Multistring analysis of wellhead movement and uncemented casing strength in offshore oil and gas wells

This paper presents a theoretical method and a finite element method to describe wellhead movement and uncemented casing strength in offshore oil and gas wells. Parameters considered in the theoretical method include operating load during drilling and completion and the temperature field, pressure field and the end effect of pressure during gas production. The finite element method for multistring analysis is developed to simulate random contact between casings. The relevant finite element analysis scheme is also presented according to the actual procedures of drilling, completion and gas production. Finally, field cases are presented and analyzed using the proposed methods. These are four offshore wells in the South China Sea. The calculated wellhead growths during gas production are compared with measured values. The results show that the wellhead subsides during drilling and completion and grows up during gas production. The theoretical and finite element solutions for wellhead growth are in good agreement with measured values and the deviations of calculation are within 10%. The maximum von Mises stress on the uncemented intermediate casing occurs during the running of the oil tube. The maximum von Mises stress on the uncemented production casing, calculated with the theoretical method occurs at removing the blow-out-preventer (BOP) while that calculated with the finite element method occurs at gas production. Finite element solutions for von Mises stress are recommended and the uncemented casings of four wells satisfy strength requirements.