Study on prediction and evaluation of sustained annular pressure in ultra-deep gas wells considering tubing multiple leakage points

To accurately predict sustained annular pressure (SAP) resulting from tubing integrity failures in ultra-deep gas wells, a new prediction model is developed by considering the effect of leakage states of leakage points and the interaction between volumes of gas leakage and liquid leakage. First, a new calculation equation for the SAP is established by analyzing the coupling relationship of multiple leakage points. Then, a tubing-casing annulus is divided into several segments and transient annular pressure is solved by iteration methods. Finally, the impact of sizes and the number of leakage points are mainly studied. The results show that the maximum error of the new model is only 3.81% by comparison with the measured data. For the case of two leakage points, when the size of the lower leakage point is larger than the upper leakage point, the recovery process has a significant point of the abrupt change with a pressure of 9.15 MPa, and the recovery trend is first rapid (the average recovery rate is 1.83 MPa/h) and then slow (the average recovery rate is 0.40 MPa/h). The number of leakage points only impacts the recovery rate. When the position of leakage point exceeds 3000 m, the impact on the recovery rate becomes relatively marginal. This means that the closer the leakage point is to wellhead, the greater the effect on the recovery rate, so it is necessary to reasonably optimize upper tubing structure to ensure its integrity. These findings lay a theoretical foundation for ensuring the wellbore safety of ultra-deep gas wells.