Research On The Mechanism Of Under-Injection And Optimization Of Injection Increase Methods Of Water Injection Wells In NP Block

At present,most of high and medium permeability oilfields in China have entered the end of development.The water cut in produced fluid is increasing and the production is decreasing,which is unable to meet the needs of petroleum industry development.Therefore,more and more low and ultra-low permeability oilfields become the focus of development gradually.However,due to the poor permeability of matrix and easy pollution of the reservoirs,problems such as high injection pressure or difficulty in water injection often arise in the process of waterflooding development.It is necessary to improve the injection performance by injection increase measures.NP block is a typical low porosity and ultra-low permeability reservoir,with small pore and throat radius,high clay content and strong heterogeneity.Since the block was put into production in 2011,the number of under-injection wells has increased year by year,and the problem of under-injection has become the main factor restricting the water-flooding development in NP block.This paper researched the mechanism of wells under-injection from two aspects: the theoretical water absorption capability of wells and the pollution factors in injection process,to analyze the influence of various factors on under-injection.Through the analysis of segmental water injection indicator curves,this paper established a method for reservoirs’ apparent starting pressure gradient calculation,which solved the shortcomings of core experiments,like insufficient representativeness and so on.The calculation method for theoretical water absorption capability established by this made the results get closer to the actual situation in oilfield site.By comparison,the actual injection volume of wells were lower than the theoretical water absorption volume,which proved that pollution have a great influence on water injection.Analysis of pollution factors in the process of injection showed that the order of influence degree is migration and blockage of particles in reservoirs,expansion of clay,blockage of solid particles in injected water,scale precipitation.Based on the analysis of influence factors and quantitative evaluation of water absorption capability at the beginning of injection,this paper established and selected a water absorption prediction model through multiple nonlinear regression.The calculation results can accurately reflect the water absorption capability which at the beginning of injection without water injection pollution.Based on the theoretical and predicted water absorption index of injection wells,this paper analyzed and figured out the effect of different injection increase methods,and established evaluation index for it.Then this paper formed a procedure for selecting injection increase methods considering the effect and economics of different methods,and developed a software to realize the integration of water absorption capability analysis quantitatively and injection increase methods selection.Field application showed that the procedure and the software is correct,and the application prospects are board.