With the increase of well depth and recovery difficulty in oil and gas development, the downhole temperature and pressure rise unceasingly. The wellbore is simultaneously subjected on the non-uniform in-situ stress and high temperature and high pressure (HT&HP) under this complicated operation environment, which will seriously threat the wellbore integrity and bring huge economy loss. So, give a comprehensive analysis of wellbore integrity and propose corresponding prevention measures are of important significance for the safe and economic operation of HT&HP wells.
In this paper, a new thermal-mechanical coupled model and analysis method for analyzing the mechanical response of casing-cement sheath-formation have been established. The model, which can be simplified to a plane strain problem, has considered the combined actions of temperature, pressure and non-uniform in-situ stress with appropriate boundary conditions. Subsequently, the corresponding equations have been deduced to obtain the mechanical responses of the model under the three kinds of actions mentioned previously. The wellbore integrity has been evaluated by proposed coefficient of failure (COF) defined by the yield criterion of Von-Mises, Drucker-Prager and Mohr-Coulomb. On this basis, the parameter sensitivity analysis of wellbore integrity is presented.
With the increase of formation temperature and non-uniform degree of the in-situ stress, the COF of casing, cement and the contact surface of casing-cement decrease. The Poisson's ratio of cement has no impact on the COF of casing and the contact surface of casing-cement, while the COF of cement increase slightly. If the temperature variation is below 170°C, the cement ring cement is out of failure, and the wellbore integrity is under control. Besides, it is advantageous for wellbore integrity to keep the elasticity modulus of cement 10GPa-20GPa.
This paper has a guiding significance for analysis and control of the wellbore integrity in HT&HP wells, which can allow the engineers to understand the casing failure and wellbore integrity more intuitively, and help them choose reasonable operation parameters to improve the operation life of HT&HP wells.