## 1 Introduction  
In the primary recovery stage, oil can be mainly produced using natural energy sources, such as reservoir pressure. As oil production progresses, reservoir pressure decreases, and economical oil production is no longer feasible. To reduce the decline of reservoir pressure and push the oil in the reservoir to the production well, water flooding (secondary recovery), which is the process of injecting water into the reservoir, has been commonly applied to most oil fields. The primary and water flooding methods typically extract no more than 10%-40% of the original-oil-in-place [1]. One of the main reasons for oil to remain after water flooding is the high mobility of the injected water induced by its low viscosity. Mobility is defined as the effective permeability of the reservoir divided by the viscosity of the phase, which is water in this case [2]. The high mobility of injected water compared to oil results in low volumetric sweep efficiency and a large amount of remaining oil. Volumetric sweep efficiency is the volume of oil in contact with the displacing agent divided by the volume of the original-oil-in-place [3]. One of the effective methods for increasing volumetric sweep efficiency and oil production is the injection of viscous fluids. Polymer flooding, which is a technique for enhanced oil recovery (EOR), is the process of injecting a viscous polymer solution into the reservoir. Polymer flooding is now considered to be a technically- and commercially-proven EOR method, especially since its large-scale application at Daqing oil field in northern China, which produced about 300,000 barrels of incremental oil per day [4]. The viscosity of the polymer solution is the key property for the successful application of polymer flooding. The required viscosity of the polymer solution, which can effectively push viscous oil from the pores of the reservoir rock to the production well, can be calculated using water saturation in the reservoir, the relative permeability of each phase (oil and water) and oil viscosity [2]. To ascertain whether the viscosity of the available polymer solution can reach the required viscosity given the injection and reservoir conditions, the apparent viscosity (also known as in situ viscosity), which is the viscosity of the polymer solution in a porous medium, such as the reservoir, needs to be calculated. Apparent viscosity cannot be measured directly. Instead, it is calculated according to [PERSON]'s equation using data from core flood experiments. However, core flood experiments require much time and specialized expertise. The apparent viscosity of polymer solutions has usually been calculated using the viscosity in a bulk solution; this viscosity of a polymer solution in the bulk state can be measured or estimated. Experimental measurement using a rheometer is the most reliable method, but it is a time-consuming process. The measurement requires experience and skill with a special apparatus for the preparation of a homogeneous polymer solution. Generally, the time required for mixing with an EOR polymer is 2-4 days per solution sample. The molecular chain of an EOR polymer can be easily agglomerated and generates a micro-gel; therefore, identifying the homogeneity of the polymer solution through filtration testing is required. Furthermore, the viscosity of a polymer solution is affected by various parameters at the same time in a complex way. Many researchers [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21] found that the viscosity of polymer solutions for EOR has a non-linear relationship with the molecular weight, degree of hydrolysis, concentration of polymer, cation concentration of polymer solution, shear rate and temperature. Considering the relatively long time required for measurement and the large number of samples induced by various influential parameters, the range of samples and the measurement conditions need to be narrow and focused. Viscosity estimation of the polymer solution is effective for these purposes.