Study On Time-varying Law Of Reservoir Physical Properties And Adjustment Of Water Flooding-Chemical Flooding In Ultra-high Water Cut Oilfield

Ultra-high water cut oilfields generally refer to oilfields with water cut of more than 90%in produced fluid.This kind of oilfield has basically gone through the development process of several decades.After such a long water injection development,the physical properties of the reservoir have changed greatly,and the contradictions within the reservoir,well pattern and between wells have become more prominent.On the one hand,the high-permeability seepage channels formed by long-term water injection scouring cause invalid or inefficient circulation between oil and water wells,making it difficult to continue to expand the swept area of injected fluid.On the other hand,the reservoir has entered the stage of ultra-high water cut,and its seepage characteristics have changed.In order to tap the potential in the future,it is necessary to control the situation of increasing water cut and accelerating production decline.The rapid increase of water cut has greatly changed the oil-water distribution,fluid percolation characteristics and water drive law in the reservoir.How to accurately analyze the reservoir evolution law in the water drive stage is an important theoretical basis for the formulation of future development plans and the improvement of oil recovery,so it is necessary to carry out the research on the physical property evolution law of the reservoir after long-term water drive.In this paper,the core oil displacement experiment,large plate experiment,3D network visualization modeling and CT scanning technology are used to study the change law of reservoir physical parameters after water flooding and chemical flooding.The change law of reservoir physical parameters with the development process of water flooding and chemical flooding is revealed from three aspects of mineral composition,permeability and porosity,and the distribution law of remaining oil is studied through a variety of scales,The displacement effect of different injection media was studied by using the large-scale plate model displacement experiment,and the influence of gravity and oil-water density difference on the development effect of water flooding and chemical flooding was analyzed.On this basis,a numerical simulation model based on time-varying reservoir parameters is established by using the numerical simulation method,and the influence of time-varying reservoir physical parameters on remaining oil distribution and development effect is systematically studied by using the model.For GD oilfield with water cut up to 95%,the injection system to continue to improve oil recovery is determined,which provides a theoretical and practical basis for the adjustment of development plan.The achievements and understandings are as follows:（1）The variation law of reservoir parameters such as porosity and permeability with increasing pore volume multiple of injected water is clarified.The research shows that the change range of porosity is only about±1%during water flooding and subsequent chemical flooding,but the change range of permeability is large.In the water flooding stage,the permeability change increases with the increase of initial permeability.For cores with748×10^-3μm² and 1496×10^-3μm² permeability,the permeability growth percentages are 61.2%and 69.48%,respectively,and the throat growth percentages are 5.04%and 6.11%,respectively;In the chemical flooding stage,the displacement experiment was carried out on the core with permeability of 1496×10^-3μm².Because the throat was blocked by clay mineral precipitation,the throat growth percentage was negative,which was reduced by 1.76%.According to the experimental results,based on the existing theories and methods,the quantitative identification and characterization methods of high permeability seepage channels are established.（2）The distribution law and occurrence state of remaining oil in the multi-scale of pore core well are described.It is clear that the main driving force of the injected water is the oil droplets with a diameter of 69.21～124.58μm,the remaining oil is broken into smaller oil droplets in a scattered distribution,while the oil droplets with a diameter greater than 124.58μm cannot be used,and there are still a large number of remaining oil droplets in the core pore space,which are mainly distributed in a network;The results show that ASP flooding can gradually peel off and produce oil droplets with a diameter greater than 124.58μm.When the water content is 98%,the micro residual oil distribution in the core pores becomes more scattered,the topological connectivity becomes worse sharply,and the dominant micro occurrence state of residual oil changes from network to porous and solitary drop;The distribution law of remaining oil in heterogeneous cores with different rhythms and washing degrees at core scale is clarified;The influence of different sedimentary rhythms and interbeds on the distribution of remaining oil at well scale is clarified.（3）A mathematical model of oil-water percolation considering gravity is established,and the movement direction of oil and water under gravity in the process of oil displacement is defined.The laboratory oil displacement experiment considering gravity was carried out.The recovery rate was 41.48%when the water content was 98.46%.The water content was reduced to 96.55%when the core was inverted up and down,and the recovery rate was 44.40%when the water content was 98.33%,and 2.92%more points were recovered on the original basis.Observing the visual flat sand filling model,it is found that due to the effect of gravity,the water flooded degree in the lower part is high,the oil in the upper part is almost unused,and the remaining oil is enriched in the upper part.Based on the conceptual reservoir numerical simulation,the mechanism of remaining oil enrichment and production affected by gravity factors is clarified.（4）Based on the plate model experiment,the influence of different injection media such as water,polymer aqueous solution and ASP flooding system on the development effect is clarified.Considering the influence of different injection media on the pressure field and saturation field,the polymer/surfactant/alkali ASP flooding system suitable for crude oil production in the study block was optimized.Further considering the difference of gravity and oil-water phase density,the mechanism of further expanding swept volume of vertical upward displacement system in the formation,producing remaining oil in the upper part of the reservoir and improving oil recovery in different media under the effect of gravity is studied.（5）The criteria of high permeability channel determination and magnitude division considering the time-varying reservoir parameters are proposed.Based on the time-varying evolution law of water drive reservoir physical parameters,a reservoir numerical simulation method considering the time-varying reservoir parameters is established,and the laws of permeability field increase,streamline field sweep area decrease,oil saturation field relative decrease in high permeability area and relative increase in other areas are clarified after time-varying.The research shows that after considering the time-varying reservoir parameters,the fluid sweep efficiency calculated by numerical simulation is reduced,the oil production of single well in typical blocks is reduced by 0.33%,and the recovery degree is reduced by 0.04%.The time-varying numerical simulation of typical blocks shows that in the area where the remaining oil distribution is mainly controlled by geological factors,the remaining oil in the main sand body accounts for 35.06%;The remaining oil in the abandoned River accounted for22.95%;The remaining oil controlled by faults accounted for 15.8%;The positive microstructure controls the remaining oil accounting for 3.38%;The remaining oil in sporadic small sand bodies accounted for 22.81%;In the area where the remaining oil is mainly controlled by development factors,the remaining oil is mainly concentrated at the top of the reservoir caused by imperfect well pattern and gravity differentiation.（6）Based on the theory of grey correlation and cluster analysis,considering the influence of permeability range,permeability variation coefficient,formation combination span,formation combination thickness and other factors,the formation combination boundary and formation combination division method are established.The injection system parameters of the study block were determined,and the water cut,oil production,recovery and other indicators were comprehensively analyzed,and the best water flooding chemical flooding development adjustment scheme was selected.