Numerical Simulation of Gas–Liquid Gravity Displacement in Vertical Fractures Under Downhole High-Temperature and High-Pressure Conditions

Gas–liquid gravity displacement poses a significant risk to drilling safety. However, the underlying mechanisms governing this process under downhole high-temperature and high-pressure (HTHP) conditions in deep and ultra-deep wells remain poorly understood. In this study, a numerical simulation method based on the Volume of Fluid (VOF) model was developed to investigate gas–liquid gravity displacement behavior under downhole HTHP conditions. The model was validated against 200 data points from visual laboratory experiments, showing excellent agreement with a relative error below 8.58%. Using this validated model, we then conducted 330 numerical simulations to systematically investigate the characteristics of gravity displacement under downhole HTHP conditions. Compared with surface low-pressure conditions, gravity displacement under downhole HTHP is markedly different, characterized by a narrower displacement window, lower gas influx (e.g., 99.5% reduction at −1500 Pa vs. surface conditions) and loss rates, and a smoother gas–liquid interface. As fracture width decreases, both gas influx and drilling fluid loss rates decline nonlinearly, and the displacement window contracts significantly. A critical fracture width for the onset of gravity displacement was identified, ranging from 0.3 to 0.5 mm depending on downhole conditions such as equivalent depth, drilling fluid density, and viscosity. Furthermore, increasing drilling fluid density expands the displacement window and increases the drilling fluid loss rate, whereas higher viscosity reduces both gas influx and drilling fluid loss rates. In contrast, fracture roughness exhibits minimal influence on gravity displacement. These findings provide practical criteria for optimizing well control strategies, thereby reducing drilling risks and improving operational safety. These findings advance the fundamental understanding of gravity displacement and contribute to a theoretical basis for improving drilling safety in deep fractured gas reservoirs.