Drilling Fluid Density Optimization for Narrow Density Window Fractured Formation

Abstract Fractures are prevalent in the Leikoupo Formation in western Sichuan, with a narrow safe density window of 0.06-0.12 g/cm 3 , leading to frequent downhole accidents like kicks and losses. Using Pengzhou X well as a case study, this research employed numerical simulation to analyze the impact of various drilling fluid densities within this window on displacement rate and bottomhole pressure. It established an optimal drilling fluid density model for fractured formations with narrow density windows. Findings reveal that as drilling fluid density increases, annulus up-return velocity decreases, potentially trapping fracture gas in the upper annulus. Optimal drilling fluid density lies within this narrow window, maintaining a dynamic balance between preventing fast gas return and fluid accumulation in fractured formations. The model applied to Pengzhou Y well yields a drilling density correlation of 0.000001036, consistent with optimized model results. This approach optimizes drilling fluid density within narrow density windows, mitigating downhole gushing and leakage accidents.