Surface diagnosis technology for the key parameters of leakage point in the tubing-casing annulus of deep gas wells

In the development process of deep gas well, the phenomenon of abnormal tubing-casing annulus pressure is caused by tubing string leakage and failure, which is likely to lead to gas leakage, wellhead jack-up, string damage and other accidents and consequently threaten wellbore safety. Superior to the traditional downhole logging method, the surface diagnosis technology can judge the tubing leakage degree rapidly, locate downhole leakage points accurately, avoid the disturbance of production string and discover and identify potential wellbore risks as early as possible. Firstly, the leakage point depth calculation model was established according to pressure balance principle. Then, the process of gas leakage was described by means of pinhole model, and the gas accumulation and pressurization model based on the annulus volume compatibility principle was constructed. Finally, the sensitivity factors of annulus pressure were quantitatively analyzed by means of control variable method. In addition, the annulus pressure increase laws were divided into four typical modes based on leakage point depth and equivalent size difference, combined with the characteristics of annulus pressure relief/recovery testing curve. Taking the parameters of one case well as the example, the physical process of the annulus pressure caused by tubing string leakage and failure is divided into three stages, i.e., pinhole leakage of pipe string, gas-liquid two-phase migration and cavitation accumulation and pressurization. As the time goes, annulus pressure and gas volume increase gradually and leakage rate decreases gradually until the pressure balance period inside and outside the leakage point approaches to the stable value. The pressure increase model of “large hole in the shallow part and fast rising at high pressure” is the highest in risk level. The inversion calculation results show that the annulus pressure of the case well is 26.1 MPa, the equivalent size of leakage point is 2.1 mm and the maximum leakage rate is 0.20 m³/min, indicating higher wellbore safety risk. It is recommended to adopt mechanical repair and chemical plugging agent for sealing and plugging.