𝛋 =𝐐 𝛍 𝐋

𝐀 𝚫𝐏

43 md

ISCO Pump

(water at constant

flow rate)

Design

1” Core

Holder

effluent

polymer

water 100 psi

confining

pressure

pis

ton

Transducer

L

H

Computer

Pressure Readouts from Polymer Floods with FP3330S (8 MDa) in Seawater

0

0.5

1

1.5

2

2.5

3

3.5

4

0 1 2 3 4 5 6

ΔP

(p

si)

Pore Volumes

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

0 1 2 3 4 5 6 7

ΔP

(p

si)

Pore Volumes

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 1 2 3 4 5

ΔP

(p

si)

Pore Volumes

Q = 0.03 mL/min

Q = 0.03 mL/min

Q = 0.03 mL/min

Q =

0.0

15 m

L/m

in

There is evidence of plugging in the second and third polymer floods, in the

form of upward drift in the pressure plateau. This is similar to what happens in

the filter paper based filtration test, but with a constant flow rate and non-

uniform pore size.

8 min sheared, 0.4 µm filtered

4 min sheared, 0.4 µm filtered

0 min sheared, 0.6 µm filtered

Residual Resistance (Post Flood with Brine)

-2

0

2

4

6

8

10

12

0 1 2 3 4 5 6 7 8

ΔP

(p

si)

Pore Volumes

Q = 0.007 mL/min

Q = 0.150 mL/min

Q = 0.100 mL/min

Q = 0.007 mL/min

Polymer is not easily removed from the core with brine, possibly due to

adverse viscosity. It appears that most of the permeability reduction is

irreversible.

• Mechanical degradation and filtration are effective tools for reducing

polymer average molecular weight and shaping the molecular weight

distribution

• Mechanical degradation can be calibrated precisely for a given polymer

under precisely controlled experimental conditions

• Filter membranes with precise pore sizes can be used to quantitatively

characterize the (non-adsorptive) plugging behavior of polymers

• The core tested plugged under the conditions explored

• This plugging appears to be robust to post-flood brine

• Accurate characterization and quantitation will require more

pore volumes of polymer to be injected

Future Plans and Goals

• Repeat the core characterization experiment for a large number of cores of

varying low permeability

• Construct relations between core properties (permeability, pore body size

distribution) and resistance factor for polymer injection with fully

characterized polymer solutions (rheology, filtration)

• Define guidelines for producing a polymer solution of maximum possible

viscosity with acceptable plugging behavior given knowledge of rock

properties

Dβ

=β*

[µm

]

κ [md]

µ [

cP]

κ [md]

Hypothetical Relations between Maximum Achievable Viscosity, Polymer Filtration Behavior, and Rock Permeability

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