Summary
Laboratory data are used to show that commercial polyacrylamides hydrolyzeto an equilibrium degree that depends on the temperature of hydrolysis but islargely independent of the brine composition. At greater than 20 ppm hardnesslevels, polyacrylamide solutions pass through a sharp cloud point as theirtemperature is raised. This cloud-point temperature depends primarily on thehardness level of the brine and the degree of hydrolysis of the polymer, withlesser dependency on polymer molecular weight and polymer concentration. Indications are that these cloudy solutions cause pluging of porous media. Therefore, a polymer solution is potentially useful only below its cloud-pointtemperature. For application in a given reservoir, the temperature andfrequently the hardness of the water are fixed. If a polyacrylaniide hydrolyzesat reservoir conditions to where its cloud point in the field water falls belowthe reservoir temperature, it is not suitable for polymer flooding in thatreservoir. Cloud-point data, in conjunction with rate-of-hydrolysis data, indicate a "safe" limit of approximately 75C [167F] for brines containing 2,000ppm hardness and above, increasing to around 88C [190F] at 500 ppm, 96C [205F]at 270ppm, and at least 204C [400F] at 20 ppm and below. Most unsoftenedinjection waters will limit polyacrylamide use to below 93C [200F]. Mostproduced waters will limit applicability to below 82C [180F].
Introduction
Partially hydrolyzed polyacrylamides used for EOR are Partially hydrolyzedpolyacrylamides used for EOR are known to be sensitive to temperature anddivalent ions. The amide groups present in these polymers will hydrolyze inaqueous solutions to an extent that depends on pH and temperature. Theresultant more hydrolyzed poly-acrylamide may have a degree of hydrolysissufficient to poly-acrylamide may have a degree of hydrolysis sufficient tocause precipitation in the reservoir or injection water used.
Several investigations have dealt with this precipitation mechanism ofpolyelectrolytes in hard brines. It is widely accepted that precipitation isthe result of interaction between divalent cations and the carboxylate groupspresent within the hydrolyzed polymer. Strong site present within thehydrolyzed polymer. Strong site binding apparently occurs between the divalentcation and two carboxylate groups on the polyion. Whether actual precipitationoccurs, however, depends on temperature. precipitation occurs, however, dependson temperature. If the temperature of a solution of polyacrylamide in watercontaining divalent cations is gradually raised, the solution generally willsuddenly turn cloudy at a well-defined temperature. If the temperature israised further, precipitation follows. precipitation follows. This cloud point, then, represents a stability limit for that particular polyacrylamide in thatparticular brine. For EOR purposes, the particular polymer/brine combination ispotentially useful only below its cloud point. As polymer flooding becomes morewidespread, there is a polymer flooding becomes more widespread, there is atendency for polyacrylamides to be applied at higher temperatures, sometimes inbrines containing significant hardness levels. It has not been clear how farthe temperature/hardness limits can be pushed without polymer precipitation. precipitation. We attempt to define some of these limits through an extensiveseries of cloud-point measurements for commercial polyacrylamides, coupled withdata on the rate of hydrolysis as a function of temperature. These are combinedto predict precipitation times as a function of hardness level and temperature, thereby providing guidelines for the use of polyacrylamides in EOR.
Experimental
Materials.
All the polymers tested were acrylamide polymers of commercialorigin, which were used as received polymers of commercial origin, which wereused as received or were thermally hydrolyzed to desired extents before use. The salts used in this study were reagent grade.
Sample Preparations. Because thermal stability of polyacrylamides isimpaired in the presence of dissolved polyacrylamides is impaired in thepresence of dissolved oxygen, precautions were taken to exclude oxygen frompolymer solutions. A glove box maintained under an polymer solutions. A glovebox maintained under an oxygen-free helium atmosphere was used for all samplepreparation and handling. The presence of oxygen in the preparation andhandling. The presence of oxygen in the box was monitored with a solution ofdiethyl zinc. A 5 % sodium chloride solution was sparged for 15 to 20 minuteswith nitrogen from which all oxygen had been removed. This purging time wassufficient to reduce the dissolved oxygen content of the water to less than 10ppb as measured by a Chemet TM dissolved-oxygen kit.
