Xanthan Gum XC Polymer

Xanthan gum is properly classed with the natural polymers, although it is actually obtained in its produced rather than in its natural form. Xanthan gum production method was developed at the Northern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, Peoria, Illinois, in 1961. Jeanes et al129 present a recent review containing 49 references on xanthan. Xanthan is a water-soluble polysaccharide produced by bacterial action (genus Xanthomonas) on carbohydrates.130 It was introduced as a drilling fluids component in the mid 1960s under the name ""XC polymer,"131 and its use has increased noticeably since 1970.

Deily et al131 reported several interesting properties of xanthan gum solutions. The polymer builds viscosity in water or salt solutions, although somewhat more gum is required for the same viscosity increase in saturated salt solution. Xanthan gum solutions display exceptional shear-thinning

Xanthan Gum Solution
Figure 11-10. Xanthan gum repeating unit structural features. (From Jeanes, et a/.129 Courtesy of U.S. Dept. of Agriculture.)

properties: Apparent viscosity is markedly lower measured at 30,000 sec 1 than measured at 1,000 sec^1. Cross-linking with chromic ion significantly increases viscosity. Raising the pH from 7 to 11 has little effect on viscosity. Degradation of polymer by short time heating to 250°F (120°C) is negligible.

Composition. Extensive research (see reference 129 for sources) has established that xanthan consists of a linear backbone of glucose residues linked (as in cellulose) with alternate residues having attached a three-unit-long side chain, as illustrated in Figure 11-10. The presence of functional carboxyl, carbonyl and adjacent hydroxyl groups in the xanthan structure makes cross-linking by chromium ions an easy way to enhance the thixotropic behavior of xanthan solutions.132 Molecular weight of xanthan is estimated to be about 5.000,000.

Production. The name XC polymer was applied because the substance is produced by the action of the plant pathogen, Xanthomonas campestris, on carbohydrates in a suitable medium. The product is characterized as an extracellular microbial polysaccharide, i.e., a polysaccharide formed as a coating on each bacterium. Commercial d-glucose from cereal grains mixed with yeast, dipotassium acid phosphate, and small amounts of necessary salts make up the medium for fermentation.

These fermentation conditions are carefully controlled. Viscosity increases markedly as fermentation proceeds. Final processing involves precipitating in isopropyl alcohol, separating, drying, and milling the product to a powder.

Processing equipment and methods are described by Patton and Lindblom133 and by Jeanes.107c

Kelco Company began commercial production of xanthan gum in 1964.

Xanthan Gum in Drilling Fluids. Xanthan gum's major application in drilling fluids is as a thickener or, more precisely, as a suspending agent. Carico134 concluded that the suspending ability of a polymer solution is directly related to the iow-shear-rate viscosity of the solution, i.e., K taken at 200 and 100 rpm, or the 3-rpm dial reading, on the Fann viscometer (see Chapter 5). Results of a simple settling test show that the suspending ability of xanthan gum surpasses that of any other polymer currently used ill drilling fluids.

The exceptional suspending ability of xanthan gum at low concentrations favors its use wherever transportation costs are high. Although xanthan gum is not a filtration-control agent, it is compatible with filtration-reducing substances, such as bentonite and CMC. Several instances of xanthan gum applications in drilling have been cited in Chapter 2.

Xanthan gum has desirable properties for workover and completion fluids. Formation damage studies on Berea sandstone cores showed satisfactory permeability restoration on backflow after injecting one-half pore volume of polymer solution.135 Carico and Bagshaw103 reviewed the properties of representative polymers used in drilling, workover, and completion operations, and stressed the importance of examining the properties under the conditions of use in order to select the polymer best suited for a particular application.

In cross-linking xanthan gum by means of chromium compounds, a definite procedure is recommended. Xanthan gum is first dispersed in water containing at least 80 ppm calcium ion. (If the water is saltier than sea water, a pilot test should be made, because cross-linking is not effected in saturated salt solution.) Chromic chloride or chrome alum powder is then dissolved in water and added to the gum solution in the amount of 20% by weight of the xanthan gum. The pH of the mixture is adjusted to 7.5-10 by slowly adding dilute caustic soda solution.

Xanthan gum is used in concentrations of 0.2 to 2 lb/bbl (0.6-6 kg/m3).

Modifications and derivatives of xanthan gum have been proposed131 1 and synergistic behavior has been claimed for mixtures with other polymers.138,130

1978 consumption of xanthan gum in the United States is estimated to have been 500 tons, with possibly an equal quantity being exported.

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