High Temperature and Hardness Stable Copolymers of Vinylpyrrolidone and Acrylamide

Abstract

The preparation and solution properties of poly(vinylpyrrolidone-co-acrylamide), synthetic water-soluble copolymer which has non-precipitating behavior in hard brines at high temperatures, is discussed. One sample of the copolymer has remained in synthetic seawater for over six years at 121°C (250°F) without precipitation. In that time the viscocity of its seawater solution has remained constant. It is recognized that while polyacrylamide is a highly effective water thickener, it rapidly hydrolyzes at high temperatures. In the presence of divalent cations, such as Ca++ or Mg++, hydrolyzed polyacrylamide will precipitate and lose viscocity. The usefulness of polyacrylamide is restricted to applications in which temperatures of less than about 75°C are encountered.

Polyvinylpyrrolidone, on the other hand, has been shown to resist hydrolysis at elevated temperatures. Unfortunately, polyvinylpyrrolidone lacks the viscosifying power to be economically useful in enhanced oil recovery processes.

We have found that copolymers of vinylpyrrolidone and acrylamide can be effective polymers for oil recovery applications. We anticipate the greatest use will be in operations in which high temperatures are encountered. These conditions are now commonly called hostile environments. Vinylpyrrolidone-acrylamide copolymers, on aging in hard brines at high temperatures, will undergo some hydrolysis of the acrylamide moieties, but the degree of hydrolysis is limited. The presence of vinylpyrrolidone in the copolymer apparently restricts the level of hydrolysis.

An equimolar copolymer of these monomers will hydrolyze about 40% of the available acrylamide in months of aging at 121°C. Aged at the same conditions, polyacrylamide will quantitatively hydrolyze. The vinylpyrrolidone must be incorporated in the copolymer to protect acrylamide as a mixture of homopolymers will precipate on aging.

Non-quantitative hydrolysis and viscosity producing power of the copolymers described herein permits design of a copolymer based on the application temperature. Lower temperature applications will require less vinylpyrrolidone in the copolymer. The preparation and general characteristics of vinylpyrrolidone-acrylamide copolymers are discussed. Particular attention is given the relationship of vinylpyrrolidone level and the equilibrium level of acrylamide hydrolysis at 121°, and 150°C.