Viscosify shear

Many microbial polysaccharides show pseudoplastic flow, also known as shear thinning. When solutions of these polysaccharides are sheared, the molecules align in the shear field and the effective viscosity is reduced. This reduction of viscosity is not a consequence of degradation (unless the shear rate exceeds 105 s 1) since the viscosity recovers immediately when die shear rate is decreased. This combination of viscous and elastic behaviour, known as viscoelasticity, distinguishes microbial viscosifiers from solutions of other thickeners. Examples of microbial viscosifiers are ... 

The concentration at which a steep rise in this curve begins has been termed as the critical or threshold concentration (2,3). Figure 6 shows such typical curves for PTF and BTF in n-hexane. Despite the fact that different shear rates are involved in capillary viscometry, it can be qualitatively said that at a given concentration, PTF viscosified n-hexane better than BTF. It is clear from Figure 6 that the critical concentration for these two compounds is above 0.7%, while analogous tri-n-alkyltin fluorides showed a critical concentration of less than 0.4% (3). This may be due to the presence of bulky Me3Si-groups nearer to the Sn-F bond, which causes some steric hindrance to auto-association. 

Welan has similar properties to xanthan gum except that it has increased viscosity at low shear rates and improved thermal stability and compatibility with calcium at alkaline pH (90). The increased thermal stability has led to its use as a drilling mud viscosifier especially for high temperature wells. The excellent compatibility with calcium at high pH has resulted in its use in a variety of specialized cement and concrete applications. 

The nature of the shear viscosifying component and possibly the magnitude of the shear viscosity can contribute to the magnitude of the DUEVs (42). This information will relate to many applications in which extrusion rates are not totally extensional in nature (e.g., automotive sealants and mobility control studies (23, 26) in which the permeability of the formation is a critical factor). For roll-applied films, the magnitude of these different deformation rates (16) are significantly different (Figures 8 and 9) and their relative contribution to a formulation s performance may be complex. 

The local stiffness of polysaccharides explains the high viscosity obtained at a given molecular weight, compared with flexible polymers, as well as the relatively low sensitivity to the external salt concentration (since Lp > Lg in many conditions). It was shown that the specific viscosify is direcfly related to the overlap parameter C[ti] at zero shear rate and expressed by the following relation ... 

These data show that the lower-molecular-weight, methyl-substituted GG are somewhat 1. poorer viscosifiers, 2. more prone to shear degradation in high shear applications, and 3 more prone to lose thickening capacity when electrolyte is added to the copolymer solution than is the unsubstituted copolymer. 

A decisive criterion for the economics of a viscosifier for polymer flooding processes is its ability to perform in the existing, often saline reservoir- and flooding waters at the respective reservoir temperature. This goes just as well for the shear viscosity as for the elastic viscosity. Data on the effectiveness of the additive and the rheological behaviour of the polymer solutions are preferably obtained using a low... 

First, it is necessary to consider why it is important to discuss aspects of the chemical structure of these materials when it is mainly the physical properties, such as viscosifying power , which are of interest when applying polymers in the field. However, on closer examination, it is found that virtually all of the important physical properties of the polymer, including flow behaviour, adsorption/retention, thermal and shear stability, can be understood in terms of the molecular structure of the polymer. Throughout the discussion in this chapter, and elsewhere in this work, it is assumed that the polymer is in an aqueous solution this is usually a brine which may contain a number of ions, such as Ca, Mg, SO ", in addition to Na and Cl ions. 

The yield stress tyiM of the mudcake in question may depend on numerous factors, among them, differential pressure, solids content, viscosifier type, chemical composition, and temperature. The critical invasion rate proposed by Fordham et al. (1991) is simply the lowest volume flow rate Q that will, for a particular mud and drillpipe size, produce viscous shear stresses at the mudcake surface in excess of Note that viscous stress and total volume flow rate,... 

In the case of extensional viscosity, unified curves have been presented for a limited number of cases and the data used for coalescence are also-limited. Because of the difficulties in measurement of extensional viscosity, the reliability of the data is often questionable. Because the original data cannot be as trustworthy as in the case of shear viscosity or complex viscoaty data, the master curves of extensional data should be looked at in the same light. In the case dt shear viscosify, the master rheograms for the filled polymers have been shown to be the same as those for the unfilled system. is, of course, true in the medium to high shear-rate region. In the low-shear-rate region, however, the effects of yield stress would dominate and the uniqueness of the curve will be... 

Viscosity is a measure of fluid resisfance to mofion, and if relates fhe strain rate to applied shear stress. A functional dependence of gas viscosify on temperature at low density is given by Chapman-Enskog based on kinetic theory (Bird et al., 1960) using Lennard-Jones potentials. The theory has been also extended to multicomponent gas mixtures. For most common applications, however, a simplified semiempirical formula of Wilke (1950) is used ... 

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