Migration Laws of Acidic Gas Overflow in High Temperature and High Pressure Gas Wells

Most of the existing ultra-deep gas wells in China are characterized by high temperature, high pressure, and high sulfur content. During the development process, they face severe challenges such as unclear mechanisms of acidic gas-induced blowouts and difficulties in wellbore pressure inversion, which pose significant risks to well control operations. To reveal the reasons behind the tendency of acidic gases to trigger blowouts and to clarify the impact of different concentrations of acidic gases on the flow behavior of annular fluids, this study considers the effects of solubility and phase changes on the physical properties of acidic gases. A method replacing critical parameters with pseudo-critical parameters is used to analyze the variation trends of gas density, solubility, and other properties along the well depth. A mathematical model for the annular flow of acidic gas overflow incorporating solubility phase change effects is established. The model is numerically solved using a four-point difference scheme, exploring the essential characteristics of gas flow in the annulus after overflow, and discussing the distribution patterns of physical properties of acidic gases, as well as dynamic parameters such as wellbore pressure and temperature along the well depth. The results indicate that the physical properties of acidic gases change significantly with well depth: the more acidic gas present in the wellbore, the smaller the deviation factor, and the greater the density and viscosity, with parameter changes exceeding 40% near the pseudo-critical point. Compared to pure methane, mixed fluids containing acidic gas experience more than 20% volume expansion near the wellhead, and the flow velocity increases by more than 10%, leading to a higher risk of a sudden pressure drop during well control. This study clarifies the migration patterns of acidic gas overflow in HPHT (high pressure, high temperature)gas wells, providing valuable guidance for optimizing well control design, improving well control emergency plans, and developing well-killing measures.