Study On Mechanism Of Shallow Mudstone Creep Casing Damage

Casing damage in major oilfields in my country is relatively serious.According to statistics in 2018,casing damage wells in the Sixth Middle Block of Xinjiang Oilfield accounted for 56.55%,and casing damage occurred in shallow formations.Casing damage has seriously affected the production of oil and gas,causing damage to the well pattern,reduced production,suspension of production,and even abandonment of oil wells,causing huge losses to the exploitation of oil fields.Therefore,in view of the serious problem of casing damage in this study area,100 wells in this block were collected and counted,the main controlling factors of casing damage were clarified,and based on the research of finite element and related mechanics theory,the theory and The method of combining numerical simulations is used to study the mechanism of casing damage,thereby revealing the law of casing damage in the Sixth Middle District of Xinjiang Oilfield,which will effectively prevent casing damage in the Sixth Middle District of Xinjiang Oilfield,reduce the investment in repairing casing wells every year,and extend the work of oil and water wells.The number of years is of great significance.Based on the statistical analysis of the basic situation of the study area,this paper is based on the traditional mudstone creep model.The traditional creep model is improved by adding water content,so that the model can better describe the change of formation water content and the physical properties of mudstone.The effect of rheology.Secondly,the elastoplastic finite element mechanical model of the casing-cement ring-formation combination is established,and the finite element simulation study of the casing damage mechanism under different formation water content provides a simple and feasible method for the study of the casing failure mechanism of complex formations.method.The specific research results are as follows:(1)After analyzing the geological situation,types of casing damage and influencing factors of casing damage in the study area,it is concluded that the main factors causing casing damage in the study area are uneven ground stress and mudstone creep,followed by cementing in the later stage.The unqualified quality aggravated the damage of the casing.(2)First,analyze the mudstone creep test curve in the study area,and fit the creep parameters of the traditional Burger model.It is concluded that the traditional Burger creep model does not fit well with the true creep test curve of mudstone.,And then improve the traditional Burger creep model,and verify the improved Burger model.Judging from the verification results,the improved Burger model has a better fitting effect than the traditional model,and can be used to study the creep characteristics of mudstone in this study area.Finally,the improved Burger model is used to calculate the external squeezing force of the casing in the study area,and the expression of the external squeezing force changing with time after the interaction between the viscoelastic rock formation and the casing under the non-uniform ground stress is obtained.(3)A three-dimensional finite element model of sand and mudstone interlayer was established to simulate the equivalent stress of casing running under different water injection pressures and different wall thicknesses.The research results show that most of the casings have stress mutation shear failure at the sand-shale interface,and increasing the wall thickness has no obvious effect on preventing and controlling the casing damage of the sand-shale layer,and the steel grade is improved,such as replacing the J55 steel grade with P110 The steel grade can effectively prevent and control the damage of the casing in this mudstone section.(4)Establish a two-dimensional finite element model of the mudstone section,and comprehensively analyze the stress of the casing at different water cuts in the formation when the cementing quality is good and the cementing quality is poor.The research results show that when the cementing quality is poor,the casing The stress increases with the increase of the formation water content,and the water content of the formation when the casing yields is much smaller than the water content of the formation when the cementing quality is good,so the poor cementing quality aggravates the casing to a certain extent.Damage to the tube.(5)Through a comprehensive analysis of the mechanism of casing damage in the study area,combined with the casing damage in the study area,corresponding preventive and repair measures are given.