Acrylamide-based water-soluble polymers are used in operations such as drilling, water flooding and water conformance. A drawback of these polymers is that the primary amide group is sensitive to hydrolysis at elevated temperatures. This fact sometimes becomes a problem in deep reservoirs, because the solutions properties change when the degree of hydrolysis of the polymer increases and, in a media of high salinity and water hardness, the precipitation of the polymer may occur.
To improve the thermal stability different copolymers have been prepared. The comonomers used for this purpose are mainly N-alkyl substituted acrylamide derivatives and of particular importance N-vinyl pyrrolidone (NVP). The aim of the present work is to evaluate the improvement of the thermal stability of cationic water-soluble polymers based on acrylamide when NVP is introduced as a comonomer, in order to know the amount and kind of incorporation of NVP required to get a polymer useful for applications under severe conditions of temperature.
Several copolymers and terpolymers of acrylamide, a cationic monomer and NVP were prepared. The products were evaluated in terms of the thermal stability of the primary amide group and the fraction of retention of the intrinsic viscosity in aging tests at 248°F. The introduction of NVP limited the hydrolysis of the primary amide groups. When the content of NVP increased in the terpolymers the hydrolysis degree reached after the aging was lower. In addition, the distribution of the monomers in the polymer chain also showed an influence in the improvement of the thermal stability. The results made possible to suggest a mechanism for the stabilization of the amide group by NVP.
These results provide information to prepare a cationic water-soluble polymer that could be used for applications where severe temperature conditions are expected.
Introduction
A disadvantage of acrylamide based polyelectrolytes in aqueous solutions is the sensitivity of the amide group to the hydrolysis reaction1,2,3,4. The hydrolysis of the amide groups takes place readily when polyacrylamides are heated even under moderate pH and temperature conditions5. This can be a limitation for some applications in the oilfield because these polymers are usually employed in environments where temperatures between 170 and 350°F can be experienced in the presence of high levels of salts and water hardness. When the amide group is hydrolyzed, a carboxylate group is produced in the polymer chain (Fig. 1), which results in changes in solution properties. In environments of high hardness the carboxylate groups can bind to the divalent cations, causing a reduction in the molecular dimensions in solution and eventually it may give rises to the precipitation of the polymer3.
Although hydrolysis of the amide groups is highly dependent on pH and temperature, this reaction is relatively slow at room temperature. However at 212°F almost all the amide groups in a polymer are known are hydrolyzed in a few days1,9. Under some conditions degradation of the polymeric chain may also occur. This leads to a reduction in the molecular weight of the polymer, which might affect even further the properties of the polymeric solutions.
To improve the thermal stability many different copolymers have been prepared2,7,8,9. The two most important are sodium-2-acrylamido-2-methylpropane sulfonate (AMPS) and NVP. In the following paragraphs some studies carried out with copolymers of Am with these two monomers are summarized.
AMPS is a secondary amide and it is known that these are less susceptible to hydrolysis than primary amides. In this monomer, the alkyl chain is a gem-dimethyl group, which can impart further protection due to steric hindrance10.
Moradi-Araghi et. al. studied a copolymer of Am with AMPS with mole percent composition Am:AMPS of 63:37 at 121°C. They found that almost all the amide groups were hydrolyzed, and concluded that AMPS did not protect Am against thermal hydrolysis. However, the rate of the hydrolysis was a function of the amount of AMPS in the polymer; the polymers with higher amounts of AMPS hydrolyzed at a slower rate8. Parker et. al.11 determined the hydrolysis of acrylamide units in a copolymer of Am with AMPS, at 90, 120 and 150°C. They mention that the hydrolysis of the acrylamide units in these copolymers are unaffected by the presence of AMPS, even the hydrolysis of AMPS is facile over 120°C.
