Numerical Simulation and Analysis of Annular Trapped Pressure during the Testing of Deepwater High-Temperature and High-Pressure Gas Wells

Abstract

During the testing of high-temperature and high-pressure (HTHP) offshore gas wells, heat transfer occurs between the high-temperature gas in the tubing and the low-temperature fluid in the annulus. This causes the annular fluid to expand, leading to an increase in annular trapped pressure, which poses a potential integrity risk. To accurately predict this pressure variation, a full-scale physical model of an offshore HTHP gas well was developed based on simplified assumptions of a constant geothermal gradient and homogeneous casing material. By integrating the Pressure–Volume–Temperature (PVT) equation of state with a transient heat transfer model of the wellbore, a section-by-section method for calculating annular temperature was proposed. A coupled prediction model for annular trapped pressure, incorporating both thermal effects and annular volume change, was then established. Using Well Y in the eastern South China Sea as a case study, numerical simulations of annular temperature and trapped pressure were performed. The results indicate that the model’s average relative error compared to experimental data is approximately 6.15%. When the production rate is 10× 104m3/d, the trapped pressures in annuli A, B, and C are 31, 25, and 21 MPa, respectively. Both temperature and pressure increase progressively from the wellhead to the bottom, with the most significant variations occurring in the shallow section. Sensitivity analysis shows that trapped pressure rises rapidly during the early stage of testing and gradually levels off over time. The annular trapped pressure is positively correlated with production rate, geothermal gradient, fluid expansion coefficient, and well depth, whereas increases in casing elastic modulus, Poisson’s ratio, and the thermal expansion coefficient of the fluid tend to reduce it. The study provides a theoretical foundation and practical support for evaluating wellbore integrity and controlling annular risks under complex conditions in deep-water HTHP oil and gas wells.OPEN ACCESS Received: 03/09/2025 Accepted: 11/11/2025 Published: 16/04/2026

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