Hydrolysis and Precipitation of Polyacrylamides in Hard Brines at Elevated Temperatures

Abstract

Summary Laboratory data are used to show that commercial polyacrylamides hydrolyze to an equilibrium degree that depends on the temperature of hydrolysis but is largely independent of the brine composition. At greater than 20 ppm hardness levels, polyaerylamide solutions pass through a sharp cloud point as their temperature is raised. This cloud-point temperature depends primarily on the hardness level of the brine and the degree of hydrolysis of the polymer, with lesser dependency on polymer molecular weight and polymer concentration. Indications are that these cloudy solutions cause plugging of porous media. Therefore, a polymer solution is potentially useful only below its cloud-point temperature. For application in a given reservoir, the temperature and frequently the hardness of the water are fixed. If a polyaerylamide hydrolyzes at reservoir conditions to where its cloud point in the field water falls below the reservoir temperature, it is not suitable for polymer flooding in that reservoir. Cloud-point data, in conjunction with rate-of-hydrolysis data, indicate a "safe" limit of approximately 75°C [167°F] for brines containing 2,000 ppm hardness and above, increasing to around 88°C [190°F] at 500 ppm, 96°C [205°F] at 270 ppm, and at least 204°C [400°F] at 20 ppm and below. Most unsoftened injection waters will limit polyacrylamide use to below 93 °C [200°F]. Most produced waters will limit applicability to below 82°C [180°F].