Explanation of Tie‐Back Cement Sheath Leakage Caused by Microannulus in Ultra‐Deep Gas Wells and Control Strategy

The tie‐back cement sheath is an important to isolate formation fluids in ultra‐deep gas wells. To address the leakage caused by micro annulus and the quantitative evaluation methods, this study establishes a full‐well characterization approach for microannulus by integrating mechanical analysis with finite element simulation. A quantitative evaluation method is developed for leakage severity based on equivalent permeability and the influence of prestress on microannulus is analyzed. Furthermore, a assessment method for cement sheath leakage is proposed by linking leakage pressure differentials with the total breakthrough pressure. The results indicate that the extreme values of wellbore pressure are the primary drivers of micro annulus initiation. Both microannulus size and equivalent permeability increase with depth and exhibit nonuniform distribution. In the case study well, the maximum equivalent permeability of the cement sheath reaches 26.93 mD, forming a channel for high‐pressure fluid migration. Prestress cementing reduces the size of microannuli. The quantified relationship between total breakthrough pressure and leakage pressure differential challenges the conventional assumption that “microannulus formation directly implies leakage,” providing a more reliable basis for leakage control. However, further work is required to resolve the key bottleneck of quantitatively characterizing breakthrough pressure gradient.