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Enhanced Oil Recovery


Enhanced Oil Recovery Optimizing Molecular Weight in Polymer Flood Fluid viscosity in a polymer flood It is surprising to find that, regardless the initial molecular ... – PowerPoint PPT presentation

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Title: Enhanced Oil Recovery

Enhanced Oil Recovery
Optimizing Molecular Weight in Polymer Flood
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 ?

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
0 5 10 15 20 25 30 35 40 Molecular weight (106
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)
  • 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

Example of chemical degradation
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

Example of mechanical degradation
  • Low MW polymers are less sensitive to shearing

Example of mechanical degradation
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

Low stability for high MW
  • The stability of high molecular weight molecules
    is very low

Very high stability
Poor stability
Low stability
Very low stability
Number of molecules
0 5 10 15 20 25 30 35 40 Molecular weight (106
Injection 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
  • 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 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

Viscosity cps
Factors Brine (TDS) Temperature Oxydo reduction
equilibrium Oxydo reduction reactions Flow Differe
ntial pressure, Shear Permeability Precipitation
(salts, sulfur, SRB, Fe) Viscosity of
injection Viscosity at different steps of
Mechanical degradation
Chemical degradation
Stable fluid
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
  • 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
  • Harder are the gels, more the pressure of
    injection is high and the mechanical degradation
  • The absorption of the polymer will depend on the
  • 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 parameters
  • There are solutions to improve these parameters
  • Addition of free radical scavengers. ITW
    (isopropylalcoholthiourea) 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 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 75C to
    90C 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
  • 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?
  • 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
  • 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 attention
  • Optimizing Polymer Molecular Weight not
    Maximizing it, to Achieve the Highest Efficiency
    in EOR Polymer Flooding
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