Optimization Design And Software Development Of Wellbore Abandonment Parameters For Offshore Oil And Gas Wells

With the development of oil and gas production,offshore oil and gas wells will gradually lose their exploitation value,so it is necessary to plug and discard the wellbore.If the plugging is not properly disposed,the fluid in the wellbore of oil and gas wells may flow upward,collude in different formations,or even leak out of the wellhead.Compared with land-based oil and gas wells,the safety requirements of offshore oil and gas wells are higher.Once oil and gas leakage,it will cause serious safety and marine environmental pollution,and pose a serious threat to nearby ships,coastal residents and marine organisms.At the same time,with the development of oil and gas production,more and more oil and gas wells begin to enter the middle and late stage of production,and are or will be faced with the problem of abandonment.Therefore,it is very important and urgent to optimize the parameters of abandonment for offshore oil and gas wells.Based on the investigation of offshore oil and gas well abandonment operations in China,the UK North Sea,the Gulf of Mexico and other countries,the basic processes and key technologies of offshore oil and gas well abandonment operations at home and abroad are summarized.It is found that the main risk point of the abandoned well shaft seal is the cement plug.The finite element model of the abandoned well cement plug is established,and the parameter sensitivity analysis of the cement plug parameters,formation parameters and other factors is made.The results show that reducing the elastic modulus of the cement plug can reduce the failure risk of the cement plug seal in the abandoned well shaft;lengthening the length of the cement plug can reduce the water in the abandoned well shaft Risk of sealing failure of mud plug;risk of sealing failure of cement plug in offshore abandoned well decreases with the decrease of cement plug radius;risk of sealing failure of cement plug in offshore abandoned well decreases with the decrease of formation pressure difference at both ends of cement plug.In addition,after forging and milling the corrosion section of casing,cement plug sealing operation and reducing the eccentricity of casing will also reduce the risk of cement plug sealing failure.Based on the seal integrity analysis of the permanent barrier,the optimization design of the permanent barrier is carried out.The analysis shows that the best material for the permanent barrier of abandoned well is high cementation and low permeability cement plug.Generally,three or more permanent barriers are needed.The main barrier is used to block the oil,gas and water in the production layer,the secondary barrier is used to block the inter channeling fluid between different layers,and the surface barrier is used to prevent the leakage of wellbore fluid to the seabed.Based on the failure mechanism of cement plug seal and the principle of length optimization design,the length optimization design model of cement plug is established.According to the optimization design model of cement plug length,the software of optimization design of wellbore abandonment parameters for offshore oil and gas wells is developed,which includes two functional modules: sealing analysis of cement plug and length design of cement plug for offshore abandonment wells.The safety analysis of cement plug sealing in abandoned wells and the optimization design of cement plug length in abandoned wells are realized respectively.In view of a platform abandoned well in Bohai Sea,the software is used to analyze the case,and the analysis results are consistent with the actual situation.In this paper,the optimization design method and software of wellbore abandonment parameters of offshore oil and gas wells are provided,which can be used for reference in the operation of wellbore sealing and abandonment of offshore oil and gas wells.