Modeling and Parameter Optimization for Bullheading Kill in Ultra-Deep Fractured Gas Reservoirs

Accurate prediction of wellbore pressure during bullheading operations in ultra-deep fractured gas reservoirs faces critical challenges, primarily stemming from dynamic bottomhole boundary estimation uncertain-ties and the complex evolution of gas-liquid flow regimes. Through a laboratory physical simulation system, this study elucidates the gas mi-gration-fragmentation dynamics under varying-viscosity kill fluid sys-tems and clarifies the dynamic equilibrium mechanism at the gas-liquid interface during killing operations. Based on these findings, a novel method for determining the critical displacement of the bullheading method was proposed. By coupling gas-liquid two-phase seepage in fractured reservoirs with wellbore multiphase flow, a transient multi-phase flow model and its numerical solution algorithm for bullheading kill operations in ultra-deep fractured gas reservoirs were developed. Field validation demonstrates that the prediction error for wellhead cas-ing pressure remains within 7%, falling within the allowable engineering error margin. Systematic analysis of kill fluid parameters (injection rate, viscosity, density) and reservoir fracture characteristics reveals that wellhead pump pressure is directly proportional to both the dis-placement rate and viscosity of the kill fluid, while being inversely pro-portional to reservoir fracture permeability. High-density kill fluids can accelerate the decrease in pump pressure but have minimal impact on peak pressure. For the case study, the critical kill rate range is deter-mined to be 9.6–65 L/s; rates below this threshold fail to effectively suppress gas influx, while exceeding it risks equipment overpressure. The proposed multiphase flow model and parameter optimization method can provide a theoretical foundation and technical support for safe and efficient well kill operations in ultra-deep fractured gas reser-voirs, significantly enhancing the scientific rigor and reliability of emer-gency well control strategies.