HPAM degradation mechanisms

FIGURE 5.30 Effect of severe shearing and resulting mechanical degradation in a Berea core on the viscosity of an HPAM sample. Source Seright et al. (1983). 

The mechanical stability of polymers was related to the polymer s conformation in some of the earlier drag-reduction studies. Above a critical stress, degradation was faster the more contracted and entangled the polymer s conformation (5-7). In petroleum applications the mechanical instability of synthetic relative to carbohydrate polymers was well-recognized. The relative stability problems (possibly related to DUEVs (8)) encountered in the use of high molecular weight hydrolyzed poly(acrylamide) (HPAM) led to the development of an inverse-emulsion polymerization technique (9). (Current research directions using this technique are discussed in Chapter 9.)... 

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

Figure 4.18. The measured changes in the MWD of an HPAM sample after mechanical degradation in a sandstone core (after Seright et a/., 1981).

It appears, then, that the mechanical degradation process is intimately connected with the molecular structure of the macromolecule and the resulting fluid rheology that arises from this structure. For a flexible coil macromolecule, such as HPAM or polyethylene oxide, the polymer solutions are known to display viscoelastic behaviour (see Chapter 3) and thus a liquid relaxation time, may be defined as the time for the fluid to respond to the changing flow field in the porous medium. It may be computed from several possible models (Rouse, 1953 Warner, 1972 Durst et al, 1982 Haas and Durst, 1982 Bird et al. 1987). The finite extendible non-linear elastic (FENE) (Warner, 1972 Bird et al, 1987a Haas and Durst, 1982 Durst et al, 1982) dumbbell model of the polymer molecule may be used to find the relaxation time, tg, as it is known that this model provides a good description of HPAM flow in porous media (Durst et al, 1982 Haas and Durst, 1982) the expression for fe is ... 

For 0.1% HPAM in both 0.3% NaCl brine and seawater in 573-md Berea sandstone, Newtonian behavior was observed at low to moderate fluid fluxes, while pseudodilatant behavior was seen at moderate to high fluxes. No evidence of pseudoplastic behavior was seen in the porous rock, even though one solution exhibited a power-law index of 0.64 in a viscometer. For this HPAM in both brines, the onset of mechanical degradation occurred at a flux of 14 ft/d in 573-md Berea. 

Examination of Figs. 6 and 7 reveals that the filter cake resistance starts at a much higher level for HPAM polymers (sohd symbols) than for xanthan polymers (open symbols). As mentioned earlier (and as will be discussed in detail in the Rheology in Porous Media and Mechanical Degradation section), much of this effect is attributed to viscoelasticity of HPAM (Jennings et al. 1971 Hirasaki and Pope 1974). 

When an HPAM solution is injected into a well with no fractures (i.e., with radial flow as fluid moves away from the weUbore), the highest flux and the greatest mechanical degradation will occur just as the polymer enters the formation. Thereafter, fluxes will decrease and no further significant mechanical degradation of the polymer will occur (Seright et al. 1981 Seright 1983). 

Rheology and mechanical degradation in porous media were quantified for a xanthan and an HPAM 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 at 24,600 psi/ft pressure gradient. 

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