salt diluted

Figure 4-30a. Effect of order of addition of salt and polymer (schematic)

Figure 4-30b. Platelets repel each other when the edges are saturated with poly electrolyte (schematic, plan view).

carry no electrostatic charge. Consequently, they have greater stability in high salinity fluids. Starch is nonionic, and as previously mentioned, is used for filtration control in salt water muds. It has the advantage of being inexpensive, but the disadvantage of being biodegradable, and a biocide must be used with it.

Other nonionic polymers include hydroxyethylcellulose (HEC) and guar gum. Like CMC, HEC is made from cellulose, but its functional group is an ethylene oxide chain, (CH2-0-CH2)n- HEC has two great advantages: it is stable in polyvalent brines, and it is almost completely soluble in acid. Consequently it is used a great deal in completion and workover fluids (see Chapter 10). Guar gum is also used in workover fluids, but is degraded by enzymes instead of acid. The colloidal activity of natural gums is reduced by high concentrations of monovalent salts, and eliminated in polyvalent brines. However, gums that have been reacted with ethylene oxide or propylene oxide (see Figure 4-31) are stable even in saturated polyvalent brines.

Compared to clay muds, polymer suspensions have low yield points relative to plastic viscosity, and no real gel strength. Structural properties may be obtained, however, with a gum known as xanthan biopolymer (because it is produced by bacterial action) by crosslinking between the chains with chromic chloride. The structure of xanthan gum is shown in Figure 11-10.

There are also cationic polymers which have positive sites along the chain, created by the dissociation of an inorganic anion. They are used in drilling fluids mostly as emulsifiers and wetting agents, as discussed in Chapter 7. However, a proprietary polyamine has recently been introduced for the purpose of stabilizing formation clays during completion and workover operations.58 The cationic group is strongly adsorbed on the clay, reducing its negative charge, and thus inhibiting swelling and dispersion.

Thermal decomposition is a major factor limiting the use of organic polymers in drilling fluids. It may be compensated for by the addition of fresh polymer, but the rate of decomposition increases with temperature, and above a certain temperature becomes excessive. Thomas59 developed a method, based on reaction rate kinetics, for determining the rate of decomposition at various temperatures. By this means he showed that the rate of decomposition of starch-based polymers increased sharply above about 225°F (107°C) and that of cellulose polymers above about 300°F (149°C), making their use uneconomical above those temperatures. He states that the method is applicable to other materials






Figure 4-31. Hydroxyalkyl guar gum molecule. (Courtesy of Stein Hall Company.)


Figure 4-31. Hydroxyalkyl guar gum molecule. (Courtesy of Stein Hall Company.)


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