Generation and development of micro-annulus under complex load environment conditions in well deviated sections and barrier establishment method

The cyclic loading and unloading of casing internal pressure is an important influencing factor causing the generation and development of micro-annulus at the wellbore cementation interface.The well deviated section bears alternating loads while facing complex conditions of deflection of in-situ stress and casing eccentricity. A numerical model of the casing-cement sheath-formation combination in the deviated well section considering the casing eccentricity and in-situ stress deflection was established. A comparative analysis was conducted on the change law of the cumulative plastic strain under conventional conditions, as well as under the conditions of in-situ stress deflection and casing eccentricity in the deviated well section. The influence of the casing's centralization degree, wellhead pressure, and mechanical parameters of the cement sheath on the the change law of the width of micro-annulus was analyzed, and the distribution law at different depths in the well deviated section was quantified. The research findings indicated that, as the degree of casing eccentricity grew, the initial plastic strain will correspondingly increased, thereby further heightening the risk of gas channeling. The width of the micro-annulus formed within the cement sheath gradually expanded as the wellhead pressure keeps rising. Under identical mechanical environment conditions, enhancing the casing centralization degree and decreasing the elastic modulus of the cement sheath are of great benefits for reducing the width of the micro-annulus. Under the condition of the same number of alternating load cycles, the width of the micro-annulus in the well deviated section will tend to increase when depth increases. Finally, a cementing method using composite slurry column structure in deviated section was proposed, and it proved to have better effect in controlling the width of micro annulus. Moreover, the research results can offer an effective theoretical reference for safeguarding the sealing integrity of the cement sheath in the deviated well section.