Trapped Annular Pressure Caused by Thermal Expansion Between Double Packers in Ultra-Deep Wells

Trapped annular pressure between double packers is caused by fluid thermal expansion. To accurately predict the trapped annular pressure in ultra-deep wells, a calculation model is proposed with consideration of the impact of fluid nonlinear properties. Nonlinear fluid properties include the variations of the isobaric expansion coefficient and isothermal compression coefficient. Through a case study of an ultra-deep well in western China, the performance of the current method is compared with the traditional method that ignores fluid nonlinearity, and the influences of annular temperature, fluid density, and sealing position on the trapped annular pressure are investigated. Meanwhile, the safety of tubing is analyzed. The result indicates that the traditional method significantly underestimates the trapped annular pressure. The annular pressure may exceed the collapsing strength of the tubing under extreme working conditions, resulting in tubing failure. Reasonably optimizing the distance between double packers and reducing the annular temperature may help mitigate the annular pressure build-up. These findings play an important role in evaluating the safety of packers and strings in ultra-deep wells.