Extreme-Density Fluid Design and Sour Gas Mitigation Using Advanced Downhole Pressure Modeling

Abstract

Abstract A challenging high-pressure, high-temperature (HPHT) drilling campaign presented severe operational challenges, including extreme pressure, high H2S (hydrogen sulfide), high CO2, (carbon dioxide) severe WBM gelation, and the need for rapid adaptation under extreme downhole conditions. Additional challenges included tight downhole operational window, high risk of losses and stuck pipe. The primary objective was to design a fluid for maintaining wellbore integrity and stability while drilling through high-risk intervals. The preparation phase addressed extreme H2S and CO2 contamination by formulating a fluid system pre-treated with background scavenging materials. A meticulous rheological design was implemented to ensure operational flexibility within a narrow pressure window while accommodating significant Exploration mud weight variations. Proprietary Downhole Pressure Simulation Software (DPSS) was employed to minimize the risk of mechanical wellbore instability and reduce the likelihood of stuck pipe incidents. To further mitigate differential sticking, the fluid was engineered for ultra-low filtrate loss. A high-temperature Flat Rheology Oil-Based System (FROBS) was selected as the primary solution to meet these demanding technical requirements. This system was rigorously tested under conditions up to 450°F and densities up to 160 pcf (pounds per cubic feet), while maintaining elevated concentrations of H2S scavengers in the background. Comprehensive planning accounted for prolonged static periods, severe sour gas contamination, stringent fluid loss control for hole stability, and adaptability to substantial mud weight increments. One of the HPHT drilling wells was initially planned for 105 pcf WBM in the 5 7/8-inch interval. However, during drilling of the preceding 8 3/8-in section, the WBM experienced severe gelation due to elevated temperature and CO2 contamination, causing extreme break-circulation pressures and necessitating early FROBS deployment. After displacement, density was increased from 105 to 148 pcf using a blend of Barium Sulfate (BaSO4) and Tri-Manganese Tetra Oxide (Mn3O4). This significant weight adjustment posed challenges, but extensive laboratory preparation ensured the high-temperature FROBS maintained performance. The adjustment was executed on-site without delays, supported by pilot testing and shearing. DPSS modeling optimized tripping times and preserved wellbore stability, enabling flawless liner installation without losses. The campaign required a fluid system resilient to severe H2S and CO2 contamination, with ultra-low fluid loss to prevent stuck pipe and a stable flat rheology for extended static periods. The engineered high-temperature FROBS, integrated with advanced downhole pressure modeling, ensured precise hydraulic control and flawless execution, eliminating prior operational failures and setting a benchmark for drilling in extreme environments.