The effect of string mechanical properties degradation on wellhead growth of offshore HPHT wells

For offshore HPHT wells, the string has been serving in a harsh environment for a long time, which leads to string mechanical properties degradation and a worse wellhead growth phenomenon. In this article, the mechanical properties of P110 and N80 strings varied with temperature through the MTS experiment. And based on wellbore temperature and pressure conduction method, a new high-accuracy wellhead growth calculation model is established combined with thermal-induced string mechanical properties degradation. The difference between the calculated results and the measured data is less than 8%. And the influence of string mechanical properties degradation on wellbore safety is studied. The results show that high temperature leads to a sudden drop in wellbore safety factors and a sharp increase in wellhead growth. And the maximum critical safety production is recommended to be 85×104m3/d when the case well is worked for 500 days. Attenuation of the string elastic modulus leads to a slight decrease in axial force and a slight increase in wellhead growth. The increase in the thermal expansion coefficient of string has a noticeable influence on wellhead growth. Reducing string thermal expansion capacity and increasing material stiffness can alleviate the wellhead growth phenomenon.