An experimental study on the role and contribution of the first normal stress difference and elongational viscosity in immiscible viscoelastic Saffman-Taylor instability

Abstract

This study experimentally examined the immiscible viscoelastic Saffman-Taylor instability (STI) in porous media. The viscoelastic displacing fluids are solutions of glycerin, water, and polyacrylamide with concentrations of 100, 200, and 400 ppm, and the immiscible displaced fluids are silicone oils with different viscosities. The viscosity of viscoelastic liquids is rather constant, so they could be considered Boger liquids The main innovative aspect of the present study is the analysis of the role and contribution of the first normal stress difference and elongational viscosity in the immiscible viscoelastic displacement. In this study, instead of using the simple geometry of the Hele-Shaw cell, some more applied results for viscoelastic STI in the low permeability transparent packed bed porous media have been presented. Another novel aspect of the current study is the analysis of the effect of different parameters, such as elasticity, flow rate, and viscosity ratio, on the viscoelastic STI for the case of immiscible displacement, experimentally. The results of the injection of Boger liquids were compared with the injection of equivalent Newtonian fluids to investigate the effect of fluid elasticity. Furthermore, the effects of flow rate and viscosity ratio on the flow instability were studied in detail by analyzing images of the propagation of finger-like patterns and the results of mixing length and sweep efficiency. The results indicate that the contribution of the first normal stress difference to the stabilizing effect of polymeric additives is less than 18% for the different scenarios of Boger liquid injection. Thus, the elongational viscosity of displacing fluid has the main role in enhancing the sweeping of the displaced phase. When decreasing the polymer concentration and increasing the flow rate, the contribution of the elongational viscosity on STI can become more important compared to the first normal stress difference.