Study on Viscosity Retention of Polymer System and Its Application in a Bohai Oilfield
Authors: Liu Bin , Yue Baolin , Zhang Wei , Chen Cunliang , Zhu Zhiqiang , Song Xin , Lv Hongtao 
Published: March 2021

Abstract
Due to the high viscosity of crude oil and high water-oil mobility ratio, water channeling is serious in a Bohai oilfield. Polymer flooding has been carried out in the oilfield, and good results have been achieved. When polymer flooding is implemented in the oilfield, only the wellhead viscosity of polymer is known, but the viscosity of polymer system in the formation is not accurately known. The viscosity of polymer system in the formation is an important parameter for polymer flooding effect and later polymer injection parameter optimization. Due to the lack of data and the difficulty of operation in offshore oilfield, it is urgent to study the viscosity retention of polymer after being sheared from the borehole. The flow of polymer solution is divided into two stages. The first flow stage is that the flow of polymer solution in the wellbore is equivalent to the flow of an equal diameter circular pipe. Assuming that the solution system is incompressible and is one-dimensional stable flow, the mathematical model of apparent viscosity is established by momentum theory and the constitutive equation of pseudo-plastic fluid. Finally, the apparent viscosity and shear rate of the solution system are calculated by the mathematical model, which keep unchanged along the flow direction in the equal diameter circular pipe. The second flow stage is that the flow process of polymer solution through the borehole is equivalent to the flow process of shrinkage and expansion in a variable cross-section pipeline. The viscosity mathematical model of the solution system after the borehole shearing is established. The viscosity retention is calculated by the mathematical model, and the influence of perforation radius and other indexes on formation working viscosity is analyzed. The results show that the viscosity retention of the polymer system is 34.1%∼36.9% by using the new model. Through the analysis of the influencing factors, it is concluded that the consistency coefficient and perforation radius have the greatest influence on the viscosity retention. By applying the calculated viscosity retention obtained by the new model to the numerical simulation, the water cut history fitting of single well is improved. Due to the same concentration injected in the whole oilfield, the effect of polymer flooding in some areas is not obvious. The viscosity of polymer in the formation is calculated by the new model. After concentration optimization and adjustment, the concentration of polymer injection in three wells increases from 800mg/L, 1000mg/L and 1200mg/L to 1500mg/L respectively, and the oil production of the surrounding production wells increases significantly, and the daily oil production increases 105m3. The new technology has been widely used in five wells of other two oilfields in Bohai Oilfield. On the basis of calculating the viscosity retention rate, good results have been achieved by adjusting the injection concentration, and the total oil increase has reached 5×104m3. There are some assumptions in the calculation of this technology. In the future, the fluid flow will be further studied under the condition of removing the assumptions.