Research On Water Injection Quality And Decompression And Augmented Injection System Of Shunbei Oilfield

Water injection development is a widely and effective secondary oil recovery technology,which is a basic measure to improve the effectiveness of oilfield development.However,in the process of water injection development,due to the incompatibility of the injected water with the reservoir fluid and reservoir rock,as well as the substandard water quality indicators,it often causes blockage of pore pipes and reduces the permeability of the reservoir.As a result,the water injection pressure increases and the water injection volume decreases,which has a serious impact on the water injection development of the oilfield.In this paper,taking the Shunbei reservoir as the research object,the experimental water samples and reservoir cores were analyzed by atomic absorption spectroscopy,X-ray diffraction and other technical means,and the compatibility,reservoir sensitivity and water quality index of water injection were studied.The reasons for the increase of water injection pressure and the decrease of water injection volume were clarified,and the research on decompression and augmented injection system was carried out.The results showed that:（1）The water sample contained a large amount of Ca²⁺,HCO₃^-and SO₄^2-scaling ions.After the injection water and formation water were mixed,there was a tendency of Ca CO₃ and Ca SO₄ scaling.The scaling amount increased with the increase of temperature and scaling ion concentration,and decreased with the increase of pressure.The accuracy of the predictive software simulation data is demonstrated by scaling experiments.Eventually,the SHB3 well was selected as the source well of the injected water,and the optimal mixing ratio of the injected water SHB3 and the formation water SHB5-9 was 9:1.（2）The content of calcite in reservoir rocks is the highest,and the proportion of illite/smectite mixed layer in clay minerals is the highest.The velocity sensitivity damage degree of the reservoir was weak and the critical flow rate was0.75～1.50cm³/min.The water sensitivity damage degree was moderately strong.The salt sensitivity damage degree was moderately weak and the critical salinity was80514.59～120771.88mg/L.The acid sensitivity damage degree was moderately weak.The alkali sensitivity damage degree was weak and there was no critical p H value.（3）According to the industry standard,the median particle size,suspended solid content and oil content of suspended solids in the water samples exceeded the standard,which was harmful to the reservoir.The orthogonal experiment showed that the suspended solid content had the greatest influence on the core permeability,followed by the median particle size of suspended solids,and the oil content had the least influence on the core permeability.At the same time,the water quality standard of water injection was determined:the median particle size of suspended solids≤1μm,the content of suspended solids≤3mg/L,and the oil content≤9mg/L.（4）Serious scaling and water sensitivity,and substandard water quality were the main reasons for the increase of pressure and the decrease of water injection volume.Through laboratory experiments,the decompression and augmented injection system was optimized:80mg/LDSF-2+2.5%DBCM+0.2%LBC-2S.The system which had good long-term stability,could effectively reduce the formation scaling trend,inhibit the expansion and migration of clay particles,reduce the oil-water interfacial tension,and reverse the wetting of oil-wet surface to water-wet surface,thereby reducing the injection pressure of water injection wells.In addition,the optimal slug size of the depressurization and injection system is 0.4PV,the depressurization rate is 26.32%,and the recovery rate is 12.31%.In this paper,the scale formation trend and reservoir sensitivity characteristics of injected water of Shunbei Oilfield are defined,water quality standards are established,and a set of decompression and augmented injection is developed,which provides reference for the future water injection development of Shunbei Oilfield.