J. Cent. South Univ. Technol. (2008) 15(s1): 107-110
DOI: 10.1007/s11771-008-325-z
Rheological properties of poly(acrylamide-co-sodium acrylate) and poly(acrylamide-co-sodium vinylsulfonate) solutions
CAO Jie(曹 杰)1, CHE Yu-ju(车玉菊)1, CAO Xu-long(曹绪龙)2, ZHANG Ji-chao(张继超)2,
WANG Hong-yan(王洪艳)2, TAN Ye-bang(谭业邦)1
(1. School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China;
2. Research Institute of Geology, Shengli Oilfield Company, Dongying 257000, China)
Abstract: Poly(acrylamide-co-sodium acrylate) (PAM/AA-Na) and poly(acrylamide-co-sodium vinylsulfonate) (PAM/VSS-Na) were prepared by inverse emulsion polymerization. The effects of CaCl2 on PAM/VSS-Na or PAM/VSS-Na aqueous solutions were investigated by steady-flow experiments at 25, 40, 55 and 70 ℃. The results show that the apparent viscosities of both solutions decrease with addition of CaCl2 or increase of temperature and shear rates. PAM/VSS-Na solution has better performance on the salt tolerance, shear endurance and temperature resistance due to containing sulfonic group in the molecules. Ca2+ concentration can affect the viscous activation energy of both solutions and the reason may be that these interactions between Ca2+ and also copolymer molecules are related to temperature and competitive in solution. These results may offer the basic data for searching the flooding systems with the ability of temperature resistance, salt tolerance and shear endurance for tertiary oil recovery.
Key words: polyacrylamide; sodium vinylsulfonate; rheological properties; salt tolerant; temperature resistant
1 Introduction
Many research groups have focused on the technology of enhanced oil recovery, and the notable progress has been made in chemical flooding, including surfactant flooding, polymer flooding, alkaline-polymer flooding and alkaline-polymer-surfactant flooding[1-6]. The practices show that the rheology and interfacial tension of injected fluid have an important effect on the oil-displacing efficiency in polymer flooding.
At present, one of the most widely used water-soluble polymers is partially hydrolyzed polyacrylamide (HPAM), which controls mobility in the reservoirs by increasing the viscosity of the injected water, and more importantly, by reducing the formation of permeability. However, HPAM has many disadvantages in oil recovery. For example, the viscosity of HPAM aqueous solution exhibits an abrupt reduction in 3% brine[7]. It suffers excessive thermal hydrolysis at high temperatures and as a result may precipitate in the presence of bivalent cations. Substituted or modified monomer, can yield a better product which may be polyacrylamides, or those copolymerized with a suitable thermally stable, salt tolerant or shear endurant[8-9]. It is established that the copolymers of acrylamide with monomers including sulfonic group offer polyelectrolyte behavior in aqueous solutions-characteristics of special interest to tertiary oil recovery[7].
In this work, poly (acrylamide-co-sodium acrylate) (PAM/AA-Na) and poly(acrylamide-co-sodium vinylsulfonate) (PAM/VSS-Na) were prepared by inverse emulsion polymerization, respectively. The PAM/AA-Na and PAM/VSS-Na solutions with or without CaCl2 were investigated via steady-flow experiments at 25, 40, 55 and 70 ℃. The aim is to offer the basic data for searching the flooding systems with the ability of temperature resistance, salt tolerance and shear endurance.
2 Expermental
2.1 Materials
Poly(acrylamide-co-sodium acrylate)(PAM/AA-Na), which is an intrinsic viscosity of 910 mL/g in 1 mol/L NaNO3 solution, and poly(acrylamide-co-sodium vinylsulfonate) (PAM/VSS-Na), which is an intrinsic viscosity of 696 mL/g in 1 mol/L NaNO3 solution, were prepared by inverse emulsion polymerization. The emulsions were precipitated by ethanol, and washed by ethanol for six times and mixture of water and ethanol for three times to remove emulsifier and unreacted monomers. The molar ratio of sodium acrylate in PAM/AA-Na is 7.2%, which was determined by con-ductimetric analysis. And the molar ratio of sodium vinylsulfonate in PAM/VSS-Na is 6.6%, which was determined by elementary analysis. CaCl2 was purchased from Tianjin Chemical Agents Company, China. The water used was distilled for three times.
2.2 Preparation of sample solutions
Three stock solutions, concentrated PAM/AA-Na, PAM/VSS-Na and CaCl2 solutions were prepared. After stirring for 2 d, PAM/AA-Na and PAM/VSS-Na solution were gently heated at 50 ℃ to ensure complete dissolution. A series of sample solutions were then prepared at natural pH by mixing different amounts of the stock solutions to obtain desired CaCl2 concentrations between 0 and 3.0 mmol/L. Sample solutions were then stirred for 12 h and left overnight to equilibrate before the viscosity measurements.
2.3 Rheological measurements
The rheological measurements were carried out on a HAAKE RS75 rheometer (Germany) with coaxial cylinder sensor system (Z41 Ti). The temperature was maintained at (25.0±0.1), (40.0±0.1), (55.0±0.1) and (70.0±0.1) ℃, respectively. For the shear-dependent behavior, the viscosity measurements were carried out at shear rates ranging from 0 to 1 000 s-1.
3 Results and discussion
3.1 Effect of CaCl2 and shear rates on apparent viscosity of both solutions
Fig.1 shows the steady state shear flow curves of 0.5% PAM/AA-Na and 0.5% PAM/VSS-Na solutions at different CaCl2 concentrations and temperatures. Fig.2 shows the relative values of apparent viscosity of 0.5% PAM/AA-Na and 0.5% PAM/VSS-Na solutions as a function of CaCl2 concentration and shear rate. From the results, it is found that apparent viscosities (η) of both PAM/AA-Na and PAM/VSS-Na solutions decrease with increasing CaCl2 concentration or temperature. Both solutions exhibit shear-thinning effect at high shear rates and behave as non-Newtonian fluids with increasing shear rates. However, the reduction of apparent viscosity of PAM/VSS-Na solution is lower compared with that of PAM/AA-Na solution as CaCl2 concentration and shear rate increase.
The decrease of the viscosity with shear rate is mainly related to the orientation of macromolecules along the streamline of flow and to the disentanglement of macromolecules with increasing shear force. In general, while in the solution of a good solvent, ionic polymers exist in the maximum possible expanded state
Fig.1 Steady state shear flow curves of 0.5% PAM/AA-Na (▲,▼) and 0.5% PAM/VSS-Na (△,▽) solutions at 25 ℃, 75 ℃(a) and 0, 3 mmol/L CaCl2 concentrations(b)
to minimize the repulsive interaction between the ionic groups of the same macroion bearing a similar charge.
And an addition of electrolyte in the ionic polymer solution induces the increase of solution ionic strength and screens the electrostatic charges. Then the macromolecule conformation reduces to the statistical coil conformation. As a consequence, a decrease of the apparent viscosity in copolymer solution containing CaCl2 in comparison with aqueous solution is observed[10].
Because Ca2+ can associate with carboxylic group, it facilitates the formation of intra-molecular associations of PAM/AA-Na, which may precipitate in the solution. Sulfonic group has weaker interaction with Ca2+ and is larger than carboxylic group, which prevents the degradation of the backbone of copolymer and also makes copolymer to expand more. As a result, the apparent viscosity of PAM/VSS-Na solution is unchanged or increased at lower shear rates and decreases much less at high shear rates compared with that of PAM/AA-Na solution.
Fig.2 Relative values of apparent viscosity of 0.5% PAM/AA- Na (▲,▼) and 0.5% PAM/VSS-Na (△,▽) solutions at 0.3 s-1 as a function of CaCl2 concentration at 25, 70 ℃(a) (relative value of each salt free solution is defined as 100%) and relative values of apparent viscosity of 0.5% PAM/AA-Na (■,▲,▼) and 0.5% PAM/VSS-Na (□,△,▽) solutions as function of shear rate at 25 ℃ at different CaCl2 concentrations(b) (each relative value at 0.3 s-1 is defined as 100%)
3.2 Effect of temperature on apparent viscosity
As well known, the increase of temperature can give rise to crimple of PAM/AA-Na molecules due to its dehydrating and destruction of the associational structure[4]. As a consequence, Fig.3 shows that the viscosities of all solutions decrease at each CaCl2 concentration as temperature increases.
It is reported that the relationship between apparent viscosities of polymer solution and temperatures satisfies Arrhenius equation:
(1)
where η is the apparent viscosity of polymer solution, ΔEη is the viscous activation energy; R is the gas constant, and T is the absolute temperature. According to this Eqn.(1), lnη and the reciprocal of temperature of the
Fig.3 Viscosity of 0.5% PAM/AA-Na (a) and 0.5% PAM/VSS-Na (b) solution at 10 s-1 as function of temperature at different CaCl2 concentrations
polymer solution should show a straight-line relationship, with slope of ΔEη /R. As a result, viscous activation energy of PAM/AA-Na and PAM/VSS-Na solutions at different CaCl2 concentrations was calculated, as shown in Fig.4. Viscous activation energy is related to the dependence between viscosity and temperature of polymer solution, and the higher the viscous activation energy is, the more the influence of temperature on the viscosity.
Fig.4 shows that the viscous activation energy of PAM/VSS-Na is less than that of PAM/AA-Na in CaCl2 solutions at each concentration, which indicates that PAM/VSS-Na and CaCl2 system has better performance on the temperature resistance. However, the viscous activation energy of PAM/VSS-Na is more than that of PAM/AA-Na in salt free solution. The reason may be that sulfonic group is larger than carboxylic group, which indicates that charged groups in PAM/VSS-Na molecules are more crowded than that in PAM/AA-Na molecules when charges are not screened by other salt ions. As a result, PAM/VSS-Na is more sensitive in pure aqueous solution.
Fig.4 Changes of viscous activation energies of 0.5% PAM/AA-Na and 0.5% PAM/VSS-Na solutions as function of CaCl2 concentration
Fig.4 also shows the viscous activation energies of both solutions increase sharply at 0.5 mmol/L CaCl2 concentration. This may indicate the interactions between Ca2+ and molecules of PAM/AA-Na or PAM/VSS-Na are related to Ca2+ concentration. Because Ca2+ could associate with amide group and carboxylic group, and could compress the electrical double layer of charged group, and capture the hydrated shell around polar groups, and screen the electrostatic charges. These interactions may be competitive and related to temperature. As a result, when CaCl2 concentration is less than 0.5 mmol/L, the viscous activation energies of both solutions are lowered because some or all of these interactions may be less owing to lacking enough Ca2+ in solutions.
4 Conclusions
1) The apparent viscosities of PAM/AA-Na and PAM/VSS-Na aqueous solutions decrease as Ca2+ concentration, temperature and shear rates increase. Both solutions exhibit shear-thinning effect at high shear rates and behave as non-Newtonian fluids when shear rates increase.
2) The reduction of apparent viscosity of PAM/VSS-Na solution is lower compared with that of PAM/AA-Na solution as salt concentration and shear rates increase, which shows that PAM/VSS-Na has better performance on the salt tolerance and shear endurance.
3) The viscous activation energy of PAM/VSS-Na is less than that of PAM/AA-Na in CaCl2 solutions at each concentration, which indicates that PAM/AA-Na is more sensitive to temperature in salt solution. And the viscous activation energy of PAM/VSS-Na is more than that of PAM/AA-Na in pure aqueous solution. This may be because that charged groups in PAM/VSS-Na molecules are more crowded than that in PAM/AA-Na molecules when charges are not screened by other ions.
4) Viscous activation energies of both solutions increase sharply at 0.5 mmol/L CaCl2 concentration, which is probably because that the interactions between Ca2+ and copolymers are competitive and related to temperature.
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(Edited by LONG Huai-zhong)
Foundation item: Project(0610005) supported by the Natural Science Foundation of Jinan City, China; Project(200603085) supported by the Postdoctoral Research Foundation of Shandong Province, China
Received date: 2008-06-25; Accepted date: 2008-08-05
Corresponding author: TAN Ye-bang, Professor; Tel: +86-531-88363502; E-mail: ybtan@sdu.edu.cn
