Research and Application of FLAT-PRO Constant Rheology Synthetic Base Drilling Fluid in Deepwater Operation

Technical problems encountered in deepwater drilling, such as low temperature, narrow drilling window, formation of gas hydrate by shallow natural gas and hole sloughing etc. have posed rigorous requirements to drilling fluids. Using GTL Saraline 185V as the base fluid, a deepwater synthetic base drilling fluid, FLAT-PRO, was developed based on optimizing the emulsifier, organo-clay, filter loss reducer and other additives. Laboratory evaluation results showed that FLAT-PRO had stable constant rheology at 4-65℃. In permeability test, a permeability recovery of higher than 90% was obtained. This performed very well even when was contaminated with 10% seawater and 10% drilled cuttings. FLAT-PRO was easy to degrade, satisfying the need of environmental protection. An offshore well, LS-A, drilled in South China Sea, with water depth of 1,699.3 m, had formation (main target) temperatures between 34.6℃and 36℃. The coefficient of formation pressure at the shoe of the φ508 mm casing string was lower than 1.14, indicating a narrow safe drilling window. How to control ECD and mud losses were the major concerns of the drilling operation. FLAT-PRO with a density of 1.03 g/cm³ was used in spudding the well, and FSVIS was chosen to maintain the rheology of the drilling fluid. During drilling, rheological parameters of the drilling fluid were maintained as follows:FV=50-70 s, YP=8-15 Pa, φ₆ reading=7-15, and φ₃ reading=6-12. 2% of HFR was used in filtration control. Drilling, tripping and wireline logging were all successful performed. Filed application showed that the deepwater constant rheology FLAT-PRO drilling fluid had stable rheology at different temperatures, good inhibitive capacity, and superior lubricity, thereby maintaining low ECD during drilling and satisfying the needs for deepwater drilling.