Study on Prediction of Sustained Annular Pressure and Annular Gas–Liquid Interface Depth in HPHT Gas Wells

To accurately predict the sustained annular pressure and annular gas–liquid interface depth during the production of high-pressure and high-temperature (HPHT) gas wells and address the issues of natural gas leakage and annular fluid loss caused by leaks in the tubing string, a prediction model for the sustained annular pressure and annular gas–liquid interface depth in HPHT gas wells is established. The model considers the expansion and compression of annular fluid, as well as the variation in annular volume under the effects of temperature and pressure. Taking the HPHT Well H21 in western China as an example, the model is verified using field-measured data, and the variation laws of sustained annular pressure and annular gas–liquid interface depth under different leak parameters and production parameters are investigated. The results show that the predicted sustained annular pressure first rises rapidly, then decreases slowly, and finally tends to stabilize. The expansion, compression, and leakage of annular fluid influence the depth of the annular gas–liquid interface, which changes dynamically in the early stage and stabilizes in the later stage. The maximum error between the measured and predicted sustained annular pressure is 2.65%, the stable sustained annular pressure is 13.97 MPa, and the minimum gas–liquid interface depth is 144.00 m. It is recommended to increase the density of the annular protection fluid, maintain an initial sustained annular pressure, and replenish it regularly, so as to ensure the safe control of HPHT gas wells under sustained annular pressure.