Injection And Production Optimization Method Study On Polymer Flooding Of Fluvial Reservoir In Offshore Oilfield

The reserves of fluvial sedimentary sandstone reservoir account for a large proportion in BH Oilfield.Due to many layers in its vertical direction,large lateral change and poor continuity,the reservoir vertical and plane development heterogeneity is severe during water flooding,resulting in low oil recovery.As the main method of tertiary oil recovery,polymer flooding has achieved good development results in fluvial reservoirs of Chinese onshore oilfields.However,due to the high mining cost and difficult operations of offshore oilfields,many polymer flooding optimization techniques are difficult to apply.Therefore,this paper full of practical significance starts from reducing the heterogeneity of vertical and plane development,and carries out optimization of polymer flooding under complex conditions of fluvial reservoir in offshore oilfield.In this paper,the numerical simulation model of certain fluvial reservoir in BH Oilfield is established.Based on the analysis of reservoir heterogeneity and residual oil distribution characteristics,multi-slug polymer flooding based on vertical heterogeneity and reasonable distribution of well group polymer dosage are proposed in order to respectively reduce the vertical and plane heterogeneity.For the multi-slug polymer flooding method,the displacement limit was obtained by laboratory experiments and numerical simulation.Meanwhile,the clustering method based on entropy weight algorithm was used to carry out the layer combination.Then,the calculation method of polymer dosage for each slug is established by using the product of permeability ratio and thickness ratio as an evaluation index.For polymer dosage reasonable distribution method,based on the sensitivity analysis of influencing factors,the parameters such as plane permeability variable coefficient,formation coefficient and residual geological reserves were selected to determine the main controlling factors by applying the oil production per ton polymer as evaluation index,methods of calculating reasonable polymer dosage of well group and well production were established.Finally,a polymer flooding optimization plan for certain fluvial reservoir in BH Oilfield was designed by combining two optimization methods.The results show that the vertical residual oil is mainly distributed in the upper of the reservoir and the layers with medium-low permeability,and there is large difference for plane residual oil.The displacement limit of multi-slug polymer flooding is that the permeability ratio is supposed to be less than 7.Matching relationship research of the combination mode and reservoir rhythm shows,when the high permeability layer is at the bottom,inject high concentration polymer slug first can further enhance oil recovery.The simulation results of the actual well group multi-slug polymer flooding optimization plan show that the upper remaining oil can be effectively displaced,reduce the vertical development heterogeneity.Injectionproduction optimization of polymer flooding results show that the weight ratio is 5:5 by using the plane coefficient of variation and the remaining reserves to distribute the reasonable amount of polymer,and the weight ratio is 4:6 by using the formation capacity and the remaining reserves for the reasonable distribution.The optimization plan of polymer flooding in the fluvial reservoir has a better development effect,and the enhanced oil recovery is 0.76% higher than that of single slug uniform polymer flooding and 7.17% higher than that of water flooding.