Polymer retention

Propagation of enhanced oil recovery chemicals through rock is critical to the success of an EOR project. Polymer retention in permeable media has been the subject of considerable study (349)... 

Lochmiiller, C.H., Jiang, C., Liu, Q., and Antonucci, V., High-performance liquid chromatography of polymers retention mechanisms and recent advances, Crit. Rev. Anal. Chem., 26, 29, 1996. 

FIGURE 16.3 Dependences of the polymer retention volume on the logarithm of its molar mass M or hydrodynamic volume log M [T ] (Section 16.2.2). (a) Idealized dependence with a long linear part in absence of enthalpic interactions. Vq is the interstitial volume in the column packed with porous particles, is the total volume of liquid in the column and is the excluded molar mass, (b) log M vs. dependences for the polymer HPLC systems, in which the enthalpic interaction between macromolecules and column packing exceed entropic (exclusion) effects (1-3). Fully retained polymer molar masses are marked with an empty circle. For comparison, the ideal SEC dependence (Figure 16.3a) is shown (4). (c) log M vs. dependences for the polymer HPLC systems, in which the enthalpic interactions are present but the exclusion effects dominate (1), or in which the full (2) or partial (3,4) compensation of enthalpy and entropy appears. For comparison, the ideal SEC dependence (Figure 16.3a) is shown (5). (d) log M vs. dependences for the polymer HPLC systems, in which the enthalpic interactions affect the exclusion based courses. This leads to the enthalpy assisted SEC behavior especially in the vicinity of For comparison, the ideal SEC dependence (Eigure 16.3a) is shown (4). 

In conclusion, the enthalpic partition processes in the columns for polymer HPLC substantially differ from the adsorption processes. Enthalpic partition can be employed for the separation of polymers of the low-to-medium polarity in combination with the alkyl bonded phases on silica gels. The extent of the enthalpic partition and consequently also of the polymer retention is controlled primarily by the thermodynamic quality of eluent toward separated species and by the extent of the bonded phase solvation. 

Side processes may bring about either an increase or a decrease in measured retention volumes. Often, the presence of side processes in the SEC system is signalized by the increased dependence of polymer retention volumes on the operational parameters such as the sample concentration, flow rate, and temperature. 

Similar to other coupled methods of polymer HPLC, for example, LC CC (Section 16.5.2), the choice of the column packing and the mobile phase components for EG-LC depends on the retention mechanism to be used. Adsorption is preferred for polar polymers applying polar column packings, usually bare silica or silica bonded with the polar groups. The eluent strength controls polymer retention (Sections 16.3.2 and 16.3.5). The enthalpic partition is the retention mechanism of choice for the non polar polymers or polymers of low polarity. In this case, similar to the phase separation mechanism, mainly the solvent quality governs the extent of retention (Sections 16.2.2, 16.3.3, and 16.3.7). It is to be reminded that even the nonpolar polymers such as poly(butadiene) may adsorb on the surface of bare silica gel from the very weak mobile phases and vice versa, the polymers of medium polarity such as poly(methyl methacrylate) can be retained from their poor solvents (eluents) due to enthalpic partition within the nonpolar alkyl-bonded phases. 

Willhite, G.P. Dominguez, J.G. Mechanisms of Polymer Retention in Porous Media in Improved Oil Recovery by Surfactant and Polymer Flooding, Shah, D.O. Schechter, R.S. (Eds.), Academic Press New York, 1977, pp. 511-554. 

Fig. 6.1. Separation of alkylphenones using replaceable polymer solution as a separation medium (Reprinted with permission from [22]. Copyright 1997 American Chemical Society). Conditions Column 50 cm (42 cm active) x 50 pm i.d., 4.00% linear polymer retentive phase, mobile phase 40% acetonitrile in buffer pH 9.1 voltage, 30 kV injection, 5 kV for 15 s. Peaks acetophenone (1), butyrophenone (2), hexanophenone (3), and octanophenone (4).

The two factors (D and DT) controlling polymer retention are strictly orthogonal D depends only on the size and geometry, whereas DT depends on chemical composition. Th-FFF has been used mainly to discriminate between chain length differences (reflected in D) within polymer families, yielding molecular weight distributions (MWDs) [15,168]. The promise of compositional differentiation and measurement based on DT has been little exploited but certainly has future potential as illustrated by the example in Fig. 10. 

The early research of Myers et al. [1,2] shows that polymer thermal field-flow fractionation (ThFFF) retention and thermal diffusion are solvent dependent. Recently, Sisson and Giddings [3] indicated that polymer ThFFF retention could be increased by mixing solvents. Rue and Schimpf [4] extended the molecular-weight range that can be retained by ThFFF to much lower molecular weights (<10 kDa) by using solvent mixtures without using extreme experimental conditions. There are several other reports on the effect of solvents on polymer retention, selectivity, and the universal calibration in FFF in last few years [5]. 

The study of Sisson and Giddings [3] shows that polymer retention in some binary solvents can be enhanced while retention is relatively unaffected or even diminished in other cases. In the case of a mixture of 30% dodecane and 70% THF, polymer retention was enhanced by 35% relative to pure THF. The low end of the molecular-weight range, which can be analyzed by ThFFF, therefore, can be expanded further down by using binary carriers. 

Rue and Schimpf [4] investigated component partition of a solvent mixture under thermal gradient. It is indicated that when one solvent is a significantly better solvent for a polymer, then the gradient of this solvent will cause the polymer to migrate in the same direction as the thermophoretic motion of this solvent. This is called solubility-based migration as a partitioning effect. When the better solvent partitions to the cold wall of the ThFFF channel, polymer retention is enhanced when the better... 

W. J. Cao, P. S. Williams, and M. N. Myers, Solvent effects on polymer retention and universal calibration in ThFFF, in preparation. 

I The driving force for polymer retention in thermal FFF is thermodiffusion, which varies with polymer composition. As a result, polymer blends and copoly-4 mers can be separated even when the molecular I weights or diffusion coefficients are identical. Further-Q more, because thermodiffusion can be measured quan-I titatively by thermal FFF through the thermodiffusion coefficient (T>r), the compositional distribution can, in g principle, be obtained directly from elution profiles, g provided the dependence of Dj on composition is known. In practice, there are several complications. 

Deuterated tracer pulse chromatography (TPC) was used to characterize the retention behavior of Tenax-GC and four polyimide-based sorbent materials. Deuterated n-hexane, ethanol, 2-butanone, nltromethane, and benzene were used as compounds to probe ive types o chemical Interactions o the compounds with the polymers. Retention properties were Investigated with dry and humidified helium carriers both with and without the Incorporation of non-deuterated test compounds. Analyte competition was shown to occur on all of the sorbents. Humidity affected the retention of the probe compounds on the polyimides to a much greater extent than on Tenax-GC. The technique was shown to elucidate subtle differences In sorbent behavior. 

This section discusses polymer rheology, polymer retention in porous media, and rock permeability reduction. 

Polymer retention includes adsorption, mechanical trapping, and hydrodynamic retention. These different mechanisms were discussed by Willhite and Dominguez (1977). Mechanical entrapment and hydrodynamic retention are related and occur only in flow-through porous media. They play no part in free powder/ bulk solution experiments. Retention by mechanical entrapment is viewed as occurring when larger polymer molecules become lodged in narrow flow channels (Willhite and Dominguez, 1977). 

In the experiment using sand pack (see Figure 5.36), when the injected polymer concentration was 600 ppm, the dynamic polymer retention at the inlet was 24.5 [ig/g and retention at the exit it was 6 [Xg/g. If the curve s trend was extended to the infinite distance, then the retention was 3.31 fig/g the... 

After a steady state is reached in a polymer retention experiment in a core, the total level of retention increases when the fluid flow rate is increased (Chauveteau and Kohler, 1974). This type of rate-dependent retention, called hydrodynamic retention, is not understood as well. Fortunately, it is generally thought to give a small contribution to the total retained material (Sorbie, 1991). 

For the preceding three mechanisms of polymer retention, mechanical entrapment can be avoided by prefiltering or preshearing the polymer or by applying the polymer in a high permeability formation. Hydrodynamic retention is probably not a large contributor in the total retention and can be neglected... 

The laboratory unit used to define polymer retention, Cp, is in mass of polymer per unit mass of solid, usually in micrograms per gram of rock (itg/g). Sometimes (e.g., in UTCHEM), the unit is in grams per 100 milliliter (cm ) of pore volume (PV), g/100 mL PV, which is equivalent to weight percent (wt.%) if the solvent (water) density is 1 g/mL and the pore volume is filled up by the solvent (water) only. In bulk static adsorption, a more fundamental measure of adsorption is the mass of polymer per unit surface area of solid, which is referred to as the surface excess, Cps, usually in milligrams or micrograms per square meter (mg/m or (tg/m ). Sometimes, in field applications, the retention unit is in mass of polymer per unit volume of rock, usually in lb/ acre-foot. 

There are large differences between the level of static adsorption of HPAM and dynamically retained level in a core or pack (Lakatos et al., 1979). These differences are the result of changes in the specific surface area of consolidated and unconsolidated packs and also the accessibility of certain portions of the pore space. These differences also depend on the extent of mechaifical retention that is present in the dynamic core flood experiment. Polymer retention in consolidated porous media cannot be determined with static bulk adsorption (batch adsorption techniques) because the process of disaggregation to obtain... 

In most cases, polymer adsorption is considered irreversible that is, it does not decrease as polymer concentration decreases (Szabo, 1979 Lakatos et al., 1979 Gramain and Myard, 1981). The irreversible effect is caused by polymer adsorption on rock. However, this is not exactly true because small amounts of polymer can be removed from porous rock using prolonged exposure to water or brine injection. Usually, however, the rate of release is so small that it is not possible to measure the concentrations accurately. It is thus more accurate to state that the rate of polymer retention is much greater than the rate of polymer removal. Retention also may occur when flow rates are suddenly increased. This process is called hydrodynamic retention, which is reversible (Green and Willhite, 1998). 

As shown in Eqs. 5.31 and 5.32, polymer retention decreases with permeability. Figure 5.46 shows an example for HPAM adsorption in Berea. It is obvious that mechanical trapping in a low-permeability rock is higher than that in a high-permeability rock. Another possible explanation is high clay content in low-permeability rocks. 

When polymer molecular sizes are larger than some pores in a porous medium, the polymer molecules cannot flow through those pores. The volume of those pores that cannot be accessed by polymer molecules is called inaccessible pore volume (IPV). In an aqueous polymer solution with tracer, the polymer molecules will run faster than the tracer because they flow only through the pores that are larger than their sizes. This results in earlier polymer breakthrough in the effluent end. On the other hand, because of polymer retention, the polymer breakthrough is delayed. In other words, if only polymer retention is considered, the polymer will arrive in the effluent later than the tracer. 

These two factors can be best explained by Figure 5.47. In the displacement shown in the figure, a constant polymer concentration is injected into the core, which has not been contacted previously by polymer. Retention causes the effluent concentration to lag behind the tracer, as shown in the first flood in the... 

FIGURE 5.47 Comparison of tracer and polymer concentration profiles in the effluent when the polymer retention mechanism is dominated (first flood) and when IPV is significant (second flood). Source Hughes et al. (1990). 

Eigure 5.48 shows the permeability effect on the maximum permeability reduction factor, Ekrmax, predicted from Eq. 5.37. This figure shows that Fbjnax decreases with permeability, which is consistent with the observation that the polymer retention decreases with permeability, as shown in Figure 5.46. The laboratory-measured permeability reduction data at the polymer concentration... 

Szabo, M.T., 1975. Some aspects of polymer retention in porous media using a C-tagged hydrolyzed polyacrylamide. SPEJ (August), 323-337. 

Agestat [C Polymers retention aid, pigment dispersant drainage aid, stabilizer, raw and waste water cither for pi r industry. 

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