A Study on the Determination Method of Top-up Volume Based on the Upward Migration Velocity of Oil and Gas

Top-Up operation serve as a critical measure to maintain formation pressure balance, prevent formation fluid influx, reduce the upward migration rate of oil and gas, extend safe operating time, and reduce the risks of overflow and blowout during well control. However, current research predominantly adopts qualitative and semi-quantitative methodologies, while Top-Up operation frequently utilize empirical criteria for fluid displacement—a practice colloquially termed “Blind Killing” (experience-based approach) in well control engineering. Based on this study, a calculation model for gas influx velocity during tripping out operations has been established. The model first determines the size of gas particles by back-calculating from the observed gas rise velocity in static drilling fluid. Then, it evaluates the variation of drilling fluid viscosity with tripping speed, considering the shear-thinning behavior of the fluid. Using the viscosity and drill string structure, the model calculates the gas rise velocity under different tripping speeds. Finally, the required loss rate to maintain wellbore safety is determined based on the calculated gas rise velocity. /Based on this model, the influence of drilling fluid properties, tripping speed, and formation fluid density on the upward migration velocity of oil and gas was investigated. The results indicate that as the tripping speed increases, the viscosity of the drilling fluid decreases due to shear thinning, leading to an increase in the oil and gas migration velocity, which in turn requires a higher loss rate to maintain formation pressure balance. This method balances the upward migration of oil and gas by controlling the mud loss rate. Compared to traditional blind Top-Up methods, the recommended drilling fluid volume in this study ensures wellbore safety while reducing fluid consumption and overall operational costs, thereby providing a theoretical foundation for safe Top-up operations.