Study On Heterogeneity And High Water Cut Genesis Of Chang 8 Reservoir In Z9 Of Heshui Area,Ordos Basin

Zone Z9 in Heshui Area with a low porosity and ultra-low permeability reservoir is located in the southwest margin of Ordos Basin.Segment 8,the second member of the Triassic Yanchang Formation is the main oil bearing series.Since 2002,it has been built with 5-spot pattern and rhombus 9-spot well pattern,and it was put into full production in 2010.A series of technologies,such as water injection,fracturing,are widely used during the development of low permeability oilfield in Changqing Oilfield,which greatly improves the recovery rate.However,due to the unreasonable use of early measures and the influence of fracture and reservoir heterogeneity on water injection development,high water cut becomes a common problem in oil wells at present.With the development time prolonged,there are more and more low production and efficiency wells caused by high water cut,so it is urgent to carry out the research on the causes of high water cut in oil wells.Through the application of regional geological data,core data,perforating data,analysis and testing data,production performance data,stimulation measures data over the years,the research on reservoir heterogeneity was carried out systematically based on geological research.In addition,taking well block,development unit and well group as units,the condition of production decline,the direction of water breakthrough,the efficiency of oil and water well measures over the years were analysed.Clarify the reasons for high water cut in oil wells and propose treatment measures,there are several results in the paper.（1）The porosity,permeability and oil saturation of Chang 8₂² reservoir in Z9 area are 1%～26.1%,0.90～2.30m D and 40.25%～55.87%,respectively.The development degree and thickness of barrier and interbed in each small layer vary greatly,the reservoir heterogeneity is strong.The thinner the barrier and interbed are,the higher the output of the oil well.The thinner the interbed is,the higher the initial well production,the smaller the decline of the oil well production.When the thickness of barrier is between 5m and 9m,the initial single well production is low and the decline of oil well is large.The barrier has a greater impact on the water injection effect of the water injection well,resulting in ineffective water injection.（2）Z9 area mainly adopts rectangular well pattern,rhombic 9-spot well pattern and5-spot well pattern for production and development.Through the analysis of production decline of three well pattern units,the production difference of each unit is large.The development effect of Zhuang 166 unit is the best,followed by Zhuang 171 unit,and the water flooded wells of rectangular well pattern are less than those of rhombic 9-spot well pattern,which is more stable for efficient oilfield development.（3）The direction of water breakthrough in zone Z9 is determined by tracer test,water flooded well direction analysis,and effective direction analysis.The water breakthrough direction determined by the tracer is mainly NEE-SWW,NNE,SEE;the water flooded well direction is NNE,NEE,followed by SEE;the effective direction of oil wells is mainly NEE-SWW and NWW-SEE.As a whole,due to the complex well pattern,numerous geological factors,and high intensity of measures,the direction of water breakthrough is relatively scattered.（4）The causes of high water cut oil wells in Z9 area mainly include static geological factors and dynamic development factors.The geological factors include poor reservoir physical properties,water cut of the formation itself,strong vertical peak water absorption and plane heterogeneity.The development factors are maily about reservoir fracture growth,water breakthrough or water-flooded due to the high intensity of measures over the years,the opening of cracks affected by injection pressure.（5）With the stimulation effect of oil and water well measures,three suggestions are put forward:improving injection-production well pattern,treating water absorption profile,refracturing and temporary shielding fracturing measures.The research results will be significant for controlling water and stabilizing oil,delaying production decline,improving oil recovery and tapping the potential of remaining oil.