Study on Casing Strength Check Under Superimposed Complex Working Conditions in Bohai Oilfield

The main types of casing damage in Bohai oilfield include casing corrosion, deformation, leakage and rupture, etc. Faults, mudstone hydration expansion, declection and corrosion are most important causes of casing damage. From the aspects of casing stress change and strength attenuation, the casing check under the superposition conditions of various complex working conditions was carried out. Firstly, in terms of casing stress change, taking a single relatively regular fault as an example, the finite element numerical simulation method was used to analyze the casing stress caused by the fault. Comprehensively considering the changes of mudstone moisture content and rock mechanical parameters, a casing force model considering the hydration expansion of mudstone was established. Secondly, in terms of strength attenuation of casing, the intensity variation law of full-size casing under different full-angle change rates was evaluated by indoor experiments. Based on metagenomic sequencing biological information, the bacteria that played the main decisive role in the corrosion of oilfield water samples were analyzed, and the dominant bacteria were determined through the corrosion results of low-grade carbon steel samples in indoor media experiments, and a prediction model for uniform corrosion and pitting corrosion long-term corrosion rate of pipes of different materials was formed. Under the complex working conditions of oilfield development, the Mises stress of casing should be lower than the casing yield strength, or the calculated safety factor needs to be higher than the minimum safety factor. The method established in this paper was used to check the 9–5/8″ casing of D17 well in a directional injection well of Bohai oilfield. The stresses of casing in this section include casing bending, mudstone hydration expansion, bacterial corrosion caused by the water injection process, in addition to the stress under conventional working conditions. Considering the above complex stress conditions, the 3Cr casing could meet the production needs of the oilfield. The research results of this paper have certain guiding significance for casing material selection and verification under oilfield development conditions.