Explaining the Sustained Annular Pressure in High Pressure Ultra-Deep Gas Well with Tie-Back Casing Through Numerical Simulation

Sustained annular pressure in the tieback casing annulus of ultra-deep gas wells significantly affects the long-term safe and stable production of the well. Based on the actual case of sustained annular pressure in the tieback casing annulus, this study analyzes the pressure variations in the wellbore caused by changes in the wellbore fluid density. A finite element numerical model was established to examine the distribution patterns of micro-annuli, and a mathematical quantitative relationship between micro-annuli and the combined permeability of the cement sheath was developed. The study also investigates the variation in gas leakage rate with changes in cement sheath permeability. The results show that wellbore pressure variations and the effective stress within the micro-annuli are the primary causes of cement sheath seal integrity failure. The micro-annuli leads to a significant increase in the permeability of the cement sheath, which in turn allows high-pressure gas to leak to the wellhead, resulting in sustained annular pressure. This study provides a theoretical basis for the control of seal integrity in the tieback casing of ultra-deep gas wells.