## 1 Introduction
### Pilot Projects of Polymer Flooding  
Polymer flooding has been performed in many pilot projects, as a response to the poor sweep efficiency of water flooding. The application of polymer flooding is expected to result in additional oil recovery due to the higher aqueous viscosity of polymer. However, formation brine salinity is a major challenge for application of the process at a field scale. The higher salinity of formation brine significantly decreases polymer's in situ viscosity and hence presents economic challenges for such projects [68; 69; 70]. These economic constraints motivate companies to perform low-salinity pre-flush to optimize the process. Water pre-flush is commonly used to decrease the salinity of the formation brine, which is expected to improve polymer flooding performance [36; 39; 41]. * A pilot study performed in Marmul Field (Oman) and reported by [PERSON] et al. [39] describes polymer flooding with low salinity as a pre- and post-flush. The salinity of the formation brine was reported as 3 g/L, and low-salinity pre-flush/post-flush with 0.6 g/L NaCl brine was injected. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**No.** & **Focus** & **Approach** & **Polymer/ppm** &
\begin{tabular}{c} **Molecular** \\ **Weight (MD)** \\ \end{tabular} & **Measurements** & **Reference** \\ \hline
1 & Mechanical degradation & Core flood & HPAM500-1000 & 12-18 & RF/RRF & [PERSON] et al. [19] \\
2 & Mechanical degradation & & Overview & & [PERSON] et at. [31] \\
3 & Mechanical degradation & & Overview & & [PERSON] [52] \\
4 & Mechanical degradation & Pipe flow & HPAM(300 & 28 & DR * (\%)) & [PERSON] et al. [4] \\
5 & Mechanical degradation & Capillary flow & PAM/1500 & N/A & Fluid deformation rate & [PERSON] et al. [58] \\
6 & Mechanical degradation & Pilot test/core & HPAM/700 & 20 & Apparent Viscosity, MWD & [PERSON] et al. [59] \\
7 & Mechanical degradation & Core flood & HPAM500-4000 & 8-18 & RF/RRF & [PERSON] et al. [18] \\
8 & Mechanical degradation & Pipe flow & HPAM/700 & 28 & DR (\%) & [PERSON] et al. [4] \\
9 & Thermal degradation & & Handbook & & [PERSON] and [PERSON] [54] \\
10 & Thermal degradation & & Overview & & [PERSON] [53] \\
11 & Thermal stability & NMR ** & H/1000-1500 & N/A & Male fraction & [PERSON] et al. [60] \\
12 & Thermal/Mech. & Capillary rheometer & PAM/500-2000 & N/A & Viscosity & [PERSON] et al. [61] \\
13 & Thermal/Harsh & Rheology, MW & HPAM2000-5000 & N/A & Viscosity & [PERSON] et al. [62] \\  & & Mechanical degradation & HPAM(800-1400 & 3.5-18.5 & Viscosity loss & [PERSON] et al. [63] \\
14 & Breitne hardness & Capillary shearing & Various/2000 & 11-26 & Degradation & [PERSON] et al. [55] \\
16 & Breitne hardness & Core flood & HPAM(3000) & 28 & Pressure & [PERSON] et al. [10] \\
17 & Mechanical degradation & Core flood & HPAM/1000 & 20 & Normalized effluent viscosity & [PERSON] et al. [64] \\
18 & Brine salinity & Viscometer & HPAM(500-3000) & N/A & Viscosity, RF & [PERSON] et al. [65] \\
19 & Harsh conditions & & Review & & [PERSON] [60] \\
20 & Harsh conditions & Viscometer, Chromatography & N/A/170-180 & 6.5-8.5 & Hydrolysis degree, MW, viscosity loss & [PERSON] et al. [67] \\ \hline \hline \multicolumn{7}{c}{DR *: degradation rate, NMR **: nuclear magnetic resonance, RF: resistance factor, RRF: residual resistance factor.} \\ \end{tabular}
\end{table} 
Table 1: Challenging factors reported in the literature over the last five years, concerning polymer viscoelasticity. A combination of polymer injection with pre- and post-flush resulted in 59% of stock tank oil initially in place (STOIIP). * [PERSON] et al. [40] presented pilot test data of the well-known polymer flooding (pilot one (PO) and pilot two (PT)) application in Daqing Field (China). The formation brine salinity was 7 g/L, and a pre-flush of low-salinity brine of 0.8-1.3 g/L was performed before polymer flooding (1000 ppm concentration of polymer). Interestingly, the produced water salinity was reported in concentrations of 2 to 4 g/L.