Molecular weight optimization

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Enhanced Oil RecoveryEnhanced Oil Recovery

Optimizing

Molecular Weight

in Polymer Flood


Fluid viscosity in a polymer floodFluid 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 ?


Average molecular weightAverage 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

0 5 10 15 20 25 30 3540

Molecular weight (10^6 Daltons)

15M 25M20M


Chemical degradationChemical 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)

• 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

Oxydizer Reducer

O2 H2S

Fe2+, Fe3+ Scavenger

Hydrocarbon Fe2+

Peroxydes Sulfato reducing bacteria

NH3


Example of chemical degradationExample of chemical degradation


Mechanical degradationMechanical 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


Example of mechanical degradationExample of mechanical degradation

• Low MW polymers are less sensitive to shearing


Example of mechanical degradationExample of mechanical degradation


Influence of time on shearInfluence 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


Low stability for high MWLow stability for high MW

• The stability of high molecular weight molecules is very low

Number of

molecules

0 5 10 15 20 25 30 3540

Molecular weight (10^6 Daltons)

15M 25M20M

Very high stability

stabilityPoorstability

Lowstability

Very lowstability


Injection time & mechanical degradationInjection time & mechanical degradation• At the beginning of the injection, the shear is given by

– The flow of polymer solution– The area of injection– The area of the front surface of the reservoir– The 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


Final viscosityFinal 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

Viscositycps

Time

FactorsBrine (TDS)TemperatureOxydo reduction equilibriumOxydo reduction reactionsFlowDifferential pressure, ShearPermeabilityPrecipitation (salts, sulfur, SRB, Fe)Viscosity of injectionViscosity at different steps of degradation

Mechanicaldegradation

Chemicaldegradation

Stable fluid


Polymer choicePolymer 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


How to correct these parametersHow 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


New types of polymersNew 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


EOR problems solved?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


Thank you for your attentionThank you for your attention

Optimizing Polymer Molecular Weight not Maximizing it, to Achieve the Highest Efficiency in EOR Polymer

Flooding

Technology

Molecular weight optimization