Summary

Screen-factor (SF) measurements are widely used in the petroleum industry to characterize polymer solutions. The measurements are easy to perform and provide information that is different from solution-viscosity measurements. However, there has been no quantitative explanation of what solution property is being measured by SF. We show that SF measures the elongational viscosity of a polymer solution. Experiments on a modified commercial screen viscometer show the relationship between SF measurements and elongational-flow measurements performed by Durst and coworkers. Durst has shown that the elongational flow field in a packed bed of spheres triggers a transition in the conformation of a flexible polymer molecule, such as polyacrylamide, from a coiled to a stretched state. This transition in conformation is accompanied by a jump in the resistance to flow by an order of magnitude. We show that conventional screen viscometers operate in the regime where the molecules are in the highly stretched state. On the basis of Durst's work, it is calculated that the SF measurement is sensitive to high-molecular-weight tails in the polymer molecular-weight distribution.

Introduction

Dilute aqueous solutions of polyacrylamide have been used extensively in EOR as mobility-control agents.1 Consequently, many studies have been conducted to evaluate the properties of polyacrylamide solutions. Two of the most common techniques are intrinsic-viscosity and SF measurements.

A major disadvantage of the intrinsic-viscosity measurement is the time-consuming and tedious experiments that must be performed to allow extrapolation of data first to zero shear rate and then to zero polymer concentration. This lengthy process must be carried out if meaningful results are to be obtained for high-molecular-weight polymers.2,3 SF measurements offer a simple, rapid characterization method for dilute polymer solutions that correlate well with end-use performance.1

Despite the common use of SF measurements, there is no consensus about what solution properties the SF measures. It is generally agreed that the SF is much more sensitive than the intrinsic viscosity, [µ], to differences in molecular weight, M, and molecular-weight distribution (MWD). In a recent experimental investigation, Seright and Maerker4 correlated SF with MWD. The MWD's of three fluorescently tagged polyacrylamide samples were obtained from ultracentrifugation experiments. A regression analysis of the SF and MWD data resulted in

  • Equation 1

where ci is the concentration of Polymer Species i, Mi is the molecular weight of Species i, and k and x are parameters.

Parameter x had values of 3.35,3.59, and 2.16 for the three polyacrylamides tested by Seright and Maerker. Their data thus establish that the SF is sensitive to the high-molecular-weight components of the polymer sample. Seright and Maerker attributed this MWD sensitivity to the viscoelastic properties of polyacrylamide solutions in the presence of large elongation rates encountered in the converging and diverging flow present in the screen viscometer.

Our analysis of the flow-resistance mechanism occurring in the screen viscometer is based on the work in Refs. 5 through 9, where investigations were made of the dramatic increase in resistance to flow through porous media that accompanies the flow-induced transition in polymer conformation from a random coil to a stretched state. The experiments presented show the relationship between the SF and Durst's observations.

Investigators5 have performed studies of the importance of the elongational flow fields that exist during the flow of dilute polymer solutions through randomly packed beds of uniform-sized beads. In these studies, polyacrylamide solutions of M=18.3×106, [µ] = 1136 cm3/g, at concentrations of 12.5,25, and 50 ppm in ethylene glycol, and in various aqueous salt solutions were passed through beds packed with 392-, 762-, 1396-, and 6556-µm-diameter beads.

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