Molecular Degradation, Injectivity, and Elastic Properties of Polymer Solutions

Abstract

Summary. New features of polymer rheology in porous media were observed when hydrolyzed polyacrylamide (HPAM) solutions were flowed through sandstone, gravel packs, and glass-bead packs at high rates. Independent measurement of elongational viscosity, with a ductless-siphon technique demonstrates that the enhanced resistance seen at high flow rates is not directly proportional to . Furthermore, significant permeability trends in resistance factor and mechanical degradation are observed when plotted vs. strain rate. When these results are translated to field flow rates and geometries, they indicate that HPAM solutions can be injected at reasonable injection pressures with minimal viscosity losses, provided that perforated completions are designed with either sufficient perforation density or perforation size. perforation size. Introduction This paper covers several independent topics related to the flow of polymer solutions at high rates through porous media. High flow rates typically occur near injection wells, causing certain problems for polymer injection. Solutions of flexible polymer molecules, such as HPAM, are mechanically degraded and lose viscosity (or screen factor) during flow through porous media at high rates. Flexible polymers also exhibit enhanced resistance to flow in porous media, polymers also exhibit enhanced resistance to flow in porous media, beyond the resistance attributable to shear viscosity, which can reduce injectivity at high rates. Both mechanical degradation and enhanced flow resistance are associated with a viscoelastic polymercoil deformation in elongational flow of sufficient strength. polymercoil deformation in elongational flow of sufficient strength. Solutions of rigid polymer molecules (such as xanthan) do not show this behavior. We present data that relate enhanced resistance and mechanical degradation of polyacrylamide solutions to flow rate and permeability in porous media. The role of elongational viscosity is permeability in porous media. The role of elongational viscosity is examined, and a technique is discussed for determining the elongational viscosity of polymer solutions. Finally, our findings are applied to the practical problem of injecting polyacrylamide solutions through wellbores with perforated completions. Flow Fundamentals Intrinsic viscosities of polymers used for mobility control in water-flooding and chemical flooding typically range from 10 to 100 g/dL, and solution concentrations are usually in the range of 200 to 2,000 ppm. In this semidilute solution regime, the polymer coils overlap and polymer/polymer interaction forces are often significant but strong entanglements do not form. Fluid properties depend on single-chain polymer dynamics. The effect of polymer/polymer interaction, when present, is a nonlinearity in the concentration dependence of certain properties, such as viscosity. Under these conditions, the Newtonian low-shear-rate) viscosity in a pure shear flow (e.g., flow in a Brookfield viscometer) depends on the average hydrodynamic radius of the polymer coil. (For infinitely dilute solutions, this relationship is given by the Einstein equation.) Thus, polymers that have very different structures-e.g., xanthan and polymers that have very different structures-e.g., xanthan and HPAM-will produce solutions of similar viscosity if the equilibrium coil dimensions and the number of polymer molecules per unit volume are similar. Flow in porous media, however, is not a pure shear flow; convergence and divergence give rise to strain components of the velocity-gradient tensor as the fluid travels through pore spaces. In straining flows of sufficient strength, flexible-coil polymers such as HPAM undergo a coil-stretch transition, resulting in highly extended conformations. Large coil extensions are accompanied by the building of large extensional stresses within the fluid, and the resistance to flow at high rates in porous media can be much greater than the Newtonian shear viscosity would suggest. Inflexible polymers like xanthan, however, have only a limited capacity for polymers like xanthan, however, have only a limited capacity for stretching, and straining flows of these polymer solutions do not build large extensional stresses. Shear stresses dominate the flow behavior of inflexible polymers in porous media, and the resistance to flow at high rates follows the shearing trends found in high- shear-rate viscometry experiments. The apparent dilatant behavior of solutions of flexible-coil polymers when flowing in porous media at high rates is well known. polymers when flowing in porous media at high rates is well known. Early workers recognized that straining flow in pore convergences can induce a viscoelastic polymer response. A strain rate for porous-media flow was defined as .........................(1) where v is the average interstitial fluid velocity and dp is the average grain diameter. The onset of dilatant behavior was found to occur when the Deborah number, defined as .............(2) where tp is the polymer relaxation time, exceeded a critical value. Interthal and Haas and Haas et al. found that the transition to high values of resistance factor (the product of friction factor and Reynolds number) at a critical flow rate can be described by a master curve when a specially defined "effective viscosity," which is a reduced resistance factor, is plotted as a function of Deborah number. Furthermore, they observed that the maximum value of effective viscosity is independent of permeability. In a subsequent work, the importance of the elongational character of flow in porous media was demonstrated by comparing the experimentally porous media was demonstrated by comparing the experimentally measured effective-viscosity curves to theoretical predictions of the elongational viscosity of a polymer solution in uniaxial extensional flow by use of the finitely extensible, nonlinear elastic (FENE) dumbbell model. The similarity of measured effective viscosity, in both porous media and screen viscometers, to the FENE dumbbell predictions has led to a direct association of effective viscosity in porous media with true elongational viscosity. We present the results of experiments designed to re-examine both the present the results of experiments designed to re-examine both the permeability dependence of resistance-factor maxima and the permeability dependence of resistance-factor maxima and the relationship between resistance factor and elongational viscosity. Irreversible degradation of solution viscosity is known to occur in conjunction with the viscoelastic response described above. This mechanical degradation occurs through polymer-chain scission induced by high fluid stresses. P. 1193