Mechanical Degradation Of Polyacrylamide Solutions In Porous Media
Authors: Morris C.W. , Jackson K.M. 
Published: December 2007

This paper presents experimental data showing that shear degradation is dependent upon polymer concentration and that the degree of degradation is dependent on the polymer stretch rate, the flow-path length, and polymer molecular weight. Shear degradation was studied by injecting polymer solutions through consolidated sandstone plugs of varying lengths and permeabilities for a broad range of injection rates. The degree of shear degradation was found to depend only on the polymer concentration and a single quantity involving the stretch rate and molecular weight of the polymer. This quantity is proportional to the product of the stretch rate and proportional to the product of the stretch rate and the flow distance raised to a power proportional to the molecular weight of the polymer. The power proportionality constant was found to be 107 for all proportionality constant was found to be 107 for all polymer concentrations studied. The dependence on polymer concentrations studied. The dependence on only these two quantities was seen to be valid in unconsolidated as well as consolidated sandstone. This observation can extend current capability of predicting shear degradation in consolidated porous predicting shear degradation in consolidated porous media and simplify the prediction in unconsolidated porous media. porous media. Shear degradation characteristics of polyacrylamide solutions were previously considered to polyacrylamide solutions were previously considered to be independent of polymer concentration. This paper presents data indicating an apparent dependence of presents data indicating an apparent dependence of shear characteristics on polymer concentration. Mechanical degradation effects are shown, for example, to increase with decreasing concentration. The higher-concentration polymer solutions, therefore, would be expected to show the least change in measured flow properties.

Introduction

Numerous studies show that partially hydrolyzed polyacrylamide solutions suffer mechanical degradation during the injection into and flow in porous media. These solutions, whether utilized in porous media. These solutions, whether utilized in secondary or tertiary oil-recovery applications, are designed to provide mobility control for the displacing fluids. The mechanical degradation of the polymer solution daring injection and flow in porous polymer solution daring injection and flow in porous media may reduce severely its ability to provide mobility control in the reservoir.

Maerker has investigated the mechanism for mechanical degradation of polyacrylamide solutions and has developed a procedure for predicting the loss of mobility control. However, the data presented by Maerker applies to relatively low-concentration solutions of a polymer; i.e., 300-ppm and 600-ppm polyacrylamide solutions were employed in the polyacrylamide solutions were employed in the laboratory experiments. This paper extends the investigation of mechanical shear degradation of polymer solutions and presents laboratory data on the polymer solutions and presents laboratory data on the effects of polymer molecular weight, polymer concentration, core permeability and flow rate. The data were correlated according to the procedure suggested by Maerker. In addition, a procedure suggested by Maerker. In addition, a study was made of the relationship between polyacrylamide solution degradation, as measured polyacrylamide solution degradation, as measured by percent screen-factor loss and percent viscosity loss, and the loss of fluid mobility control in Berea sandstone cores.

EXPERIMENTAL PROCEDURE

The mechanical degradation of polyacrylamide solutions was induced by forcing the solutions through 1 in. diameter core plugs Of consolidated sandstone for a wide range of injection rates. The core plugs cut from North Burbank Unit and McCabe sandstones were 1 in. in diameter and 3 in. in length and exhibited appreciable permeability variations. Berea cores, 18 in. in length, were used in the study of mobility control with sheared polymer solutions.

This study was made with commercially available partially hydrolyzed polyacrylamides described in partially hydrolyzed polyacrylamides described in Table 1. The molecular weights of Polymers A and B were approximately 15 and 10 million, respectively. Two primary concentrations were used in the solutions: 500- and 2,000-ppm polymer by weight. Both types of 2,000-ppm solutions were prepared by adding the dry powder to the water while mixing with a 3-in. Teflon propeller at 1,000 rpm for 5 minutes.