Modeling and Analysis of Sustained Annular Pressure and Gas Accumulation Caused by Tubing Integrity Failure in the Production Process of Deep Natural Gas Wells

Sustained annular pressure caused by tubing leakage seriously threatens the safe production of deep gas wells. Therefore, it is necessary to fully understand the characteristics of sustained annular pressure and find potential methods to reduce risk. However, most models are about annular pressure caused by thermal expansion and cement integrity failure. Therefore, this paper establishes a model based on the energy conservation law, gas pressure-volume-temperature (PVT) properties, and volume consistency law, to study sustained annular pressure caused by tubing leakage. The results indicate that the pressure and gas volume in tubing-casing annulus increase simultaneously and gradually slow down after tubing leakage happens. The decrease of bottom-hole pressure can reduce the risk of annular pressure, which can be realized by a downhole choke. Other measures can also control the rising speed of annular pressure, including enhancement of production rate, increase of the initial length of the gas column and annular liquid with high compressibility, but the impact of formation energy and annular gas volume should be considered. Sustained annular pressure caused by shallow leaking point has a faster rising speed, longer rising period, and higher pressure value. Large leaking point leads to a remarkable increase of leaking rate and pressure rising speed. The integrity of the upper tubing string should be strengthened.