| In recent years,more and more attention has been paid to the development of offshore low permeability reservoirs.Water injection development can increase the oil recovery rate of offshore low permeability reservoirs,but it also causes the increase of water injection pressure and the decrease of water injection volume due to some factors,this is not conducive to normal oilfield production.Weizhou W oilfield is a low permeability reservoir in the Beibu Gulf of South China Sea,taking this reservoir as an example,study the problems of rising water injection pressure and decreasing water injection volume during water injection in low permeability offshore reservoirs,identify the main influencing factors,suggestions for reducing water injection pressure and increasing water injection volume are given,provide guidance for water injection development of Weizhou W oilfield,provide reference for similar oilfield water injection development.In view of the problems existing in the research area,the main research contents and understandings of this paper are as follows:(1)Based on the investigation of the factors affecting the increase of water injection pressure and decrease of water injection volume in low permeability reservoirs,combined with the reservoir characteristics such as mineral type,clay mineral content,pore throat characteristics and production data such as water injection and water absorption index,the potential influencing factors of sensitivity damage,incompatible water quality and poor reservoir connectivity in the study area are analyzed.(2)Through the five-sensitive and stress-sensitive laboratory evaluation experiments,the changes of fluid flow rate,salinity,acidity and alkalinity and the stress of overlying strata in the reservoir are analyzed.After the expansion,dispersion and migration of particles,the changes of permeability and the damage to the reservoir are analyzed.The experimental results show that the study area has strong speed sensitivity damage,moderate weak water sensitivity,salt sensitivity,alkali sensitivity and stress-sensitive damage,weak acid sensitivity damage.It is considered that the sensitivity damage dominated by speed sensitivity is one of the reasons for the increase of water injection pressure and the decrease of water injection volume.(3)Through the determination of ion concentration,water type judgment and compatibility analysis of formation water and injected water in the study area,it is analyzed whether precipitation will occur after the mixing of injected water and formation water,resulting in reservoir plugging.The experimental results show that the salinity of injected water and formation water is quite different,and the water type is inconsistent,and calcium carbonate precipitation will occur.It is considered that the scaling problem caused by incompatibility of water quality exists in all water injection wells.(4)Through the analysis of static connectivity from the perspectives of sedimentary facies,reservoir profile and seismic data,the dynamic connectivity is studied from the perspectives of production data,material balance method and numerical simulation,and the connectivity of the study area is clarified by combining dynamic and static data.The analysis shows that there is poor connectivity in the study area,and poor connectivity between injection and production wells is one of the reasons for the increase of water injection pressure and the decrease of water injection volume.(5)In view of the reservoir blockage caused by sensitivity damage and other factors,it is suggested to use acidification and ultrasonic technology to remove the blockage,and a set of acid formula is obtained through experiments.In view of the incompatibility of water quality,it is suggested to optimize water quality to make it higher than the critical salinity.In view of the poor reservoir connectivity,it is recommended to use fracturing to cross low permeability areas to achieve the goal of reducing pressure and increasing injection. |
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