Study On Sealing Performance Of Extreme-line Casing Joint

With the increasing development of oil and gas field under complex and harsh conditions,the premium connections of casing have been widely used.With the premium connections,extreme-line casing is widely used in deep well workover and sidetrack drilling at home and abroad.In order to study the sealing performance of extreme-line casing,it is necessary to study the sealing performance of extreme-line casing correctly for deep well and ultra deep well.(1)Based on the theory of thick wall cylinder,the theoretical model for calculating the torque of sealing surface and shoulder surface is established by considering the influence of the wring and the friction of sealing surface and shoulder surface.Based on the theory of thick wall cylinder and contact mechanics,the theoretical model of premium connection interference fit of extreme-line casing is established for four main sealing structures.Four affecting factors,such as taper of cone,thickness of casing wall,spherical radius and elastic modulus of casing,are considered.The influence of four factors on the sealing contact stress of the main seal structure is analyzed.It is found that the taper of cone should be reasonably selected,the main seal structure with the internal ball to ball should be selected and reducing the elastic modulus of the material are beneficial to the sealing performance of the extreme-line casing.(2)Combined with torque calculation model and the interference fit contact stress theory model,the variation of contact stress of cone to cone seal under different operational mode is calculated and analyzed,the influencing laws of the contact stress of the main seal of the extreme-line casing under the conditions such as making up,axial tension,internal gas pressure,axial compression and wellbore temperature increase are summarized.In terms of operating conditions,reducing the maximum axial tensile load,selecting reasonably axial compression load and ambient temperature are beneficial to the sealing performance of the extreme-line casing.(3)The performance evaluation method of extreme-line casing premium connection is established for four main sealing structures,and the sealing performance of extreme-line casing premium connection under six working conditions is evaluated.Combined with contact mechanics theory and gas streaming theory,the evaluation method of leakage rate of extreme-line casing premium connection is established.Based on the theoretical model of gas leakage rate,the effects of the average contact stress,the contact length and the roughness of the sealing surface on the gas leakage rate are considered.It is found that the reasonable choice of the average contact stress,the contact length and the reduction of the roughness of the sealing surface are beneficial to the sealing performance of the sealing surface.(4)A two-dimensional finite element model of extreme-line casing premium connection is established by an example calculation for a CST-ZT ? 139.7mm ×7.72mm extreme-line casing premium connection.The variations of Mise stress and contact stress of CST-ZT ? 139.7mm × 7.72mm extreme-line casing premium connection are simulated according to different working conditions.The calculation results of the mechanical model of the interference fit contact stress of extreme-line casing premium connection are compared with those of the simulation results.By optimizing and improving the structural parameters of extreme-line casing,the sealing contact performance between new premium connection and the original premium connection of the extreme-line casing under the same working condition is compared.(5)Aiming at the research of sealing performance of extreme-line casing joint,based on the theory of sealing contact stress,the analysis software for sealing performance of extreme-line casing is developed by Python language.The software can calculate the sealing contact stress of extreme-line casing,analysis of contact stress law of seal and evaluation of sealing performance of extreme-line casing.By comparing the results of finite element simulation and analytical solution,the rationality of the analytical solution is verified.The results of this paper provide an important theoretical basis for the design and rational use of the extreme-line casing.