Pore-scale experiments reveal distinct flow field of polymer flooding with viscoelasticity loss by high salinity

Abstract

It has been confirmed that high salinity significantly reduces the oil recovery from polymer flooding, which is generally considered to be due to the loss of polymer viscoelasticity. However, how the lost viscoelasticity affects the polymer flow field remains unclear. Therefore, the micro-PIV technique and rheological experiments were utilized in this study to investigate the pore-scale flow field of polyacrylamide (HPAM) under different salinities (NaCl). The results showed that as the salinity increased from 0 g/L to 10 g/L, the storage modulus and loss modulus of HPAM decreased by 84.5%− 97.9% and 87.3%− 98.3%, respectively. Regarding the pore-scale flow field, the average velocity of HPAM under low (0 g/L) and high (10 g/L) salinities was 33.4% lower and 19.5% higher than that of water flooding, respectively. Besides, high salinity resulted a higher velocity ratio between main stream and margin regions (11.8) than that of water flooding (2.0), which suggested a greater heterogeneity and a lower sweep efficiency of the flow field in pore networks. Thus, the lower sweep efficiency with high salinity is one of the dominant causes for the reduction in oil recovery. Furthermore, the breaching in main stream and the plugging in margin regions indicated that the shear resistance stability of polymer solution was diminished under high salinity. Consequently, the flow velocity in the main stream with a higher shear rate increased, whereas the margin region with a lower shear rate was plugged, which reduced the sweep efficiency. This study provides a further understanding of the porous media flow mechanisms in polymer applications.