Flow of polymer solutions through porous media-Prediction of mobility reduction from ex-situ measurements of elasticity

Abstract

When injected at high flow rates in porous medium, polymer solutions exhibit a resistance to flow which is a signature of chain conformation and size. For biopolymers, which exist in solution as rigid rods, mobility reduction follows the shear thinning behavior measured in shear flow on a rheometer. For flexible coils, such as hydrolyzed polyacrylamide, flow thickening is observed in porous medium whereas bulk viscosity presents a shear thinning behavior. These differences are the result of the complex flow experienced in the porous medium combined with the visco-elastic properties at large strains of the solutions. In this study, we investigate the effect of physical chemistry parameters such as salinity, polymer concentration, molecular weight and degradation state on the mobility reduction in porous medium at high flow rates. We show that parameters describing the mobility reduction curve (flow rates corresponding to the onset and to the maximum of the mobility reduction curves, value of the maximum mobility reduction) are not correlated with bulk viscosity but rather with screen factor. This old and rough measurement, widely used in the 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 measurement is assessed through comparison with measurements performed on a newly developed extensional viscometer, which consists in measuring the pressure drop when the fluid is injected through a hyperbolic contraction (in which strain rate is constant at the centerline). A correlation “Screen factor” vs. “Extensional Viscosity” is obtained. Hence, from the knowledge of the mobility reduction curve in one porous medium in one set of conditions, it is possible to predict the mobility reduction in any other set of conditions from ex-situ measurements of screen factor or extensional viscosity. At last, the inadequacy of traditional small strain visco-elastic measurements to characterize the elastic behavior of polymer solutions at large strain is discussed. These results give some insight on the behavior of polymer solutions in injectivity conditions along with the way to characterize their elastic properties from bulk measurements.