Study On Expansion Process Of Mono-diameter Expandable Tubulars With A Thread Joint

As oil resources increasingly scarce,deep well and ultra-deep well are becoming the main battlegrounds of oil production.The mono-diameter expandable tubular technology can make no diameter loss when drilling wells,which is the ultimate goal of drilling.However,the difficulties of breakage and leakage at the thread joint in the expansion process limited the development of this technology.The mechanical property evolution of the material and the residual stress distribution after expansion process can have much effect on the structural integrity of the thread joint.Therefore,the present thesis,mainly focused on the thread joint of the mono-diameter expandable tubular.The stress evolution during expansion process,together with the residual stress and the seal performance after expansion process were studied by numerical and experimental methods.All these have done provide a reference for the material selection and geometry design of the technology.The main work and conclusions of the thesis are summarized as below:(1)A 2D axi-symmetric model was built using finite element software ABAQUS to study the expansion process of J55 thread joint with a 10% expansion ratio.The results show that the axial stress in the inner wall is tensile.As the cone travels,the axial stress at the outer surface turns from tensile to compressive.As for the circumferential stress,large tensile stress generates at the inner surface,while compressive stress generates in the thread zone.Stress concentration generates in the root zone of the thread joint,indicating that it is a weak part.Expansion cone with an angle of 9° or 10°,sizing section length of 65 mm could be selected as the optimized configuration.(2)Experiments were conducted based on J55 thread joint with a 10% expansion ratio.The results show that expansion force experiences a small fluctuation during expansion process.After expansion process,the thickness becomes thinner and the length becomes shorter.In addition,the micro-hardness,yield strength and ultimate strength improve while the elongation along the axial direction does not change obviously.The microstructure of J55 steel is composed of pearlite and ferrite.After expansion process,the pearlite is more intense and uniform(3)The expansion process of four different material thread joints with a 23% expansion ratio was studied by finite element method.The results show that the smallest expansion force required for expansion process is austenitic stainless steel S30408,followed by TWIP,J55 and N80 steels.The expansion force increases as the yield strength increases.According to the simulation results,only S30408 and TWIP steels can be expanded successfully among the four materials.To conclude,TWIP steel is suggested to be used for the mono-diameter expansion process based on the mechanical property and price.(4)The expansion process of TWIP thread joint with a 23% expansion ratio under different temperature was studied by finite element method.The results show that expansion force decreases with increasing temperature.The stress distribution is almost the same during expansion process no matter what the temperature is,but the stress decreases as temperature increases.The axial and radial stresses are dominated by compressive stresses while the circumferential stress is dominated by tensile stress.The thread joint is easier to be broken and the seal performance decreases slightly as temperature increases.