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Figure 4-29. Acrylamide-sodium acrylate copolymer, live sites on the edges of the clay platelets. The reason the 30% hydrolyzed copolymer is the most effective for shale preservation is probably that the charged sites on the chain match the spacing of the clay platelets.

The mechanism of water clarification is interesting. The copolymer does not by itself cause flocculation; at least enough salt must be present to initiate floc-culation, and then the polymer chains will bind the flocculated particles together.55 This point may be demonstrated by changing the order in which salt and the copolymer are added to a suspension of clay in fresh water (see Figure 4-30A). If the copolymer is added first, the chains are adsorbed around the edges of individual platelets, and are therefore not available for linking between platelets when the salt is subsequently added. Consequently, the platelets separate if the salt concentration is diluted below the flocculation value. On the other hand, if the salt is added first, then the chains can link between adjacent platelets and hold the floes together when the suspension is diluted.

In order to act as flocculation aids, polyelectrolytes must be added in very small quantities, between about 0.001 % and 0.01 %. If added in larger amounts, they act as protective colloids and raise the flocculation value of the system. It is believed that at concentrations above 0.01 %, the edges of the clay platelets are saturated with polyelectrolyte chains and thus have a strong repulsive charge (see Figure 4-30B).

Another acrylic copolymer, vinyl acetate-maleic acid, is used as a bentonite extender to increase the yield of commercial bentonite.56 Between 0.1% and 2 % of the copolymer is added to the bentonite. When the bentonite is dispersed in fresh water, the copolymer chains form links between the dispersed platelets, increasing the viscosity and yield point.

A number of nonionic polymers are used for various purposes in drilling fluids. Nonionic polymers have no dissociable inorganic radical, and therefore

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