Research On Damage And Fracture Performance Of Defective Oil And Gas Well String

Oil and gas well tubing sting is the equipment used to form oil and gas wells and work in oil and gas wells.It is the only channel for transporting drilling fluid,oil and gas and transferring power.Oil and gas well tubing sting works in a fluid-filled narrow wellbore.It is subjected to tensile,internal and external pressure of the drilling fluid,bending moment,torque and other static loads,and also bears various impact loads generated by frictional collision with the wellbore wall.At the same time,due to the drilling fluid and corrosion medium,oil and gas well tubing sting is subjected to severe corrosion and become the weakest link of drilling equipment.In addition,during manufacturing and operation,various surface damages of tubing sting can be easily caused,forming a crack source.In a complex load and corrosive environment,cracks will continuously propagate until a fracture failure occurs.The domestic and foreign scholars have conducted a lot of research on the failure of the tubing string and mainly concentrated in the drill string strength,failure reason,structure improvements and so on.How there are less studies in fracture behaviors of the cracked tubing sting and their own materials.S135 drill pipe steel and coiled tubing steel are commonly used high strength and high toughness materials for oil and gas tubing string.Their fracture properties directly affect the ability of tubing string to resist crack initiation and propagation in complex drilling conditions.Therefore,establishing an effective fracture damage model for S135 drill pipe and coiled tubing material will play an important role in promoting the evaluation of fracture characteristics and engineering application of tubing string.In recent years,cohesive zone model has developed into a simple and effective model for studying fracture failure of materials,and has been applied to simulation and analysis of the ductile fracture of metals.In this paper,the material properties of S135 drill pipe steel and coiled tubing materials are tested,and the fracture behavior obtained from the experiment is used to determine the cohesive zone model parameters and validate the FE model by the inverse analysis based on the improved LM optimization algorithm.Fracture behaviors of the coiled tubing containing cracks is studied by cohesive zone model when the coiled tubing is pulled in wellbore and winds the drum,and it provides the basis for the safety assessment of oil and gas tubing sting containing defects.The main work of this paper is as follows:1.Combining the material properties test and numerical calculation results,an inverse analysis method,which is used to determine the optimal cohesive zone model of the material,is established based on the improved LM optimization algorithm;2.According to the tensile test and compact tensile fracture test of S135 drill pipe stee,the optimal cohesion parameters of the five cohesive zone models are determined by inverse analysis based on the improved LM algorithm.And it provides basis and reference for establishing tfracture damage models of oil and gas well tubing string;3.According to the coiled tubing characteristics of thin wall and small diameter,an arc fracture specimens of coiled tubing is designed and its tensile fracture tests are carried.Combining the finite element calculation results and the test results,the optimal cohesive zone of coiled tubing is determined by inverse analysis based on the improved LM method;4.The ductile fracture behaviors of the coiled tubing containing through-wall cracks and surface cracks are studied by the cohesive zone model of the coiled tubing when the coiled tubing is pulled in wellbore and winds the drum;the critical crack shape of coiled tubing rupture under axial ultimate tensile force is determined;and the critical load of coiled tubing is determined;and it provides the basis for the safety assessment of oil and gas tubing sting containing defects.