Study On Conversion Time Between Different Methods Of Chemical Enhanced Oil Recovery In Offshore Oilfield

There are many limitations in offshore oil fields,such as short platform life,high investment risk and so on.In order to obtain better economic benefits,different EOR methods need to be implemented continuously in order to maintain the high speed exploitation of oil field.Some blocks in Bohai oil region of China have achieved good development results through the combination of well pattern infilling and polymer flooding.The reservoir sedimentary types in Bohai oil region include lacustrine delta sedimentation and meandering river sedimentation.In lacustrine delta sedimentary reservoirs,the sand bodies are distributed sequentially,the crude oil viscosity distribution is relatively uniform,and the permeability gradient between layers is small.In the meandering river sedimentary reservoir,the sand body is discontinuous,the crude oil viscosity is widely distributed,and the permeability level between layers is greatly different.There are also significant differences in EOR methods suitable for the two types of reservoirs.At present,the combination model of EOR methods suitable for different sedimentary reservoirs has not been established.In particular,the optimization methods of conversion time for different EOR methods have not yet been formed.In this paper,two types of typical sedimentary reservoir blocks in Bohai Sea are selected and numerical simulation models are established respectively.The well pattern infilling and chemical flooding methods such as polymer flooding,surfactant flooding,binary compound flooding and gel flooding were considered.Then the mechanism of chemical flooding was characterized.Based on numerical simulation,the quantitative statistical method of sweep coefficient and washing efficiency was established.The sweep uniformity index was defined to characterize the sweep difference.The distribution characteristics of remaining oil and response remaining oil under different EOR methods are described.On this basis,the replacement mechanism of different EOR methods is analyzed,and the replacement model is proposed.The combination sequence of two major and four minor EOR methods in different sedimentary reservoirs is established.Based on the parameter analysis of the influence on the replacement development effect of enhanced oil recovery,the quantitative index of conversion time is defined as "oil recovery rate decreasing range",and the conversion time is optimized by taking the recovery rate and economic net present value as evaluation indexes.Finally,the conversion time optimization method is applied to typical blocks of delta sedimentary reservoir and meandering river sedimentary reservoir,and different recovery recovery methods are developed to replace combined development schemes.Then the development effects of different schemes are predicted.The study shows that the replacement sequence of chemical flooding after well pattern infilling can improve the recovery rate more greatly.The delta sedimentary reservoir should adopt the early primary infilling mode,and the meandering river sedimentary reservoir should adopt the secondary infilling mode.The optimal succession sequence of different chemical flooding methods after well pattern infilling was optimized,namely gel flooding--binary compound flooding,polymer flooding--binary compound flooding,and binary compound flooding--gel flooding.Based on the evaluation results of the enhanced recovery rate and the NPV,it is suitable for the early well pattern infilling method for class I reservoirs with low permeability.For II-1 reservoirs with strong planar heterogeneity and small difference in interlayer permeability,it is suitable for the replacement combination of infill + polymer flooding + binary compound flooding,And the earlier the succession time,the better the economic benefits.For II-2 reservoirs with weak planar heterogeneity and large difference in interlayer permeability,it is suitable for the replacement combination of infill + binary compound flooding + gel flooding.The optimal succession time of binary flooding is 90% of the peak oil recovery speed of pre-infill water flooding,and the earlier the replacement time of gel flooding,the better the development effect.For II-3 reservoirs with significant difference in interlayer permeability,the combination of infill + gel flooding + binary flooding is suitable.When the vertical permeability gradient is large,the earlier the succession time is,the higher the economic benefit is.The application study shows that the oil recovery of delta sedimentary reservoir can be improved by 8.4% compared with the water-flooding development plan by using the replacement combination of primary well pattern infilling + polymer flooding+ binary compound flooding.In the meandering river sedimentary reservoir,secondary well pattern infilling + gel flooding + binary compound flooding replacement combination can improve the oil recovery by 9.5% compared with the water-flooding development plan.