Research on optimal timing range for early polymer injection in sandstone reservoir

In this study, lab research was performed to obtain a method to determine the timing for early polymer injection in oilfields, to increase applicability of lab research in actual oilfield management and to identify the optimum timing for polymer flooding to ensure efficient development of offshore oilfields. This study optimized the methods for lab experiments on oil displacement, analyzed the characteristics of two-phase flow permeability experiments, identified the effect of early polymer flooding, and found the method to decide the timing for early polymer flooding applicable to real-world oilfield management. The research result shows that there is optimum timing for early polymer flooding: in the case that a reservoir’s oil viscosity is 70mPa ⋅ s, the optimum timing is when the water cut is within the range from 0% to 40%. The water cut increase rate and the water/oil mobility ratio are key factors to the performance of polymers in mobility control: the best control effect can be achieved when the water/oil mobility ratio is below 1 and before the water cut increase rate reaches the maximum. Before polymer flooding, injection of a certain volume of water can unclog the channels, give full play to the effect of waterflooding, reduce the pressure of polymer injection, and cut the loss of effective viscosity of the polymer solution. In actual oilfield development, it is feasible to predict and monitor changes in the oil productivity index and take the timing of sharp decrease of the index as the optimum timing for polymer flooding.