Correlation of Mobility Reduction of HPAM Solutions at High Velocity in Porous Medium with Ex-Situ Measurements of Elasticity

Abstract

Summary When injected at high flow rates in a porous medium, polymer solutions exhibit a resistance to flow that is a signature of chain conformation and size. For biopolymers, which exist in solution as semirigid rods, mobility reduction follows the shear-thinning behavior measured in shear flow on a rheometer. For flexible coils, such as hydrolyzed polyacrylamide (HPAM), flow thickening is observed in a porous medium, whereas bulk viscosity presents a shear-thinning behavior. This difference is the result of the complex flow experienced in the porous medium, combined with the viscoelastic properties at large strains of the solutions. In this study, we investigate the effect of physicochemical parameters such as salinity, polymer concentration, molecular weight, and degradation state on the mobility reduction in a porous medium at high flow rates. All the experiments are performed on a short-length, 4-darcy sintered ceramic core. The bell shape of the mobility-reduction curves (mobility reduction vs. flow rate) is characterized by three parameters: the onset rate of flow thickening (QC), the maximum of mobility reduction (Rmmax), and the flow rate at which this maximum occurs (Qmax). Curves are rescaled by use of the two groups, Rm/Rmmax and β×Q, where β accounts for the shift in Qmax when physicochemical conditions are varied. After rescaling, all the normalized mobility-reduction curves are superposed. We show that the two parameters Rmmax and β are not correlated with the bulk viscosity of the solutions but rather with their elasticity evaluated through screen-factor measurement. This old and rough measurement, widely used in the enhanced-oil-recovery (EOR) community to evaluate “solution elasticity,” is an indirect measurement of the extensional viscosity of polymer solutions. The pertinence and the physical meaning of this rough measurement are assessed through comparison with measurements performed on a newly developed extensional viscometer [EVROC™ (Extensional Viscometer/Rheometer On a Chip), RheoSense, Inc., San Ramon, California, USA], which consists of measuring the pressure drop when the fluid is injected through a hyperbolic contraction (in which the strain rate is constant at the centerline). A correlation of “screen factor” vs. “extensional viscosity” is obtained. These results give some insight on the behavior of polymer solutions in injectivity conditions along with a method to characterize their elastic properties from bulk measurements. Finally, the inadequacy of traditional small-strain viscoelastic measurements to characterize the elastic behavior of polymer solutions at large strain is discussed.