Comparative studies of rheological properties of polyacrylamide and partially hydrolyzed polyacrylamide solutions

The present work is concerned with experimental results of rheological characteristics of polyacrylamide (PAM) and of partially hydrolyzed polyacrylamide (HPAM) (degree of hydrolysis up to 80%) in aqueous and aqueous/sodium chloride solutions with changing experimental conditions such as polymer concentration, temperature, solvent quality, and shear rate applied. It has been observed that the all‐aqueous and aqueous/NaCl solution of PAM and of HPAM exhibited the non‐Newtonian behavior with shear‐thinning and shear‐thickening areas. The onset of shear‐thickening at depends mainly on the degree of HPAM hydrolysis, as well as on solution concentration, temperature, solvent quality, and polymer molecular weight. Rheological parameters from power law (Ostwald de Waele model) and activation energy of viscous flow (E_a) are determined and discussed. The changes in apparent shear viscosity during aging of solutions of PAM and HPAM are also described. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2235–2241, 2007     

Hydrolysis and thermal stability of partially hydrolyzed polyacrylamide in high-salinity environments

Partially hydrolyzed polyacrylamide (HPAM) is the water-soluble polymer most often used in flooding applications in the petroleum industry. However, in aqueous solutions at high temperatures, HPAM undergoes hydrolysis of the lateral amide groups, and the presence of salts in the solution can lead to precipitation of this polymer. Therefore, a method was developed to monitor the thermal stability of HPAM solutions in different saline environments and varying temperatures. The proposed test method involved measurements of intrinsic viscosity as a function of time and determination of the degree of hydrolysis of the HPAM by elemental analysis. The results obtained indicated that the presence of divalent cations (Ca⁺² and Mg⁺²) negatively influenced the intrinsic viscosity of the solutions and in some systems led to precocious precipitation of the polymer in environments with higher concentrations of these cations. The hydrolysis reaction of the amide groups to the acrylate groups of the HPAM chain was significantly affected by rising temperature: at 50 °C, hydrolysis occurred, but not as significantly as at 70, 85, 90, and 95 °C. Hydrolysis up to 84% was observed for solutions processed at 90 °C. The results also indicated limits of hardness for the brine at some temperatures: 1353 ppm for 95 °C and 2867 ppm for 70 °C. For brine containing 13,610 ppm or more of divalent cations, hydrolysis and precipitation of the polymer were not observed at 50 °C. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47793.     

Slow-gelling Cr+3/polyacrylamide solutions for reservoir profile modification: Dependence of the gelation time on pH

Systematic studies of the gelation of both buffered and unbuffered aqueous Cr⁺³/polyacrylamide solutions show that the gelation time is a strong function of pH, increasing by about one order of magnitude per unit decrease in pH for polyacrylamides less than about 7.5% hydrolyzed. The gelation rate also depends strongly on the Cr⁺³ concentration, the degree of polymer hydrolysis, and temperature. At 25°C, gelation delays of more than 10 months have been obtained; the maximum delay observed at 60°C is about 1 month and at 90°C 1 d. The resistance of buffered gel solutions to change in pH results in significantly longer gelation times at 90°C and low pH than for identical unbuffered solutions; at lower temperatures, however, the differences are insignificant. Most importantly, the use of low pH to control the gelation time of Cr⁺³/polymer solutions provides an attractive, inexpensive substitute for the environmentally unacceptable Cr⁺⁶/reductant method currently employed for profile modification treatments.     

Ein Beitrag zur Temperaturabhängigkeit der Viskosität wäßriger Polyethylenglykollösungen

An automatic falling ball viscosimeter of the Höppler type with pneumatic rotary actuator and electrical time measuring was used to determine the viscosity of aqueous polyethylene oxide solutions between the temperature range of 20 to 80°C. For these investigations polyethylene oxide fractions were used having molecular weights of 600 to 20 000 g/mol. The concentration of the solutions was up to 5 base mol-% of polymers. Parameters according to Eyring- and Vogel-equations were evaluated by the measured viscosity data. This evaluation led to the result, that the parameter T₀ decreased linearly with increasing molecular weight and concentration. The effect was explained by the water-structure-breaking by means of the polymer molecules. The logarithmic intrinsic viscosity in the solutions containing water and polyethylene oxide of a molecular weight of 6 000 g/mol decreased rapidly with the temperature and led to a value of ?8.6 · 10^?3 for d In [η]/dT.     

Effect of micro-aggregation behavior on the salt tolerance of polymer solutions

Polymer solution for oil displacement is mostly used in the middle and late stage of water flooding reservoir development, and reservoir groundwater conditions are often one of the main conditions restricting polymer application. Therefore, it is necessary to develop salt tolerance of polymer solutions with different aggregation behaviors, so as to facilitate the synthesis and optimization of suitable polymer systems. The differences in the micro-aggregation behavior of three polymers with different molecular structures were explored. On this basis, the effects of divalent metal cations on the properties of the polymer solutions were analyzed by assessing the micro-aggregation behavior, apparent viscosity, hydrodynamic size, and shear rheological characteristics. The results showed that the linear partially hydrolyzed polyacrylamide (HPAM) was seriously affected by divalent cations, and the viscosity decreased obviously. The aggregation behavior of the polymer changed by hydrophobic association can enhance the salt tolerance of the solution. The hydrophobically modified partially hydrolyzed polyacrylamide (HMPAM) with “chain beam” aggregation behavior has strong intermolecular connection, which enables it to withstand the content of calcium and magnesium ions of 1100 mg l⁻¹. When the content of calcium and magnesium ions exceeds 600 mg l⁻¹, dendritic hydrophobically associating polymer (DHAP) will destroy the interaction between molecular chains, resulting in the decrease of apparent viscosity and hydrodynamic size. For polymer flooding in high-salinity reservoir, salt tolerant polymer system can be constructed by optimizing molecular weight and hydrophobic group content.     

Rheology and polymer flooding characteristics of partially hydrolyzed polyacrylamide for enhanced heavy oil recovery

Polymer flooding characteristics of partially hydrolyzed polyacrylamide (HPAM) solution with the addition of NaOH were examined in homogeneous glass‐bead packs. The heavy oil recovery in unconsolidated sandstone formations by applying the alkali‐polymer flooding was observed. Experimental results showed that HPAM solution was sensitive to temperature, salinity, and alkali, finding that alkali‐polymer solutions are more effective in improving viscosity than conventional polymer solutions. The solution of 0.5 wt % NaOH mixed with 1500 ppm HPAM (12 mol % hydrolysis degree) was found to be the optimal choice, which gives rise to the highest viscosity on the rheological characterization. Flood tests using the alkali‐polymer solution showed an increase in oil recovery by 30% over water‐flooding when the water‐cut reached 95%, indicating that alkali‐polymer could be more effective in improving sweep efficiency than polymer flood. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Molecular weight characterization of poly(acrylamide-co-sodium acrylate). I. Viscometry

Mark–Houwink constants for polyacrylamide and poly(acrylamide-co-sodium acrylate) in 0.2M Na₂SO₄ were measured using eight fractionated samples of polyacrylamide and 26 hydrolyzed polyacrylamide samples. The dependence of K and a on the copolymer compositions was found for the range of acrylate content 6 ～ 40 mol %. A relationship between intrinsic viscosity and acrylate content in the form of square root law was found. Molecular weights of copolymer samples with various compositions were estimated using viscometry with Mark–Houwink equations established in this work. The molecular weights of narrow MWD copolymer samples could be measured with an error of ±5%, whereas those of broad MWD copolymer samples with an error of ±8%.     

Eine η-c-M-beziehung für polymerlösungen

A simple equation is developed for the dependence of the viscosity upon molecular weight and concentration of polymer solutions. The validity of such a simple equation is given if the plot of log η_sp = f (log (c · [η])) is a straight curve for all viscosity values. This is demonstrated for aqueous polyacrylamide solutions. The covered region is M = 12300-6900 000 (g/mol) and concentrations c = 0,1?5 (g/100cm³). The established η-M-c-relationship is:      

Untersuchung zur Flockungsfähigkeit von Polyacrylamid-co-acrylaten (0 – 100 mol-% Acrylamid)

The flocculation activity of poly(acrylic acid), poly(acrylamide-co-sodium acrylates) and polyacrylamide, which were prepared in our laboratory, was investigated. The flocculation speed of polyacrylamide was found to increase with increasing molecular weight. The aqueous polyacrylamide solutions as well as the samples stored in the solid state showed, when M_η ≥ 1,5 · 10⁶ g/mol, a time-dependent decrease of the intrinsic viscosity and simultaneously a decrease of flocculation speed. This was accompanied by a diminution of the radius of gyration. These time-dependent facts were caused by a conformational change of the molecular structure and not by molecular degradation. Linear polyacrylamides were better flocculation agents than branched ones (20 branches) with the same total molecular weight. Poly(acrylamide-co-sodium acrylates) exhibited a maximum of flocculation activity at a comonomer degree of about 70 mol-% acrylate.     

Enzymatic degradation of polyacrylamide in aqueous solution with peroxidase and H2O2

Polyacrylamides are often used in water‐based hydraulic fracturing for natural gas and oil production. However, residual polymer remaining in the fractured rock can limit production. A novel approach for degrading partially hydrolyzed polyacrylamide (HPAM) was investigated using hydrogen peroxide and horseradish peroxidase (HRP). This sustainable HRP/H₂O₂ system degraded the polymer in solution, reducing its viscosity in both pure water and brine solutions. Molecular weight measurements confirmed that the viscosity reduction was due to a significant degradation of the polymer backbone and not primarily by other mechanisms such as amide hydrolysis or rearrangement, and so forth. The reduction in viscosity and molecular weight was first order with respect to H₂O₂ concentration. The kinetics of viscosity reduction and molecular weight are closely correlated which would allow the quicker and simpler viscosity method to help engineer future processes. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44560.     

Conformation of polyacrylamide in aqueous solution with interactive additives and cosolvents

The viscosity of polyacrylamide (PAM) dilute aqueous solutions with NaCl, glucose, and SDS as additives was measured by Ubbelohde viscometry. There was linear relationship between reduced viscosity vs. PAM concentration in aqueous solutions. The Huggins constant k and intrinsic viscosity [η] were used to study the conformation of the polymer chains and the degree of polymer–solvent interaction. In addition, the viscosity of diluted PAM solutions in water with acetone, ethanol, DMF, and ethylene glycol as cosolvent was measured. It was found that the polymer chain conformation contracted as the acetone, ethanol, and DMF cosolvent composition ratio increased, but there was no distinguishing difference between water–ethylene glycol compositions. The solution properties of PAM were used to estimate the swelling properties of PAM gel in the same external conditions, as gel is formed by crosslinking of linear polymer. In good solvent the polymer chain should be expanded, and gel is expected to have large swelling ratio. In water cosolvent systems, when the linear polymer chain underwent coil–globule transition, PAM gel should have volume phase transition under corresponding external conditions. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3122–3129, 2003     

Flow behavior of polyacrylamide solution. III. Mathematical treatment

The flow behavior of polyacrylamide solutions was systematically determined over a wide range of temperatures (20–50°C) and concentrations (20–50 ppm) by using a coaxial cylinder viscometer. The results indicated that the rheological behavior of low-concentration polyacrylamide solution behaves similar to non-Newtonian fluids at all these concentrations. The effect of temperature on the consistency coefficient and flow behavior index of polyacrylamide solution of the different concentrations followed an Arrhenius-type relationship. Moreover, the effect of concentration on consistency coefficient and flow behavior index followed an exponential-law relationship at the temperatures used. The rheological constants for the Arrhenius and exponential-law models were determined. The combined effect of temperature and concentration on the coefficient of dynamic shear stress can be represented by a single equation: shear stress = 2.446 × 10⁻⁷exp(0.0639C + 3613/RT)(shear rate)^{2.337 exp(−0.00707C−245/RT)}. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2784–2789, 2001     

Capillary breakup extensional rheometry of associative and hydrolyzed polyacrylamide polymers for oil recovery applications

High molecular weight polymers used for heavy oil recovery exhibit viscoelasticity that can influence the oil recovery during chemical enhanced oil recovery. Different polymers having similar molecular weight and shear rheology may have different elongation flow behavior depending on their extensional properties. Displacing slugs are more likely to stretch than shear in tortuous porous media. Therefore, it is critical to seek an analytical tool that can characterize extensional parameters to improve polymer selection criteria. This article focuses on the extensional characterization of two polymers (hydrolyzed polyacrylamide and associative polymer) having identical shear behavior using capillary breakup extensional rheometer to explain their different porous media behavior. Maximum extensional viscosity at the critical Deborah number and Deborah number in porous media classified the associative polymer as the one having high elastic‐limit. Extensional characterization results were complemented by significantly higher pressure drop, marginally increased oil recovery of associative polymer in porous media. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46253.     

Big effects of small nanoparticles on hydrophobically modified polyacrylamide in an aqueous solution

In this study, we investigated the self-assembly properties of small-sized (3–5 nm) oppositely charged SiO₂ nanoparticles (NPs) and hydrophobically modified polyacrylamide (hm-PAM) in aqueous solutions by rheological and structural analysis. The results show that the addition of small NPs to the hm-PAM solution remarkably enhanced its viscosity and viscoelasticity, and the viscosity remained high at elevated temperatures. Specifically, the low shear rate viscosity of the NP–hm-PAM solutions increased by up to three orders of magnitude at a very low loading of NPs (10⁻² wt %). Moreover, there appeared to be no sediment in the dispersed system after over 18 months of storage. Scanning electron microscopy and transmission electron microscopy measurements indicated that additional physical crosslinks were formed after the addition of the small silica NPs to the hm-PAM solutions. With the experiments and published results combined, we proposed a simple model of electrostatic and hydrophobic interactions for the crosslinking structure of the NP–hm-PAM composite. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47269.     

Statistical and rheological evaluation of the effect of salts in partially hydrolyzed polyacrylamide solutions

Partially hydrolyzed polyacrylamide (PHPA) is the most common anionic copolymer used in Enhanced oil recovery (EOR), but the use of this polymer presents some limitations in the presence of divalent cations. The objective of this work was to evaluate the thermal stability of PHPA in brines containing Na⁺, Mg²⁺, and Ca²⁺ cations (isolated or combined). In this study, the PHPA used at a concentration of 2500 mgL⁻¹ has a MW of 20 × 10⁶ Da. The stability of the polymeric solutions was monitored through rheological analyses for 180 days at 70°C, in the absence of oxygen, using a central composite rotational design. In the absence of dissolved oxygen and cations, the PHPA solution was basically the same as that observed over a period of 30 days. It was observed that Ca⁺² ion had the greatest influence on reducing the viscosity of PHPA in all cases. Polymeric solutions with Total dissolved solids (TDS) greater than 1000 mg L⁻¹ showed complete degradation of the polymer in 180 days. The statistical data corroborate the rheological results, showing that only the main effect of Na⁺ was not statistically significant and that the concentrations of Mg²⁺ and Ca²⁺ presented statistically significant effects in their linear components and quadratic components.     

Thermoviscosifying polymer used for enhanced oil recovery: rheological behaviors and core flooding test

Polymer flooding represents one of the most efficient processes to enhance oil recovery, but the poor thermostability and salt tolerance of the currently used water-soluble polymers impeded their use in high temperature and salinity oil reservoirs. Thermoviscosifying polymers (TVPs) whose viscosity increases upon increasing temperature and salinity may overcome the deficiencies of most water-soluble polymers. A novel TVP was studied in comparison with traditional partially hydrolyzed polyacrylamide (HPAM) in synthetic brine regarding their rheological behaviors and core flooding experiments under simulated high temperature and salinity oil reservoir conditions (T: 85 °C, and total salinity: 32,868 mg/L, [Ca²⁺] + [Mg²⁺]: 873 mg/L). It was found that with increasing temperature, both apparent viscosity and elastic modulus of the TVP polymer solution increase, while those of the HPAM solutions decrease. Such a difference is attributed to their microstructures formed in aqueous solution, which were observed by cryogenic transmission electron microscopy. Core flow tests at equal conditions showed an oil recovery factor of 13.5 % for the TVP solution versus only 2.1 % for the HPAM solution.     

Unperturbed dimensions of polyacrylamide in salt-water-methanol mixtures

The unperturbed dimensions of polyacrylamide are determined by light scattering measurements, in methanol-water solutions and correspond to: R_G?=0.39±0.01 M^0.5. This result is compared to that obtained from viscosity measurements in aqueous solution, through Stockmayer-Fixmann method. A possible explanation based upon the recent blob theory of polymers is proposed for the observed differences.     

Mark–Houwink–Sakurada constants and dilute solution behavior of heterodisperse poly(acrylamide-co-sodium acrylate) in 0.5M and 1M NaCl

Mark Houwink coefficients and molecular size parameters are presented for 31 unfractionated samples of polyacrylamide and hydrolyzed polyacrylamide in 0.5M and 1M aqueous NaCl. Polymers investigated include polyacrylamide, plus 10, 20, 50, 70, and 100 mol % poly(sodium acrylate). Their solution characteristics were studied by gel permeation chromatography, intrinsic viscosity, and light scattering techniques. A density function and heterodispersity index was assigned to each polymer sample due to the polydispersity of the molecular weight distributions. This technique improves the intrinsic viscosity–molecular weight correlation for heterodisperse commercially synthesized polymer systems. The intrinsic viscosity of these commercial compounds will yield useful estimates of their molecular weight, in spite of their obvious heterogeneity.     

Synthesis,characterization, rheological behavior,and shear response of hydrophobically modified polyacrylamide and network structure of its microhydrogel

Octylphenol polyoxyethylene acrylate (OP‐10‐AC) was synthesized, and then OP‐10‐AC was copolymerized with acrylamide (AM) to form hydrophobically modified polyacrylamide P(AM/OP‐10‐AC) through micellar copolymerization. OP‐10‐AC content and rheology behavior of P(AM/OP‐10‐AC) were investigated in detail. Especially, under the conditions of different test methods, P(AM/OP‐10‐AC) showed interesting shear responsive behavior. The results of rheology study show that OP‐10‐AC content, polymer solution concentration, salt solution concentration, and different test methods powerfully influenced shear viscosity of aqueous solutions of P(AM/OP‐10‐AC). In addition, according to the dynamic shear experimental results, the critical hydrogel concentration range (CHCR) could be confirmed for aqueous solutions of P(AM/OP‐10‐AC). Above CHCR, these polymer solutions were essentially a kind of microhydrogels, which could explain the effect of concentration and hydrophobe content on their shear viscosity and viscoelasticity from the microstructure's point of view. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

The effect of high molecular weight polyacrylamide on the sedimentation behavior of kaolin in the presence of hydroquinone

The effect of high molecular weight polyacrylamide (PAAM; 5×10⁶) and hydroquinone (HQ) on the sedimentation rate of kaolin suspensions was investigated by sedimentation, viscosimetric and UV spectrophotometric measurements. The sedimentation rates decreased exponentially with increasing HQ concentrations. PAAM solutions were prepared by dissolution of the same quantity of PAAM in aqueous HQ solutions with different concentrations. Viscosity of these solutions was observed to decrease exponentially with increasing HQ concentrations. Both the decrease in sedimentation rate and the viscosity were attributed to the reduction of the size of the macromolecular coil resulting from the breaking of intra‐ and intermolecular hydrogen bonds by HQ molecules. UV spectrophotometric results of the PAAM and HQ solutions were also in good accordance with the sedimentation and viscosimetric results.