Evolution Law Of Remaining Oil In The Extra-high Water Cut Reservoir In The X-6^th Block Eastern Of Daqing Placanticline

During the field development process,a development approach is adopted to enhance oil recovery,and water flooding development is widely utilized by oilfields as the main method of replenishing formation energy and maintaining production pressure difference in the secondary recovery stage.The difficulty of remaining oil development increases after the old oil fields arrived at high or super-high water cut stage.In solving the problems in old oilfields development,the primary task is to study the formation and distribution of remaining oil,with the study on the dynamic evolution law and distribution characteristics of remaining oil as the most critical task,which determines the formulation and adjustment effect of subsequent measures for potential exploitation.In order to further improve the oil recovery of old oil fields,the Eastern X-6th block of Xingshugang oilfield in Daqing placanticline,a part of Daqing Placanticline integrated oil reservoir,was selected as the target area.The object of study was PI3₃ single sand body formed by low curvature meandering distributary channel deposition in lacustrine shallow water delta.There were several continuous sealed coring wells,and a series of experiments were carried out,such as core casting thin section observation,scanning electron microscope test,relative permeability tests,and water displacement test,from which abundant test data were obtained.From PI3₃ single sand body under water flooding and subsequent ASP flooding,a large amount of production data was obtained.With the dynamic and static data,the research achieved the following results:（1）In terms of the water injection propulsion during the water flooding development stage,the water break time of the first oil well row and the middle oil well row in the basic well pattern as well as the spacing of each well row were analyzed,and the results showed that the water injection front stopped within the pressure relief range of the first oil well row that it intersected,instead of moving forward.When the production pressure difference remained at 0.62MPa,the injection level was pushed at 12m per month increasing with the greater production pressure difference.This demonstrates the influence of well pattern type on the seepage path and sweep area of injected water.（2）In terms of the evolution and distribution characteristics of remaining oil in the water flooding stage,5 types of remaining oil saturation test points were established within the range of basic well pattern,namely injection well row,injection and production well row,production well row,production well row,and intermediate well row.The results showed that the remaining oil saturation value was the lowest between injection and production wells,which indicated that the degree of oil displacement caused by injection water was affected by the distribution of oil and water wells.According to the value of geological reserves and cumulative oil production,the recovery degree was 47.8%when the water production rate reached the critical value of water flooding economy.The numerical simulation results showed that the remaining reserves in the upper part of the sand body are distributed in sheet shape,while the remaining reserves in the lower part are mainly distributed in strip shape between the production well rows,and a certain amount of remaining reserves also existed in the injection and production well row area.（3）With respect to the development characteristics in ASP flooding,the development history of ASP flooding was divided into three periods by the change of water production rate:the pre-effective period,the sustained-effective period,and the post-effective period of the displacement agent.6 parameters were used to define the production characteristics,including the sustained-effective time of oil-displacement agent,the slope of displacement characteristic curve in the sustained-effective period,the decreasing amplitude of water production rate,the cumulative water-oil ratio,the cumulative oil production,and the proportion of effective production time.The quantitative evaluation formula of displacement agent displacement effect was established by principal component analysis,and therefore the quantitative comparison of displacement effect among various well groups was realized.（4）With respect to the evolution and distribution characteristics of remaining oil in the ASP flooding stage,the well pattern was divided into four positions,namely the position near the water injection well,the position between the water injection wells,the position between water injection and production wells,and the position near the production well.The comparative results from comparison wells and coring wells showed that the remaining oil saturation had the greatest decrease near the injection wells and the minimum change range between the injection wells,which testifies to the plane difference of displacement degree of oil displacement agent within the range of well pattern.The remaining oil saturation at each location of PI3₃ single sand body indicated that there movable residual reserves existed within the sweep range of oil displacement agent.The recovery degree reached 68.5%after ASP flooding,and the residual reserves in the relative pressure balance area between injection wells were irregular banded enrichment.（5）As for the interpretation of formation mechanism and control factor of residual oil,in the case of identical reservoir physical properties,the greater the difference of oil-water viscosity ratio was,and the faster the relative permeability of water phase increased with the water saturation,and the earlier the water breakthrough time of oil wells was,and the worse the development effect of oil wells became.Under constant fluid physical property,the oil-water two-phase flow occurred earlier at the position with higher reservoir physical property development degree,and the water-oil fluidity ratio at this position accelerated with greater water saturation,thus aggravating the heterogeneity of fluid distribution in the reservoir.Under the joint influence of fluid physical property difference and reservoir physical property inhomogeneity,the common injection water fingering and circumfluence flow in the reservoir was the mechanism that contributed to the formation of different types of remaining oil.Under the joint influence of well pattern type determining the direction of fluid flow,formation physical properties determining the fluid flow ability,and property of oil displacing agent determining the difference of oil-water flow ability,the dynamic change of interchangeable oil-water two-phase flow ability controlled the evolution law of remaining oil.