section 9 - polymer chemistry

7/17/2019 Section 9 - Polymer Chemistry

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polymer

chemistry

section 9

p o l y m e r



S e c t i o n 9

polymer chemistry

contents

introduction ..........................................................................................................................1

polymer types .......................................................................................................................1

polyacrylate, polyacrylamide, and phpa ............................................................2

carboxymethylcellulose (cmc) and polyanionic cellulose (pac) ................2

hydroxyethylcellulose (hec) ...................................................................................4

starch .............................................................................................................................4

guar gum .......................................................................................................................5

xanthan gum ...............................................................................................................5

other naturally derived polymers ........................................................................5

polymer uses .........................................................................................................................5

viscosity .........................................................................................................................6

bentonite extension ..................................................................................................6

flocculants .....................................................................................................................6 deflocculants ...............................................................................................................6

surfactants ....................................................................................................................6

filtration control .........................................................................................................7

shale stabilisation ......................................................................................................7

polymer limitations ............................................................................................................7



S e c t i o n 9

polymer chemistry01

introductionA polymer is a molecule consisting of a series of repeating units. The

number of units can vary from several to tens of thousands with

corresponding variance in chain length and molecular weight. The

polymer can be linear or branched and can be synthetic or naturally

derived.

The lower molecular weight polymers are used as deflocculants;whereas, the high molecular weight molecules are used as viscosifiers

and flocculants.

The repeating unit need not always be the same. Copolymers consist of

two or more different groups joined together and may be ‘random’ or

‘block’ depending on how the groups are distributed on the chain.

The two major mechanisms for manufacturing polymers are

condensation, which alters the makeup of the repeating units, andaddition which utilises the presence of a double bond in the reacting unit

to form a long chain. The addition process will generally yield higher

molecular weight polymers than will condensation. The condensation

process produces a polymer in which the repeating units contain fewer

atoms than the monomers from which they were formed. Frequently,

water is formed as a by-product of the process. The process requires two

or more compounds which react chemically and does not depend upon

the presence of a double bond for propagation of the chain.

This mechanism is susceptible to interruption by impurities or any outsideinfluence which would reduce the efficiency of the process.

Many commercially available polymers are not readily soluble in water.

This is an undesirable property for drilling fluid chemicals. Fortunately,

many of the polymers available have been chemically treated in order to

make them water-soluble. The solubility of these polyelectrolytes will be

affected by the chemical makeup of the drilling fluid, pH, salts and

presence of divalent cations, etc.

polymer typesEach type of polymer has its own characteristics in terms of how it

functions in a particular type of drilling fluid. Therefore, selection of the



S e c t i o n 9

polymer chemistry 02

correct type of polymer is critical to good performance. Below are listed

five general types of commonly used polymers.

polyacrylate, polyacrylamide, and phpaPolyacrylates are used as dispersants (low mol. weight), fluid loss reducing

agents (medium mol. weight) or flocculants (high molecular weight).

Simple ones are cheap, but calcium sensitive. Copolymers are common,

e.g. vinyl sulphonate – vinyl acrylate copolymers used as high temperaturefluid loss reducing agent.

High molecular weight, partially hydrolysed polyacrylamides (PHPA)

are very effective shale stabilisers, clay extenders, flocculants, and

encapsulating colloids. More correctly they are block copolymers of

polyacrylamide (c. 70%) and polyacrylate (c. 30%). The presence of excess

PHPA can be monitored via various tests and is reported on the mud

report form.

carboxymethylcellulose (cmc) and polyanioniccellulose (pac)

These are widely used as viscosifiers and fluid loss reducing agents.

Carboxymethyl cellulose and polyanionic cellulose are both produced

by carboxymethylation. The quality of the product is determined by the

degree to which the reaction is carried out (degree of substitution) and

by whether the salt by-product is removed or not. Viscosity can be eitherhigh or low depending on chain length.

n CH2 = CH - C = N CH2 - CH -

C = N n

Hydrolysis

alkali

Acrylonitrille

(Monomer)Polyacrylonitrile (Polymer)

PHPA Polymer

CH

CH

Co _2

2

Na+

CH

CH

Co _2

2

Na+

CH

CH

Co _2

2

Na +

CH

CH

CONH2

2CH

CH

Co _2

2

Na+

CH

CH

2

CH

Co _2

Na+

CONH2



S e c t i o n 9

polymer chemistry03

When cellulose is reacted with sodium monochloroacetate, a sodium

methylacetate group is substituted on one of the three hydroxyl groups.

Cellulose Structure

HO H

HH

OH H1

CH2OH CH2OH

CH2OHH OH

4

OO

H

H

OH

OH

OH

H.OH

OH

OH Hn OH

H H

H

H

H

H H

H

H H

H4

OO

CH2OH

O

O

O

Carboxymethylcellulose Structure

HO H

H

H

OH H1

CH2O CH2O

CH2OH OH

4

OO

H

H

OHOH

OHH.OH

OH

OH Hn OH

H HH

HH

H H

H

H H

H4

OO

CH2O

O

O

O

CH2COO-Na+ CH2COO-Na+

CH2COO-Na+ CH2COO-Na+

The degree of substitution (DS) refers to the number of hydroxyl groups

upon which substitution takes place divided by the number of repeating

units in the molecule.

The degree of substitution will range from zero to a maximum of

three. Generally, CMC will have a DS in the range of 0.4 to 0.8 with 0.45

being required for solubility. The degree of polymerisation (DP) will

range from 500 to 5000. The polymers with the greater DP will impartmore viscosity to the fluid. High DS on the other hand, will permit more

tolerance to salts and cation contamination. Thermal degradation

accelerates above 250° F.

Polyanionic cellulose is similar to CMC but generally has a degree of

substitution (DS) of about 1.0. The PAC materials generally are more

expensive than CMC due to higher processing costs, but show a greater

tolerance to hardness and chlorides. PAC begins to thermally degrade at

250° F.



S e c t i o n 9


hydroxyethylcellulose (hec)Hydroxyethyl cellulose. This is used mainly as a viscosifier for completion

fluids. Its non ionic nature means that it is not affected by salt. Key factors

are purity and high acid solubility.

HEC is formed by causticising cellulose and reacting it with ethylene oxide

which replaces one or more of the hydroxyl groups present on the cellulose

molecule.

Although HEC is non ionic, it is still water soluble due to the hydroxy

ethyl groups. HEC imparts high viscosity to water or brines but exhibits

no gel strengths. It is prone to degradation through shear or heat and

has a maximum thermal stability of about 225° F.

starchWidely used as fluid loss reducing agents, particularly in salty muds. Can

be potato or grain derived. Quality and temperature stability can be

improved by various processes.

The starches are pre-gelatinised in order to permit them to readily hydrate.

Starches are peptised chemically or by exposure to heat. The peptisation

ruptures the microscopic sacks which contain the amylose and amylopectin

allowing them to contact with water and hydrate.

Starches are used mainly for fluid loss control and are effective in a large

range of fluid systems, such as seawater, saturated saltwater, KCl muds

and lime muds. Starches are thermally stable to about 250° F. Starches,

unless chemically modified are not resistant to bacteria and require a

biocide to prevent fermentation, except in saturated salt and high pHmuds.

Hydroxyeathylcellulose Structure

Cellulose +

Ethylene oxide

H HH

H OH

OH

H

HH H

H

H

n

H

H HO

C C

Hydroxyethylcellulose

O

O

O

O

CH2OCH2CH2OCH2CH2OH

CH2OCH2CH2OH

OCH2CH2OH

OH

H



S e c t i o n 9

polymer chemistry05

guar gumViscosifier used to make spud muds. Derivatives (eg.hydroxy propyl guar)

may be used in certain completion /workover fluids.

Guar gum is manufactured from the seed of the guar plant. Guar is a

naturally occurring non ionic polymer used as a viscosifier in waters

ranging from fresh to saturated salt (NaCl). High levels of hardness and

alkalinity will slow or even eliminate the hydration process and can causea significant decrease in viscosity.

Guar has a maximum thermal stability of about 200˚ F and a biocide is

necessary to retard fermentation.

xanthan gumExcellent viscosifier giving shear stable rheology with progressive gels.

It is derived from bacteria.

Xanthan Gum is a biopolymer and is a product of the action of a bacteria

(Xanthomonas Campestris) on sugar. It may be used in a variety of brines

and salinity levels. Xanthan gum begins to degrade thermally at

temperatures of about 225 - 250˚ F. Xanthan gum is the only polymer that

provides thixotropy, i.e., formation of gel structures.

other naturally derived polymersOther products which may loosely be described as polymers include:

Lignosulphate - used as a dispersant

Lignite - used a fluid loss reducer and dispersant

Lignin (esp. polyanionic lignin) - used for fluid loss control

Tannin/ Quebracho - used as dispersants

polymer usesSome of the major uses of polymers in drilling fluids are:

ƒ Viscosity

ƒ Bentonite Extension

ƒ Deflocculation

ƒ Filtration Control

ƒ Shale Stabilisation



S e c t i o n 9


viscosityViscosity is due to the interactions between the polymer molecules and

water, between the polymers themselves and between polymers and

solids.

The longer the molecules the greater the viscosity. The interaction

between the polymers, water and solids can be disrupted by applying

energy or shear. The result is that the higher the shear, the lower the

viscosity.

bentonite extension The bentonite extenders work by cross-linking bentonite particles to

increase the physical interaction between particles. There is a narrow

band of concentrations which allow this cross-linking to occur, but above

which a viscosity decrease may occur.

flocculants These polymers are characterised by a anionic high molecular weight

which will enable the polymer to bridge from particle to particle. The ionic

groups of the polymer will allow it to absorb strongly on the ionic sites of

solids and form an aggregate. The aggregates will settle or be removed by

shakers or centrifuges.

It is possible to have either total or selective flocculation. Selective

flocculation removes some of the drill solids.

deflocculants The deflocculants or thinners are usually negatively charged polymers.

These products absorb onto the edges of clay particles resulting in anoverall negative charge.

Deflocculants are anionic polymers. Polymer deflocculants are shorter

molecules with a greater charge density.

These characteristics facilitate adsorption onto the clay particle without

causing cross-linking. These polymers are sensitive to divalent cations

and are less effective when hardness exceeds about 400 mg/l.

surfactants These are discussed in the “OBM Fundamentals” section. They are polymers

with a polar, water loving end and a non polar oil soluble end.



S e c t i o n 9

polymer chemistry07

These polymers will stabilise emulsions either direct or indirect depending

on the length of each entity.

filtration control Three mechanisms can be envisaged for polymers to act as fluid loss

additives.

a. Deflocculants. These pack down the filter cake forming a flatter, lesspermeable medium.

b. Viscosity of the filtrate. The thicker the liquid phase being forced

through the filter cake, the lower rate of filtration.

c. Colloidal particles. Compressible colloidal particles will deform to plug

pores in the filter cake.

Often a combination of mechanisms will provide the most effective

control.

Starches, CMC, PAC, and hydrolysed polyacrylates are effective filtration

control agents.

Anionic polymers control filtration by viscosifying the water phase to

restrict fluid flow through the filter cake. Non ionic materials such as the

starches, some anionic materials such as PAC and CMC, work by hydrating

and swelling and physically plugging pores in the filter cake.

shale stabilisation

Shale stabilisation is provided through polymer attachment to thepositively charged sites on the edge of clay particles in shales. This

attachment minimises water invasion into the clay particle and reduces

hydration and dispersion. These polymers have been used with success

in conjunction with salt and potassium-based muds for added inhibition.

polymer limitationsPolymers have many advantages, particularly for formulating drilling

fluids from sea water or salt saturated brine or for making highly

inhibitive muds such as the KCl/PHPA systems. They do have limitations

however.



S e c t i o n 9


Rheological Characteristics

Linear polymers such as CMC, PAC, HEC , produce almost ideal power law

fluids with poor viscosities at low shear rates and flat, low gel strengths.

Thus suspension properties are poor. Xanthan gum is the exception and

has good suspension characteristics. Note, however, that gels increase as

the low gravity solids build up.

Tolerance to ContaminantsMost polymers tolerate salt or KCl very well, but the anionic ones e.g.

CMC or PAC can be precipitated by calcium if the pH is high. Cement is

the worst contaminant. Calcium values over 1000 mg/l (as in, for

example, gyp muds) can be tolerated if the pH is below 10. Polymer yields

are higher in fresh water than in saturated salt.

Temperature

The polysaccharides have relatively poor temperature stability max. BHT

250 - 300˚ F depending on grade). This can be increased by using stabilisers.

The synthetic polymers can tolerate much higher temperatures (350˚ F

– 500˚ F).

Bacteria

Starch, guar gum and Xanthan gum are quite easily attacked. The use

of a biocide is recommended. CMC and PAC are more resistant to attack.

Shear Degradation/ Absorption

Some polymers do undergo shear degradation and are absorbed onto

cuttings and drill solids. Consequently their viscosifying effects arereduced (but inhibition is increased). High molecular weight, linear

polymers such as PAC and PHPA are most susceptible.

Cost

Polymer muds are generally more expensive in terms of cost/bbl than

bentonite muds. However, the advantages obtained by their use (better

hole stability, ROP etc.) will normally outweigh the extra cost.

section 9 - polymer chemistry

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