Remarks on retention mechanisms

Polymer adsorption at a liquid/solid interface is a very well-established phenomenon and has an enormous associated literature (Lipatov and Sergeeva, 1974 Parfitt and Rochester, 1983). On the evidence from porous medium flow experiments it appears that mechanical entrapment is also a reasonably well-established mechanism for polymer retention in flow through porous media. Hydrodynamic retention is a rate-dependent effect which is rather less well understood. However, this retention mechanism is not a very large contributor to the overall levels of polymer retention in porous media and, although interesting, is probably not a very important effect in field-scale polymer floods. The important point to note is that it must be understood sufficiently well in laboratory floods so that core flood results can be interpreted correctly concerning polymer adsorption and entrapment retention mechanisms. 

Mechanical entrapment is a more likely mechanism for polymer retention for lower permeability materials where the pore sizes are smaller (Szabo, 1975 Dominguez and Willhite, 1977) and appears to increase at residual oil compared with the fully water-saturated situation (Szabo, 1975). It is also a function of the polymer/porous medium combination being studied. As noted above, the effective hydrodynamic size of the polymer relative to the pore size distribution is very important in determining the relative importance of the entrapment mechanism. In fact, this phenomenon is undesirable and, if possible, should be screened out in the choice of polymer (note that the brine also plays an important role here through solvent effects). That is, this should be viewed as a type of poor filterability of the polymer solution and hence should be avoided by such means as prefiltering the polymer or preshearing it to reduce the molecular size. In any such treatment of the polymer solution to reduce the retention by mechanical entrapment, it is important to maintain other target properties such as solution viscosity. 

Adsorption is a more fundamental property of the polymer-rock surface-solvent system and cannot be avoided in the same way except by trying another polymer. Thus, adsorption is the most important mechanism that should be studied in evaluating a given polymer flood application. Retention via mechanical entrapment should be treated as a screening variable and should be avoided. Hydrodynamic retention is generally small and can be neglected in most practical applications. Because of these views, most of the rest of this chapter will concentrate mainly on adsorption. This approach 

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