Applicable Evaluation Method On Environmental Cracking For Tubing And Casing

With the rising global demand for oil and gas resources, the situation of the oil and gas exploration has been increasingly complicated. Corrosion of tubing and casing remains a key obstacle for sustaining operational success in the acidic environment of high temperature, high pressure, and high salinity formation water with high levels of H2S, CO2, CH4 and other corrosion medium. The difficulty of oilfield development is increased due to the perforation, leakage, and rupture failure of tubing and casing caused by hydrogen damage resulting in environmental cracking. Currently, the experimental method of hydrogen damage and environmental cracking are based on the evaluation criteria of fracture mechanics. The test results of stress ring A method and DCB method are so poor in reproducing and paralleling data, and have great data discreteness that the application of the experimental results is severely limited and cannot supply evaluation principle for oilfield construction, strength design and environmental cracking.Based on the petroleum engineering, wellbore mechanics, fracture mechanics and the theory of electrochemical corrosion and strength, the dynamic multiphase flow autoclave, constant load, three-point bending test device is designed and manufactured at the pressure of 70 MPa and temperature of 200℃. The high temperature and high pressure (HTHP) constant load stress corrosion test method. HTHP slow strain rate tensile (SSRT) stress corrosion method, and HTHP three-point bending stress corrosion method are used to test the strength and toughness loss of well pipes C110-1 and Cl 10-2 under HTHP static load (80% σs,constant tensile stress or shear stress) and HTHP dynamic load (3.5×10-4mm/s strain rate) while the intensity and plastic loss is calculated caused by hydrogen damage. The characteristic of environmental cracking caused by hydrogen damage is represented and the reliability equations between hydrogen permeability coefficient and strength loss is formulated. The strength decay, plastic loss, and accelerating corrosion rate induced by hydrogen are studied by the method of orthogonal combination experiment while the environmental cracking is also considered. The Kic of the well pipes under different corrosion environments is calculated, and the environmental cracking evaluation method and strain energy criterion are established according to the characteristics of SCC, strength decay and plastic loss of the well pipes.Evaluation study on strain aging phenomenon of well pipes under different conditions has been conduced while the mechanism of hardening, embrittlement and strain aging caused by hydrogen damage is studied. The strength loss based on tensile strength, yield strength and hardening exponent of the well pipes as a result of hydrogen damage is analyzed. The toughness loss is determined on the basis of hydrogen damage affecting impact toughness, elongation, section shrinkage, surface damage and fracture appearance of the well pipes. According to the test of hydrogen permeation, the relationship between hydrogen damage and hydrogen permeability coefficient or concentration of diffusion hydrogen is determined so that reliability equation about hydrogen damage and strength loss is established.According to the fourth strength theory and strain energy damage, the L16(45) orthogonal experiment is designed to analyze the stress-strain curves of well pipe slope down in acidic environment detected by HTHP constant load stress corrosion test, HTHP SSRT stress corrosion test and HTHP three-point bending stress corrosion test. With the results of well-pipe hydrogen damage in service condition, strain energy criterion is determined while a reliable and stable applicability evaluation method for environmental sensitive cracking of well pipes is put forward depending on experimental and theoretical results.The stress intensity factor calculation method of well pipes in different acidic environment is proposed by improving strength design, applicability evaluation and enhancing mechanical properties of the well pipes. The size of the stress field of the crack tip and characteristic of environmental cracking under static and dynamic load service conditions are analyzed. The applicability evaluation on well pipes in factory and in wellbore are not only important but also necessary. The evaluation method and procedures for environmental sensitive cracking applicability of well pipes are established to supply the method and theoretical basis for the material selection and fitness evaluation of well pipes.