Simulation Evaluation And Control Technology Of Fracture Casing String Failure In Cluster Well

With the depletion of conventional energy,shale gas as an unconventional energy has become a hot spot for exploitation due to its clean,efficient and long life.In order to improve the recovery rate of shale gas,large-volume fracturing technology is widely used in shale gas extraction.However,the failure problem of fracturing casing string is becoming more and more serious,which seriously restricts the efficient exploitation and utilization of shale gas.Through the study and analysis of casing damage mechanism,it is found that the wellbore temperature change and non-uniform load have significant effects on casing damage during fracturing.This paper mainly studies the failure of fracturing casing string from the following aspects:By establishing a cluster wellbore model,the axial force distribution of the fractured casing string in the wellbore is calculated.And the influence of axial force and dogleg degree on the compressive strength of the fractured casing string is analyzed.Based on the field data,the wellbore temperature field model in the fracturing process was established,and the variation of wellbore temperature during the fracturing process was calculated.Considering the service condition of the fracturing casing string,the finite element model of formation-cement ring-casing is established according to the reservoir parameters in southwest China.It is found that under the action of coaxial load,the reduction ratio of the compressive strength of the fracturing casing string decreases with the increase of the steel level of the casing material.When the steel level of casing string material remains unchanged,increasing the wall thickness is beneficial to improving the safety performance of casing string in service.Fracturing string of casing in the process of fracturing fluid injection,wellbore temperature has a great change.The strength of the casing string is affected by temperature significantly,as the wellbore temperature decrease,outside attack resistance strength of the casing has been reduced.The wellbore temperature decreased by 62.3?in winter and 53.6 ?in summer.The squeeze the outer casing intensity fell by 16.8% and 13.5% respectively.Under the same axial stress,the extrusion strength of high grade steel casing is less affected by temperature drop.Non-uniform load has a significant impact on the safety of fracturing casing string.With the increase of non-uniform coefficient,the risk of casing string failure increases significantly.The greater the degree of non-uniformity of the load,the greater the radial displacement of the casing in thedirection of horizontal minimum force,the more serious the deformation of the casing,the greater the ellipticity of the casing,the more likely the casing will be crushed.The results of this study have certain guiding significance for the failure study of fracturing casing string in shale gas Wells,the design of casing string and the safety evaluation of casing string under fracturing conditions.