Correlation Between Mechanical Behavior Of Free Casing And Wellhead Growth

Most HTHP gas wells are characterized by deep burial,high yield,acid gas and complicated geological conditions.With the gradual production of HTHP gas wells,wellhead growth,wellhead pipeline damage,completion string safety problems caused by the free casing gradually highlighted,and has become one of the important problems affecting the safety.In order to accurately predict the wellhead growth of HTHP gas wells,evaluate its safety and guide the safety production of gas wells,it is urgent to study the mechanical properties of free casing.A test method of mechanical bite strength and cementing strength of casing-cement mantle was proposed.The shear test of cement ring with different cementation area was carried out,finding the relationship between axial load and axial deformation of cement ring.The mechanical bite strength,cementing strength and the proportion of cement ring casing were calculated.The formation mechanism of the free casing was clarified.Based on the conservation of energy and momentum conservation law,the calculation model of wellbore temperature field and pressure field around HTHP gas wells are established.Then the calculation model of the axial stress of the composite stiffness system of the tube and casing is established,considering the effect of annular fluid expansion,casing thermal effect,string buckling,trajectory of wellbore and packer on axial stress.Combined with the mechanism of wellhead growth,a method of predicting wellhead growth in HTHP gas wells is establishedA theoretical model is established to calculate the length of free casing in HTHP gas wells.A multi-objective optimization algorithm of particle swarm optimization,the composite stiffness mechanics model and the calculation model of the wellbore temperature and pressure field have been cited in this model to calculate the length free casing in HTHP gas wells.And.a new method for calculating the length of free casing in each layer has been formed.We have verified the accuracy and reliability of the new method by the calculation example of wellhead growth in the Y gas well in Sichuan Basin.The mechanical calculation program of wellhead growth and stress of wellhead caused by free casing was compiled.The program is based on the mechanical model of composite stiffness and the calculation model of wellbore temperature and pressure field.The temperature field and pressure field of X gas well in Sichuan Basin were simulated and compared with the measured data.In this paper,the effects of production,depth of annulus fluid,parameters of annulus fluid and parameters of cement ring on the annulus pressure were analyzed.The law of yield and free casing length influencing wellhead growth has been reveal.The axial stress of casing string with different length of free casing length has been simulated and the correlation between free casing combination and axial stress and safety factor has been analyzed.The axial stress of casing string under different yield was simulated and the correlation between yield and axial stress and safety factor has been analyzed.The calculation results show that the increased yield lead to the increasing of wellhead growth,X gas well wellhead growth is most sensitive to the length change of production free casing,followed by the length change of intermediate free casing,minimum the length change of surface free casing.Casing axial stress is closely related to free casing length and yield,the length of a free casing in each layer,relatively long sleeve or relative short will significantly reduce the safety of casing,the yield increase would significantly reduce the safety of the casing.