Viscosifiers

Microbial exopolysaccharides are widely used in industry as viscosifiers and as gelling agents. In this section we will consider, in general, the rheology of exopolysaccharides in solution and their ability to form gels. Specific properties of individual microbial exopolysaccharides and applications which exploit these characteristics are considered later in this chapter. 

Many microbial polysaccharides show pseudoplastic flow, also known as shear thinning. When solutions of these polysaccharides are sheared, the molecules align in the shear field and the effective viscosity is reduced. This reduction of viscosity is not a consequence of degradation (unless the shear rate exceeds 105 s 1) since the viscosity recovers immediately when die shear rate is decreased. This combination of viscous and elastic behaviour, known as viscoelasticity, distinguishes microbial viscosifiers from solutions of other thickeners. Examples of microbial viscosifiers are ... 

At low ionic strengths, Tm increases exponentially with ion activity. The effect of high concentrations of salts or miscible solvents depends on the influence they have on hydrogen-bonding and may increase or decrease Tm. In the case of xanthan gum, the value of Tm can be adjusted from ambient to over 200°C by the addition of appropriate salts. Table 7.2 presents Tm values for some industrial viscosifiers. 

An oil-based drilling mud can be viscosified with maleated ethylene-propylene elastomers [919]. The elastomers are ethylene-propylene copolymers or ethylene-propylene-diene terpolymers. The maleated elastomers are far more effective oil mud viscosifiers than the organophilic clays used. On the other hand, specific organophilic clays can provide a drilling fluid composition less sensitive to high temperatures [491]. 

Neutralized sulfonated asphalt (i.e., salts of sulfonated asphalt and their blends with materials such as Gilsonite, blown asphalt, lignite, and mixtures of the latter compounds) are commonly used as additives in drilling fluids. These additives, however, cause some foaming in water or water-based fluids. Furthermore, these additives are only partially soluble in the fluids. Therefore, liquid additives have been developed to overcome some of the problems associated with the use of dry additives. On the other hand, with liquid compositions containing polyglycols, stability problems can arise. Stable compositions can be obtained by special methods of preparation [1407]. In particular first the viscosifier is mixed with water, then the polyglycol, and finally the sulfonated asphalt is added. 

Ethoxylated methylcarboxylates Propoxyethoxy glyceryl sulfonate Alkylpropoxyethoxy sulfate as surfactant, xanthan, and a copolymer of acrylamide and sodium 2-acrylamido-2-methylpropane sulfonate Carboxymethylated ethoxylated surfactants (CME) Polyethylene oxide (PEG) as a sacrificial adsorbate Polyethylene glycols, propoxylated/ethoxylated alkyl sulfates Mixtures of sulfonates and nonionic alcohols Combination of lignosulfonates and fatty amines Alkyl xylene sulfonates, polyethoxylated alkyl phenols, octaethylene glycol mono n-decyl ether, and tetradecyl trimethyl ammonium chloride Anionic sodium dodecyl sulfate (SDS), cationic tetradecyl trimethyl ammonium chloride (TTAC), nonionic pentadecylethoxylated nonylphenol (NP-15), and nonionic octaethylene glycol N-dodecyl ether Dimethylalkylamine oxides as cosurfactants and viscosifiers (N-Dodecyl)trimethylammonium bromide Petrochemical sulfonate and propane sulfonate of an ethoxylated alcohol or phenol Petrochemical sulfonate and a-olefin sulfonate... 

For modestly viscous oils—those having viscosities of approximately 20-100 centipoise (cP)-water-soluble polymers such as polyacrylamides or xanthan gum have been used to increase the viscosity of toe water injected to displace oil from toe formation. For example, polyacrylamide was added to water used to waterflood a 24 cP oil in toe Sleepy Hollow Field in Nebraska. Polyacrylamide was also used to viscosify water used to flood a 40 cP oil in the Chateaurenard Field, France. With this process, toe polymer is dissolved in toe water, increasing its viscosity. 

Additives that assist the creation of a fracture include viscosifiers, such as polymers and crosslinking agents temperature stabilizers pH control agents and fluid loss control materials. Formation damage is reduced by such additives as gel breakers, biocides, surfactants, clay stabilizers, and gases. 

Figure 17-24. Monomers in a copolymer for viscosifying diesel N,N-dimethyl-acrylamide and N,N-dimethylaminopropyl methacrylamide.

It was discovered that viscosifying the acid showed a remarkable improvement in acid fluid loss control. The enhancement was most pronounced in very-low-permeability limestone cores. The nature of the viscosifying agent also influenced the success. Polymeric materials were more effective than surfactant-type viscosifiers [682]. 

G. P. Zhang and H. C. Ye. AM/MA/AMPS terpolymer as non-viscosifying filtrate loss reducer for drilling fluids. Oilfield Chem, 15(3) 269-271, September 1998. 

While starches are commonly used, they are relatively poor viscosifiers. Acids and bacterial enzymes readily attack the acetal linkages resulting in facile depolymerization. Both formaldehyde and isothiazolones have been used as starch biocides (17). Development of improved high temperature water viscosifiers for drilling and other oil field applications is underway. For the... 

Other viscosifiers described in the literature include acrylate copolymers such as poly(acrylamide-co-dodecylmethacrylate)... 

The use of ionomers such as lightly sulfonated polystyrene as an oil-based fracturing fluid viscosifier has been studied (70). 

The most commonly used oil-based viscosifiers are phosphate esters of various types (56,71,72). 

Despite its limited stability in acid (1), guar gum has been used to thicken 3-15% hydrochloric acid (121). An allyl ether -guar gum adduct has been proposed for use as an acid viscosifier (122). Zr(IV) crosslinked CMHEC has been used to thicken hydrochloric acid (81). 

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