Xanthan degradation mechanisms

These hydroperoxide radicals then cause propagation reactions as follows POO + RH- POOH+ R  

The above sequence of reactions will occur until oxygen has been fully consumed. The produced hydroperoxides may decompose rapidly or over a longer term, producing more free radical species. Some of the subsequent reactions with the chemically very active radicals may result in backbone scission within the polymer which, as a result, greatly reduces the polymer viscosity the overall affect is to chop up the polymer molecule, hence losing its solution viscosity. The third step in radical reactions is termination, when radicals react to produce stable products, which may be represented as follows  

Wellington (1980) discussed the interaction of hydroperoxides with iron as a transition metal catalyst which produces more free radicals which may initiate further radical-induced decomposition of the polymer. In order to remove the oxygen from solution, reducing agents such as sulphite and bisulphite may be present however, these two may react with iron to produce radicals. For example, the reaction between bisulphite (HSO ) and Fe as follows  

As well as the physical mechanism which may affect the degradation of biopolymer at elevated temperatures. Ash et al. (1983) discuss a number of other mechanisms including hydrolytic, free-radical and enzymatic mechanisms. With reference to the hydrolytic stability of xanthan. Ash et al. (1983) noted that the acetate group in xanthan is removed very readily at pH 12 even at room temperature. They also noted that a lower acetate content of xanthan has the effect of lowering the transition temperature. It has previously been reported that the viscosity of xanthan solutions is either increased (Jeanes et al, 1961) or is almost unaffected (Nisbet et al, 1982) by this loss of acetate groups. Ash et al (1983) concluded that the effect on solution viscosity of the loss of acetate may indeed be small but the 

A very extensive study of xanthan stability at elevated temperatures was carried out by Seright and Henrici (1986). They developed ampoule tests in which undetectable levels of dissolved oxygen could be established (less than 2 ppb). A range of experiments were carried out to examine the relative importance of different pathways for xanthan degradation, including 

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The main objective in this section is to look at the degradation mechanisms for xanthan and partially hydrolysed polyacrylamide in more detail. The mechanisms of both short-term oxidative attack and longer term hydrolysis and/or precipitation are discussed. A study of these mechanisms gives some helpful pointers towards molecular modifications which may improve the stability of new polymers for oil recovery applications. 

Mechanical degradation is based on the elongational flow of the polymer solution. It occurs in ionic PAM because of its relatively high molecular weight, while xanthan seems to be stable to mechanical degradation. 

Ryles (1988) investigated the stability of xanthan and found that its stability followed a pattern similar to that of polyacrylamide in terms of temperature however, the mechanisms are quite different. Xanthan s stability was independent of divalent metal ion concentration, but apparently it was related to the conformational transition temperature. The degradation of xanthan was a func-... 

FIGURE 5.31 Effect of severe shearing in a Berea core on the viscosity of a xanthan solution very little mechanical degradation was evident. Source Seright et al. (1983). 

Colloid o and xanthan gum are affected by the severity of shearing to a small extent if the pressure drop is less than 100-150 psi. At 350-500 psi range, major degradation takes place resulting in a viscosity loss of 30-80%. Colloid is the more resistant of the two and two levels of mechanical degradation seem to take place for this polymer. 

In addition to the level of filtration that is performed in preparing polymer solutions, care must be taken in the amount and intensity of mixing that is applied in making up the original solution in order to avoid uncontrolled mechanical degradation of the polymer molecules in solution. In this respect, synthetic polymers are much more sensitive than xanthan and other... 

In these experiments, xanthan would probably be mechanically stable, but HPAM would probably show some mechanical degradation. 

For applications in which enhanced-oU-recovery (EOR) polymer solutions are injected, we estimate injectivity losses (relative to water injectivity) if fractures are not open. We also consider the degree of fracture extension that may occur if fractures are open. Three principal EOR polymer properties are examined that affect injectivity (1) dehris in the polymer, (2) polymer rheology in porous media, and (3) polymer mechanical degradation. An improved test was developed to measure the tendency of EOR polymers to plug porous media. The new test demonstrated that plugging tendencies varied considerably among both partially hydrolyzed polyacrylamide (UPAM) and xanthan polymers. 

Rheology and mechanical degradation in porous media were quantified for a xanthan and an UPAM polymer. Consistent with previous work, we confirmed that xanthan solutions show pseudoplastic behavior in porous rock that closely parallels that in a viscometer. Xanthan was remarkably resistant to mechanical degradation, with a 0.1% xanthan solution (in seawater) experiencing only a 19% viscosity loss after flow through 102-md Berea sandstone at a pressure gradient of 24,600 psi/ft. 

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