Viscosity of HPAM

Figure 1. Effect of calcium on zero shear rate reduced specific viscosity of HPAM.

Han, J., Kong, B.L., Lu, X.H., 2006a. Smdy on changes of hydrolysis degree and solution viscosity of HPAM at different reservoir temperatures. Oilfield Chemistry 23 (3), 235-238, 255. 

To examine the effect of alkalis on the viscosity of HPAM, the viscosity of polymer solutions was measured as a function of shear rate at various alkali concentrations. Viscosity measurements were repeated on the same solutions after two weeks (336 h) and four weeks (696 h) from initial mixing. Figure 13 depicts the variation of the low-shear relative viscosity with sodium hydroxide concentration at polymer concentration = 1,000 ppm and a temperature of 20°C. After approximately one hour from initial mixing, the low-shear relative viscosity decreased with sodium hydroxide concentration to a limiting value. This result is similar to the trend previously observed with sodium chloride and is due to the shielding effect of the sodium ion. The influence of sodium hydroxide on the low-shear viscosity measured two weeks (336 h) from initial mixing was more dramatic where higher viscosities were obtained at low alkali concentrations. Low-shear viscosity measurements after four weeks (696 h) were very similar to those obtained after two weeks. 

Surfactant slugs are frequently used in EOR processes to mobilize residual oil by changing rock wettability or by reducing oil/water interfacial tension. To increase the efficiency of such processes, polymers can be either co-injected with the surfactant slug or as a chase. In both cases, surfactant and polymer mixing is to be expected. The effects of Triton X-100 (a nonionic surfactant) and Neodol 25-3S (an anionic surfactant) on the viscosity of HPAM solutions were examined by Nasr-El-Din et al. [41]. 

Figure 3.12. Intrinsic viscosity of HPAM versus salt concentration for soft and hard brines (after Sandvik and Maerker, 1977).

NajSjO 50 ppm HPAM-Dow Pusher 700 700 ppm Fresh water 50°C 9-9.1 Knight (1973) Viscosity of HPAM constant over 33 days. Loss of screen factor only 4%. About 30% loss of SF seen without Na2S204 in 47 days... 

Figure2. Effect of calcium on intrinsic viscosity and Huggins constant of HPAM.

Mineral Flocculation for XPS Analysis. A stock solution of 500ppm HPAM in water was prepared and left to diffuse and condition for 1 hour due to the viscosity of the solution. 1 g of the mineral was made into a slurry with 10 cm distilled water and this was then made up to 50 cm in a measuring cylinder. The solution pH was then altered to 9.0 with the relevant amount of 0.1 M NaOH... 

The cloud point slope correlation with the Intrinsic viscosity of the nonlonlc Brlj-10 mlcroemulslon with added polyacrylamide shows that polyacrylamide does not have any specific interaction with the mlcroemulslon, but rather a nonspecific repulsive Interaction similar to that shown by PVP, PEG and dextran. The nature of the Interaction between HPAM and anionic water-contlnueous... 

Table 5.2 compares KAPAM viscosity with the viscosities of the two HPAM polymers, HPAM 2B838 and MO-4000, in different saline waters (their salinities are shown in Table 5.3 Luo et al., 2(X)2). MO-4000 is a Mitsubishi product. Some of the physiochemical properties of the polymers are shown in Table 5.4, according to Daqing Industry Specification Q/DQ0977-1996. We can see that KAPAM viscosities were 22 to 81% higher than those of the other two polymers at the same concentration of 1000 mg/L. In addition, the viscosities in Daqing waters showed that the higher the salinity, the higher the incremental percent of KYPAM viscosity over the others. Liu (2003) reported similar laboratory measurements and had similar results.

The intrinsic viscosity of a homogeneous PAM solution increases when NaCl is added to the solution. When CaCh is added, the viscosity increase is even more obvious. However, HPAM viscosity decreases when a monovalent salt (e.g., NaCl) is added. The reason is that the added salt neutralizes the charge in HPAM side chains. When HPAM is dissolved in water, Na dissipates in the water. -COO in the high molecular chains repel each other, which makes them stretch, hydrodynamic volume increase, and viscosity increase. When the salt is added, -COO is surrounded by some Na, which shields the charge. Then -COO repulsion is reduced, the hydrodynamic volume becomes smaller, and the viscosity decreases. When divalent salts—CaCb, MgCla, and/or BaCla—are added in an HPAM solution, their effect is complex. At low hydrolysis, the solution viscosity increases after it reaches the minimum. At high hydrolysis, the solution viscosity decreases sharply until precipitation occurs. 

It is known that pH affects hydrolysis. Therefore, HPAM viscosity is pH-dependent. pH increases initially when alkali is added. However, adding alkali eventually will result in the decrease of HPAM viscosity owing to the salt effect. Mungan (1969) reported the effect of pH on HPAM viscosity. HCl was titrated against the original stock polymer solution with pH about 9.8 (pH of oilfield brines is usually in the range 7.5-9.5). The polymer concentration was... 

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). 

Biological degradation refers to the microbial breakdown of macromolecules of polymers by bacteria during storage or in the reservoir. Although the problem is more prevalent for biopolymers, biological attack may also occur for synthetic polymers. It has been found that HPAM can provide nutrition to sulfate-reducing bacteria (SRB). As the number of SRB increases, HPAM viscosity decreases. For example, when the number of SRB reaches 36000/mL, the viscosity loss of HPAM of 1000 mg/L is 19.6% (Luo et al., 2006). 

In the dead ends (inaccessible pore ends) with the normal line of its oil-water interface perpendicular to the flow direction, the residual oil is immovable because it is constrained by the rock configuration. In the experiments shown in Figure 6.16, the cores were flooded with water, glycerin, and HPAM. The pore diameter along the flow streamline was 250 pm. The viscosity of the glycerin or polymer was 30 mPa s. We can see that the portions (depth) of the dead pore flushed by water and glycerin were about the same, although the... 

Figure 13.20 shows the polymer (HPAM) effect on 0/W emulsion stability. In low polymer concentrations, the stability was not very sensitive to the concentration. When the concentration was above 150 mg/L, the stability was significantly improved. 0/W emulsion stability is controlled by the strength of the water film between oil droplets. The existence of polymer in water significantly increases the water film s strength and water phase viscosity. Therefore, HPAM has a significant effect on 0/W emulsion stability. It has also been observed that emulsion stability increased with polymer hydrolysis (Kang, 2001). 

The presence of polymers at the interface between oil and water makes for excellent stabilization of emulsions (1,4). Figure 13 shows the interfacial shear viscosity of the interfacial film between a model oil and NaOH solution or polymer solution at 45°C. The model oil consisted of 20% Daqing crade oil injet fuel. The contents ofNaOH, ORS41, and a biological surfactant in the NaOH solution were 1.2, 0.5, and 0.15%, respectively. The concentration ofpolymer hydrolyzed polyacrylamide (HPAM) in the solution was 150mg/L. It can be seen that the interfacial shear viscosity of the system with the polymer is three times higher than... 

The effect of sodium hydroxide on the low-shear viscosity can be explained as follows HPAM undergoes further hydrolysis in the presence of strong alkalis (base hydrolysis). As the polymer is hydrolyzed, the number of the carboxylate groups (i.e., the number of negative charges) on the polymer chain increases. Consequently, the electrostatic repulsion increases, and the chain size increases. This increase in the polymer chain size enhances the viscosity of the polymer solution in deionized... 

Figure 23 shows that the screen factor monotonically increased with polymer concentration for both xanthan materials. However, the Statoil polymer showed higher screen factors, especially at higher polymer concentrations. At high shear rates, shear viscosities obtained by extrapolating the data shown in Figure 22 are slightly lower than those obtained from the screen viscometer shown in Figure 23. This trend indicates that the elastic properties of xanthan gum are not as significant as those of HPAM.

One major disadvantage of HPAM is its high sensitivity to salts [41]. This is not so for hydrophobically associating polyacrylamide. Figure 45 shows the effect of salts on the apparent viscosity at 1.3 s for HPAM and hydrolyzed copolymer of N-octylacrylamide/acrylamide. All polymers have the same degree of hydrolysis at 18%. The two associating polymers contained hydrophobe contents of 1 and 1.25 mol%. The addition of hydrophobe reduced the sensitivity to salts, especially at the higher hydrophobe content examined. 

The viscosity of a polymer solution is related to the size and extension of the polymer molecule in that particular solution larger molecular species are generally associated with higher solution viscosities. In this section, the issue of the molecular size of the polymer is discussed mainly in the light of viscosity-related properties of the polymer solution. Relationships are developed that apply to both random coil molecules, such as HPAM, and more rigid macromolecules like xanthan. A number of other quantities are related to viscosity these include the relative viscosity, specific viscosity, reduced viscosity and inherent viscosity, which are defined in Table 3.1. (Billmeyer, 1971 Rodriguez, 1983). Clearly, all of these quantities are related to the polymer concentration in solution, and a more fundamental quantity which will be defined is the intrinsic viscosity, [ ]. The intrinsic viscosity is the limit of the reduced viscosity or inherent viscosity as the solution concentration of polymer tends to zero as shown below. 

The relative viscosity of both hydrolysed HPAM, a polyelectrolyte, and unhydrolysed PAM, a neutral molecule, are shown in Figure 3.11 as a function of salt concentration (Martin and Sherwood, 1975). As expected, the salt only affects the charged molecules. The repulsion between the backbone charges is screened by the local double layer formed by the small electrolyte species. At higher salt concentrations, the screening effect is more marked, and consequently the viscosity is lower. The effect of divalent ions. 

Figure 3.11. Relative viscosity of PAM and HPAM in sodium chloride brine polymer concentration 600mg/l, temperature 25°C, shear rate 7.3s" (—PAM, unhydrolysed -—...

It is convenient to have a general correlation or data bank of the solution viscosity of polymers as functions of concentration, shear rate and the level of salinity (NaCl) or hardness (Ca ). Correlations for these quantities have been presented for HPAM by French et al. (1981). Auerbach (1985) has presented similar correlations for the concentration/viscosity relationship for commercially available xanthans with varying levels of pyruvate in different salinity brines. 

Figure 3.13. The effect of pH change (by HCl addition) on the viscosity of two HPAM samples shear rate 50s" S concentration 2500ppm (after Mungan, 1969).

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