Dynamic Variable-Density Well Killing Model and Real-Time Decision Method for Complex High-Pressure Formations

Summary In high-pressure formations, gas influx often leads to high shut-in standpipe and casing pressures. Due to complex geological conditions and uncertainties in the pressure system, the variable-density well killing method is often used for safe well control. In this paper, a general matrix boundary of killing fluid density and volume was proposed, and a dynamic variable-density well killing model suitable for complex high-pressure formations was developed. The accuracy and reliability of the model were validated using the field measurement data and the recognized well control simulator—Drillbench, with good agreements. The developed model can impose any sequence of killing fluid densities and dynamically update subsequent well killing parameters when control strategies change. It overcomes the limitations of previous models, which required predetermined killing fluid densities before well killing and lacked the capability to adjust parameters in real time. The calculated results indicate that, under high shut-in standpipe and casing pressures, if the formation has enough pressure-bearing capacity, implementing aggressive followed by gradual killing fluid density increments, increasing large-density fluid volume, and reducing killing fluid density adjustment frequency enables rapid standpipe and casing pressures reduction. For formations with difficult-to-estimate pore pressure and low pressure-bearing capacity, the control strategies are opposite, and the lost circulation materials should be added to the killing fluid during early well killing circulation to improve the formation’s pressure-bearing capacity. Additionally, before the killing fluid reaches the drill bit, increasing the flow rate is the only means of reducing high casing pressure. Moreover, during the annulus gas removal, the change in casing pressure is consistent with that in pit gain. So, a simplified real-time casing pressure calculation method was proposed with the standpipe pressure as the primary control target, which overcomes the empiricism and blindness of conventional casing pressure control under the influence of manual control errors, pressure wave propagation delays, and secondary kicks. By integrating variable-density well killing parameter predesign with real-time decision, this research provides a more effective theoretical guidance for safe well control in complex high-pressure formations.