Enhanced Oil Recovery

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Enhanced Oil Recovery
Optimizing Molecular Weight in Polymer Flood
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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 ?

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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
Daltons)
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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

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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

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Example of mechanical degradation

• Low MW polymers are less sensitive to shearing

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Example of mechanical degradation
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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

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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
stability
Number of molecules
0 5 10 15 20 25 30 35 40 Molecular weight (106
Daltons)
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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
  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

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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 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
degradation
Mechanical degradation
Chemical 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

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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

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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
• 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
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Thank you for your attention

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