Delivering a Dependable Zonal Isolation Barrier for an Extreme HPHT Well in Offshore Vietnam - Lessons Learned from Cementing High Pressure High Temperature Well

Abstract

With recent discoveries, offshore Vietnam continues to provide promising prospects for hydrocarbon production, especially with rapid exploration and deployment in the offshore HPHT fields in the last decade. However, Vietnam has some of the most complex and hottest wells in the region. An operator was planning to break the barrier of the hottest well in offshore Vietnam. Bottom hole pressure and temperatures for this well were predicted to be around 12,000 psi and 200 deg C, respectively. Designing and delivering a dependable zonal isolation barrier was paramount to the success of the well. The well architecture included the primary cementing job for six casing strings - 30-in. conductor casing, 20-in. surface casing, 16-in. casing, 13 5/8-in. casing, 9 7/8-in. casing, and 7-in. production liner. The well was for exploration purposes and was to be abandoned by seven (7) cement plugs in cased hole across various depths. A total of 4,200 bbls of cement slurry with a wide density range from 12.0 lbm/gal for the surface casing to 18.0 lbm/gal for the production liner, were tailored and pumped in the well. Rigorous slurry testing was conducted up to 198 deg C and 13,000 psi downhole pressure for the production section slurries. To provide for proper hole cleaning a tailored spacer was designed and tested for stability under the same downhole pressure and temperatures. A total of ~1,000 bbl of spacer were pumped in the well, having a density range from 10 lbm/gal to 17.2 lbm/gal. To maintain the robustness of the slurry design in order to handle any changes in well parameters various sensitivity tests were performed at different temperatures, retarder concentrations and with mud contamination levels (predicted by computational fluid dynamic modelling). For the production section, an aggressive slurry with less than a 15 min transition time was designed to avoid any gas migration in the setting cement. This case study, techniques and lessons learned can be applied to similar wells around the globe especially in the challenging environments of extreme HPHT.