Enhanced Oil Recovery
Optimizing
Molecular Weight
in Polymer Flood
07/2006 - This document is the property of SNF. It must not be reproduced or transfered without prior consent
Fluid viscosity in a polymer flood
• It is surprising to find that, regardless the initial
molecular weight of the polymer injected, the
molecular weight of the polymer in the produced
water is low, around 4 to 10 million. The questions
that leap to our minds are:
–
–
–
–
What is the viscosity of the brine inside the reservoir ?
What is the efficiency of the motor fluid ?
Which are the parameters affecting this efficiency ?
How can the polymer flood be optimized ?
07/2006 - This document is the property of SNF. It must not be reproduced or transfered without prior consent
Average molecular weight
• Polyacrylamides are formed of very long chain of polymers,
several million Daltons, and the average molecular weight does
not represent the length of the chain. A polymer is a mixture of
long and short chains. In fact, even with very different average
MW, a part of the molecules are the same.
Number
of
molecules
15M
0
5
10
15
20M
20
25M
25
30
35
40
Molecular weight (10^6 Daltons)
07/2006 - This document is the property of SNF. It must not be reproduced or transfered without prior consent
Chemical degradation
• The mechanism of chemical degradation is the
formation of free radicals that cut the chain
• These free radicals are formed by the reaction of an
oxidizer with a reducer (redox)
Oxydizer
Reducer
O2
H2S
Fe2+, Fe3+
Scavenger
Hydrocarbon
Fe2+
Peroxydes
Sulfato reducing
bacteria
NH3
• The high molecular weight chains are more sensitive
to chemical degradation, especially those above 15
million MW
• The best polyacrylamide stability is found in a
reducing media
07/2006 - This document is the property of SNF. It must not be reproduced or transfered without prior consent
Example of chemical degradation
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Mechanical degradation
• High shear will cut the polymer chains in pieces
• High MW giving a certain viscosity with few chains
will be more sensitive to shear than a low MW
providing the same viscosity with more chains
• This shear degradation is amplified by the formation
of free radicals as a mechanism of degradation
07/2006 - This document is the property of SNF. It must not be reproduced or transfered without prior consent
Example of mechanical degradation
• Low MW polymers are less sensitive to shearing
07/2006 - This document is the property of SNF. It must not be reproduced or transfered without prior consent
Example of mechanical degradation
07/2006 - This document is the property of SNF. It must not be reproduced or transfered without prior consent
Influence of time on shear
• The shear at the injection side is normally very
limited in time (seconds to minutes)
• It is also dependent, at the beginning of the
injection, on the structure of the reservoir
• During injection time, solids come from the injected
water or are precipitated
– (CaCO3, MgOH, FeS, S2 or biological molds)
• The solids increase the shearing time by forming
channels, where the polymer solution can be under
high degradation conditions for a length of time
07/2006 - This document is the property of SNF. It must not be reproduced or transfered without prior consent
Low stability for high MW
• The stability of high molecular weight molecules is
very low
Very high
stability
Poor
stability
stability
Number
of
molecules
Low
stability
15M
0
5
10
15
Very low
stability
20M
20
25M
25
30
35
40
Molecular weight (10^6 Daltons)
07/2006 - This document is the property of SNF. It must not be reproduced or transfered without prior consent
Injection time & mechanical degradation
• At the beginning of the injection, the shear is given by
–
–
–
–
The
The
The
The
flow of polymer solution
area of injection
area of the front surface of the reservoir
permeability
• Each polymer contains a certain amount of insoluble particles which
swell in the brine about 50 times forming a large amount of gels,
increasing with time and coating the injection surface
• There are two types of gels
– Soft gels which are degraded to soluble polymers at a certain pressure; and
at this pressure, there is an equilibrium between injected gels and degraded
gels and the injection well is permanently cleaned
– Hard gels which are not sensitive to pressure and plug the injection surface
• With time, due to coating of the injection surface, the area of injection
decreases till the injection pressure from the gel is achieved, then the
mechanical degradation becomes stable (in the range of 1 to 7 bar)
• But the injection pressure increases with the MW as the amount of gel
and hardness of gel increases with MW
07/2006 - This document is the property of SNF. It must not be reproduced or transfered without prior consent
Final viscosity
• The mechanical degradation is generated directly at
the injection point
• The chemical degradation happens a few hours after
injection and polymer solution is then stable
Viscosity
cps
Factors
Mechanical
degradation
Chemical
degradation
Brine (TDS)
Temperature
Oxydo reduction equilibrium
Oxydo reduction reactions
Flow
Differential pressure, Shear
Permeability
Precipitation (salts, sulfur, SRB, Fe)
Viscosity of injection
Viscosity at different steps of degradation
Stable fluid
Time
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Polymer choice
• Choosing the best polymer for polymer flooding is a
very complex question. The efficiency of the polymer
depends on:
– The amount of oxygen at the injection and the addition of a
scavenger in large amounts will not correct it. Oxygen and
scavengers form free radicals
– The stable pressure of injection which depends on the flow,
permeability, polymer, viscosity…
– The quality of the polymer
• For the same injected viscosity the high MW products will give a
lower viscosity in the reservoir
• Harder are the gels, more the pressure of injection is high and
the mechanical degradation
• The absorption of the polymer will depend on the anionicity
– A copolymer with very narrow distribution of anionicity will absorb
less than a co-hydrolyzed polymer and less than a post hydrolyzed
polymer. The absorption will take place mainly near the injection
points
07/2006 - This document is the property of SNF. It must not be reproduced or transfered without prior consent
How to correct these parameters
• There are solutions to improve these parameters
– Addition of free radical scavengers. ITW
(isopropylalcohol+thiourea) is presently the best but
expensive and difficult to handle (flash point, volume)
– Increase of concentration of high MW products to match the
same viscosity in the reservoir as the medium MW. But this
will decrease injectivity and it is necessary to decrease the
flow to avoid fracturation. It is easier to adapt the MW to the
field conditions
– Avoid oxygen in the polymer dissolution system by a very
careful conception of the equipment
– Keep all water circuit in reducing status
– Decrease to a minimum all chemical injections
– Avoid H2S formation by SRB by biological control
07/2006 - This document is the property of SNF. It must not be reproduced or transfered without prior consent
New types of polymers
• Many types of polymers are known from the 70s in laboratory
developments. Some of them are now produced in pilot or
industrial quantities. The main ones are:
– Thermostable polymers which increase the stability of the
polyacrylamides from 75°C to 90°C with new monomers
• FLOPAAM AN 125-132
– Associative polymers with a main polyacrylamide chain and
statistic repartition of hydrophobic groups. There is an association
of these hydrophobic groups in a specific brine to give a high
viscosity
• SUPERPUSHER
– Star polymers with 3 or more branches on a central polymer group.
These polymers are normally associative to have a high viscosity
• ST5030
– Comb and T shape polymers with a main hydrophobic chain and
end hydrophobic chain
– Block associative polymers with multiple hydrophobic groups inside
an hydrophylic chain
– Structured polymers with hydrophilic branches in a main
hydrophilic chain
– Soft or Movable gels are totally insoluble yet injectable gels
mainly used in profile modification but with high potential in EOR
• FLOPERM 2000
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EOR problems solved?
• The new polymers bring as many questions as they solve
problems
– Size of associative polymers compared with the permeability of the
reservoir
– Association with oil
– Absorption on the oil wetted parts
– Very high variation viscosity against the salinity of the reservoir
with possibility of plugging with salinity increase
– Very quick loss of viscosity by dilution
– Sensitivity to Calcium, Magnesium and precipitation
– Thermal stability
– Mechanical stability
– Dissolution problems
– Very high viscosities before dilution
– …
Today the potential is high and depends on the risks taken
07/2006 - This document is the property of SNF. It must not be reproduced or transfered without prior consent
Thank you for your attention
Optimizing Polymer Molecular Weight
not Maximizing it, to Achieve the
Highest Efficiency in EOR Polymer
Flooding
07/2006 - This document is the property of SNF. It must not be reproduced or transfered without prior consent