Safety Analysis On Casing And Cement Sheath During Three Ultra Gas Well Completion

In recent years,with the rapid increase in demand of oil and gas from all walks of life in our country,the oil production turned into complicated.The complicated factors such as high temperature,high pressure and so on of the three ultra gas wells in the western region have brought new challenges to the exploration and well completion.Well completion is a improtant link to the oil production,which is connected with the development of oil and gas production.We analyzed form three aspects in the light of the diffculties of well cementation in three ultra gas well and the influence of different factors on strength and structure integrity of casing and cement sheath during well complement.Firstly,analyze the influence of different factors on the cementing quality from displacement efficiency,borehole conditions and drilling fluid properties through the literature review and field data.Through the grey relational grade analysis,we can conclude the main factors and secondary factors which affect the cementing quality.Established a few kinds of cementing quality prediction model through theoretical analysis,and also the improved prediction model is created based on the shortage of the single prediction models.Created a set of cementing quality prediction software which is applicable to the three ultra gas well by using Microsoft C# programming language and the database.Through example calculation,analyzing and comparing the prediction accuracy and computational efficiency of different prediction models,can conclude that the grey neural network combination forecasting model has high accuracy and learning efficiency,and can be well applied to the engineering application.Secondly,through establishing the finite element model of casing-cement-layer formation system,analyzed the influence of different factors on the micro gap of the first interface while the casing pressure is unloaded,we can concluded that the difference of the casing pressure before and after unloading is the most significant factor affecting the micro gap size.And also the micro gap size is proportional to the elastic modulus of the cement and layer and also the eccentricity of the casing,is inversely related to the cement thickness and casing thickness.So based on the calculation results,we proposed the useful suggestions to improve the integrity of casing and cement sheath and to reduce the size of the micro gap in the first interface.Finally,established the finite element model of casing with double-pitting corrosion by combining the knowledge of mechanics and the finite element software,through the finite element calculation,we can concluded the influence of the pitting spacing,the pitting section and the depth of the pitting on the strength of the casing under different pitting corrosion conditions.With the analysis,we can get the double-pitting limit axial spacing and limit circular angle.Using the dichotomy can calculate the limit influence range of the double-pitting under different pitting section in order to provide the theoretical basis for casing strength design.