Synthetic Water Dispersible Polymers

Composition and Structure. Numerous completely synthetic water-dispersible polymers have been made from monomeric materials.""* A very broad range in composition and properties is possible in forming homopolymcrs. copolymers, and copolymers whose homopolymers are not water soluble. Many products are synthesized by direct polymerization; some by a second reaction.

As an example of polymer synthesis, consider acrylic polymers consisting of carbon, hydrogen, oxygen, and nitrogen. Many water soluble polymers can be made from these elements. Starting with acrylonitrile (CH2:CHCN), acrylic acid can be formed, then polymerized, and sodium polyacrylate made by neutralization with sodium hydroxide. Or polymerized acrylonitrile treated with sodium hydroxide can form a polymer containing amide

(CONH2) and sodium carboxylate (COONa) groups. Or beginning with acrylonitrile, acrylamide (CH2CHCONH2) can be formed and then polymerized, forming neutral polyacrylamide. Or, a copolymer can be formed from acrylic acid and acrylamide.

Conversion of amide groups to carboxyl groups is called hydrolysis. The percent hydrolyzed, whether the polymer is nonionic, cationic, or anionic, and the molecular weight are significant factors affecting shale preservation1 iff (see Chapter 8). Polyacrylamide is available in molecular weights ranging from as low as 1,000,000 to as high as about 15 million.

In addition to acrylamide, the subject of the above example of polymer synthesis, other repeating units, as illustrated in Figure 11-13, are used in preparing a host of water dispersible polymers. No attempt will be made here to discuss preparation methods or properties of these varied materials. Only certain applications in drilling fluids will be considered.

Acrylic Polymer Applications. The performance of synthetic polymers in drilling fluids is affected not only by the composition, structure, and molecular weight of the specific polymer, but also by the composition and temperature of the system to which it is added. A given polymer may be a flocculant in low concentration and a waterloss reducer at a higher concentration. These variable factors make necessary an extensive testing program before putting a new product on the market.

Scanley102 reported a study of the effect of the molecular weight and the carboxylate/amide ratio of the two types of acrylic polymers on the viscous and filtration properties of four different mud compositions. In general, for one polymer type, filtration rate decreased, and viscosity and gel strength increased, as molecular weight increased from 180,000 to 390,000. No general conclusions were drawn on the effect of the carboxylate/aiiiid ratio. A hydrolyzed polyacrylamide, having a molecular weight of about 250,000, showed lowest water loss with carboxylate contents below 50%, and least effect on viscosity and gel strength at about 23% carboxylate.

Scanley stated that acrylic polymers should be more stable to temperature and microbial attack than polysaccharides, because the acrylic polymers have a carbon-to-carbon backbone (see Figure 4-29). Cowan159 found that the API filtrate, 75°F (24°C), of polyacrylate-treated mud did not increase after 16 hours heating at 400°F (204°C), but the filtration rate measured at 400°F (204°C) was high. As an explanation for this observation, Cowan proposed that the acrylic polymer coiled at the high temperature and uncoiled when cooled.

The use of sodium polyacrylate for filtration control in some areas has been limited by its sensitivity to calcium ions. Soluble calcium compounds should be removed by soda ash before the polymer is added to the mud.

In drilling hard rocks, with water as the drilling fluid, certain acrylic polymers are very effective flocculants and make "clear water drilling" possible.1"11 161 (See Chapter 2 for field experience and Chapter 4 for colloidal behavior.)

Copolymers of vinyl acetate and maleic anhydride contribute several desirable features in low solids muds.*3'162,103 As "bentonite extenders," the thickening properties of bentonite are enhanced, thus improving hole cleaning. Better separation of drill cuttings occurs with low solids muds. Friction losses in turbulent flow are reduced by the polymer. Faster drilling rates result in a significant reduction in cost per foot.164

In a study of shale stabilization by acrylic polymers and potassium chloride solution, Clark et al158 found that 20% to 40% hydrolyzed polyacrylamide, having molecular weight greater than 3,000,000, was more effective than polyacrylamides of lower molecular weight, or higher or lower degrees of hydrolysis (see Chapter 8).

Low-molecular-weight copolymers of styrene and maleic anhydride and their sodium salts are reported to reduce viscosity and gel strengths of muds, and the effects persist after heating 72 hours at 250T (122°C).165

Several polymer compositions are reported to afford stable muds at high temperatures. In some instances, however, the exposure at the high temperature (less than SOOT, 260°C) was relatively short and the properties after aging were measured at room temperature. Superior heat stability (10 hours at 482 F, 250 C) is claimed for a mud containing "an open chain poly-n-vmyl carboxylic acid amide"166 Sodium polyacrylate of molecular weight less than 2,500, when added to a 17 lb/gal (2.0 g/cm3) lignosulfonate-treatcd mud, substantially decreased thermal degradation, as shown by tests in the Fann consistometer at a maximum temperature of 448T (230CC).U'7 A copolymer of styrene sulfonic acid and maleic anhydride having a molecular weight before sulfonation of between 1,000 and 5,000 prevented excessive thickening of a clay-water mud heated for 16 hours at 485 F (250 C).lftS Applications of any of the above named polymers in deep drilling has not been reported.

The application of acrylic polymers in bentonite processing was discussed earlier in this chapter.

As has been shown, acrylic polymers have varied applications, and the concentration range is wide, from as low as 0.01 lb/bbl (0.03 kg/'m3) as a flocculant, to as high as 3 lb/bbl (8 kg/m3) for filtration control. Consumption of acrylic polymers in 1978 is estimated to have been 2,500 tons.

Alkylene Oxide Polymer Applications. The development of a calcium surfactant mud as a replacement for lime mud for drilling deep, hot holes was noted in Chapter 2. The nonionic surfactant is phenol 30-mol ethylene oxide adduct, having the formula C6H50(CH2CH20)3nH.

The polymer is adsorbed on clays, and shows a marked inhibiting effect on the dispersion of shale cuttings.169 * 17 0 -171 •17 2 •173 Asa solution also containing a defoamant, nonylphenoxyethanol (CQH10C6H4OCH2CH2OH), the product DMSH is effective in maintaining satisfactory flow properties in muds exposed to high temperatures.159 Lignite and DMSK were used to control flow properties and filtration while drilling to 25,600 ft (7803 m) with mud weighing 18.7 lb/gal (2.24 g/cm3), in a record-setting well in the Lousiana Gulf Coast, having bottomhole temperatures approaching 500 F (260 C).174

A 30-mol ethylene oxide adduct of nonyl phenol, C9 H,t) C6 H4(>(CH2 CH2O)30 H, is used if needed to emulsify oil in the surfactant mud.

Polymers containing alkylene oxides have been recommended as components of completion fluids. Examples are a suspension consisting of polyethylene oxide, calcium carbonate, and a wetting agent,175 and a composition consisting of a high-molecular weight (1,000,000 to 10,000,000) polyalkylene oxide and sodium or calcium lignosulfonate.176

The ethoxylated alkyl phenols are the principal alkylene oxide polymers used in drilling fluids. Concentration ranges from 1 to 10 lb/bbl (3 to 28 kg. m-\). Estimated 1978 consumption was 2,000 tons.

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