Organic Polymers

As related to drilling fluids, the term organic polymer is applied to the several varied and versatile substances which are composed of a number of repeating or similar units, or groups of atoms (called monomers) consisting primarily of compounds of carbon. Organic colloidal materials are used in drilling ffuids to reduce filtration, stabilize clays, flocculate drilled solids, increase carrying capacity, and (incidentally) to serve as emulsifiers and lubricants. Several improvements in mud performance often result from the addition of a single product.

The colloidal properties of organic polymers greatly affect the role of organic polymers in drilling fluids. The organic polymers useful in muds have a strong affinity for water. They develop highly swollen gels in low concentrations. Some are strongly adsorbed by clay particles and offer protection from flocculation by salts. Although these polymers do not swell as much in salt water as they do in fresh water, they nevertheless provide slimy particles of such size as to resist the flow of water through a filter cake. These versatile polymers make practical the use of low-solids, nondispersive drilling fluids. Although properly classed as organic polymers, substances such as the lignosulfonates, lignite derivatives, and compounds that are used primarily because of their surface active properties are not discussed in this section. The colloidal properties of organic polymers are discussed in greater depth in Chapter 4.

Organic polymers used in drilling fluids may be broadly classified according to their origin and composition. Some, such as the starches and guar gum, occur naturally, and are ready for use after slight processing. Others, such as xanthan gum, employ natural processes in their production. Slill other polymers, such as derivatives of the starches and gums, and sodium carboxymethylcellulose, might be called semi-synthetic. Another class of petrochemical derivatives, such as the polyacrylates and ethylene oxide polymers is purely synthetic.

Figure 11-4. Schematic examples of linear and branched polymer chains. Such chains may also be crosstinked. (From Scanley,^ World Oil.)

The repeating units (monomers) that make up the polymer may be the same, or two or more monomers may be combined forming copolymers. Structurally, the polymer may be linear or branched (see Fig. 11 -4),102 and these structures, either linear, branched, or both, may be cross-linked, i.e.. tied together by covaient bonds. Further variations in the structure of polymers exist because of the possibility of combining two monomers in several different ways, as illustrated in Fig. 11 -5.'02 These figures cannot show the complexity of the structure, because it has three dimensions. The structure may coil or uncoil, depending upon its environment. Much has been learned about the effect of the composition and structure on the properties of polymers, and polymer chemists have been able to synthesize polymers for many applications.

The great diversity in composition and properties of the organic polymers offered for use in drilling fluids requires a critical examination of the factors involved in the selection of a product for a specific application.1,13 Among the factors affecting performance are the effects of shear conditions, temperature, dissolved salts and alkaline materials, microorganisms, and time of use on the polymer. If the application takes place in completion operations, both the solubility of the polymer in acids and the possible plugging action of the polymer on the productive formation must be considered. Other factors involved are ease of handling and mixing, possible environmental effects, and cost of the polymer,

The chemical characteristics of the organic polymers currently applied in drilling fluids are reviewed in detail in the cited references.104 IOii l!ls'10-10!i

Starch

Starch was the first organic polymer used in substantial quantities in mud. Beginning in 1939 with salt water muds in West Texas,10" starch use for the control of filtration spread rapidly to other areas and applications wherever mud problems related to filtration were experienced. The widespread use of starch decreased as other polymers (notably CMC) were introduced and as high-pH muds were replaced by gyp muds, with the accompanying requirement of a biocide to prevent fermentation of the starch. Starch is still the most economical substance for reducing filtration of strongly-alkaline and salt-saturated muds for shallow drilling.

Composition. Starch is the principal component of the seeds of cereal grains (such as corn, wheat and rice) and of tubers (such as potato and tapioca). The starch granule is composed of carbohydrates. The assigned formula is (CeH,0O;H,O)n. Glucose is formed on hydrolysis of starch. Small amounts of nitrogen, fatty acids, and phosphoric acids are also present. The carbohydrate constituent consists of two polysaccharides: amylose and amvlopectin. Amylose is composed of long, straight chains of a-glucose

Figure 11-5. Schematic examples of types of copolymer structures. (From Scan/ey,102 World Oil.)

Figure 11-6. Formula of amylose, showing straight chain, repeating glucose units.

Figure 11-6. Formula of amylose, showing straight chain, repeating glucose units.

residues, as shown in Fig. 11-6, with molecular weights ranging from 10,000 to 100,000. Amylopectin, the chief component, consists of a mixture of branched-chain molecules, as illustrated in Fig. 11-7 and Fig. 11 8. Amylopectin has a molecular weight of 40,000 to 100,000.

Processing. Corn is the principal source of starch used in drilling muds. The starch granules are separated from the remainder of the kernel, and then must be gelatinized or pasted before the starch can be easily dispersed in water. The gelatinization process involves the rupture of the granules and a many-fold enlargement of the particle under the influence of heat, chemical agents, or both.110,111 Among the chemical agents which may be involved in the gelatinization process are urea, barium peroxide, phosphoric acid, and hydrochloric acid. Heat may be applied to a slurry which is subsequently dried, or a paste may be passed between steam-heated rolls.112,113

Starch in Drilling Mud. Starch is used in drilling mud solely to reduce filtration, and is conveniently added to mud through the cone-jet hopper.

Starch is subject to fermentation by many microorganisms (yeasts, molds, bacteria) and, unless the mud is saturated with salt or the pH is about 12, a biocide should be added if the mud is to be in use for several days. The ambient temperature affects the microbial decomposition rate; if the mud is cold, or very hot (above 160 F, 70 C). the rate is slow. Paraformaldehyde, or other biocide, may be incorporated in processing the starch, or may be maintained in the mud at a concentration of about 0.2 to 0.5 lb/bbl (0.6 to 1.4 kg/in3) to prevent fermentation.

Starch is degraded by heat and by agitation. With continued circulation in a hole at temperatures of 200°F (93 C) and above, starch breaks down rapidly. The resulting product continues to affect the viscosity of the liquid, but loses the sealing action of starch in the filter cake. Consequently, filtration rate and cake thickness are markedly greater under static bottom-hole conditions than are indicated by tests at the surface temperature.

Figure 11-7. Schematic structure of amylopectin

ClIiOH

ClIiOH

II OH II OH

Figure 11-8. Formula of a branch point in the amylose structure

II OH II OH

Figure 11-8. Formula of a branch point in the amylose structure

As are many other organic polymers, starch is co-precipitated with calcium when caustic soda is added to mud containing dissolved calcium salts. When a reduction in calcium-ion concentration is considered necessary, diam-monium phosphate can be used. In the initial preparation of the mud. any adjustment in calcium-ion concentration that involves precipitation should be made before the polymer is added.

Starch is used in mud in concentrations of 2 to 10 lb/bbl (6 to 28 kg m l). Consumption in 1978 was about 13,000 tons. About 1,500 tons of paraformaldehyde were used as a preservative.

Modified Starch. Numerous modifications and derivatives of starch have been proposed for use in drilling and workover fluids. A fermentation-resistant starch product was prepared by blending moist starch (about 20% water) with 3% paraformaldehyde and 3% bis (2-hydroxy, 3,5-dichlor-ophenyl) sulfide, and passing the mixture under pressure through a heated extruder.114,115 This product caused a smaller increase in viscosity than the usual pregelatinized starch and was unusually effective in retarding disintegration and dispersion of shale. These properties are desirable in "non-dispersive muds,"116 as discussed in Chapter 2.

A material identified as "hydrolyzed cereal solids," consisting of 75 - 85% hexasaccharides and higher saccharides, reduces gel strength and yield point of lime muds.117'118 This material is a useful additive in "shale control mud"119,120 (see Chapter 2). In lime muds having both high calcium-ion and high chloride-ion concentrations, filtration is high and is not reduced by ordinary pregelatinized starch. According to T. O. Walker,121 filtration can be lowered by adding a cationic starch (i.e., a tertiary aminoalkyl ether starch), or a quaternary ammonium starch.

Several derivatives of starch have been recommended as filtration-control agents in clay-free compositions. Among these starch products are cyanoethylated starch122, amino starch ether,123 hydroxypropyl starch ether,1:4 and quaternary ammonium salts of starch.125 Current use of any of these derivatives is small.

Guar Gum

Guar gum, like starch, is a natural polymer requiring only a little processing before use. The source of guar gum is the seeds of the guar plant, a hardy, annual, nitrogen-fixing legume, cultivated in Texas. The plant develops pods, each of which contains five or six seeds. The endosperm containing the gum makes up about 40% by weight of the seed.

Composition. Guar gum is a nonionic, branched-chain polysaccharide—a galactomannan.108" The proposed structure of the repeating unit is shown in

Figure 11-9. Formula representing the repeating unit of guar gum. n ranges from 400 to 600.

Fig. 11-9. The molecular weight is around 200,000. On the average, every other mannose unit in the straight chain has a galactose branch, as illustrated. Each repeating unit has nine potentially reactive OH groups that could form guar derivatives with substances such as ethylene oxide, and the like. In the preparation of derivatives, however, relatively few hydroxyl groups react, so the new guar polymer retains the basic structure with some improved characteristics for specific applications (e.g. hydraulic fracturing).

Processing. The hull, endosperm, and germ of the seed are separated by multi-stage grinding and sifting. After the endosperm is separated from the hull and the germ, it is finely ground and packaged as guar gum.

Guar in Drilling Mud. Guar gum produces viscous solutions in either fresh or salty water at concentrations of 1 to 2 lb/bbl (3 to 6 kg/m3). Consequently, guar is used in low solids muds.126 Guar gum is used to lower filtration rate and improve hole stability. It degrades rapidly at temperatures above 150°F, which limits its application to shallow wells. Its effect on viscosity decreases with rise in temperature.

Like starch, guar gum is attacked by microorganisms unless protected by high pH, high salinity, or a biocide. Enzymes, normally present in the gum or introduced from the environment, break down the gum with the formation of acidic substances. In using guar gum in water well drilling, acid development is regarded as evidence that the filter cake on the water-bearing formation is being destroyed. By adding methylene blue along with the gum, the disappearance of the blue color shows that acidic substances have developed.127 Unfortunately, in some instances, the gas produced by the decomposing gum confuses the standard test for coliform bacteria and the water is, perhaps mistakenly, declared unsafe for drinking.

Guar gum flocculates drill cuttings when added in low concentrations while drilling with water. Adequate settling area must be available. The concentration and colloidal activity of the cuttings must be low; therefore, the flocculation process is ineffective in fast-drilling shale, such as occurs in the Gulf Coast.

Borate ions act as cross-linking agents with hydrated guar gum, producing extremely viscous suspensions at low gum concentrations. For example, a viscosity of 6,000 cp is obtained at pH 910 with a solution of 0.25% guar gum and 0.05% sodium tetraborate. The reaction is reversed by reducing the pH to neutral, and thickening again occurs when the pH is raised to 9 10 Loss of circulation in shallow holes can sometimes be stopped by introducing a hydrated guar-borate slurry, which gels in the loss zone.

Guar derivatives formed by the reaction with alkylene oxides, such as ethylene oxide or propylene oxide, are recommended as components of clay-free workover fluids.128

1978 consumption of guar gum and its derivatives in drilling fluids is estimated to have been around 200 tons.

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