Objective For the flash calculation of natural gas, the dual dew point phenomenon under supercritical conditions can cause the water dew point calculation to converge to a false solution, through numerical analysis and the adoption of appropriate methods to avoid convergence to false solutions. The water dew point curve under supercritical conditions is plotted based on single-point flash calculations. This provides operational guidance for the design and production of natural gas extraction and transportation.

Method The water dew point has a nonlinear relationship with the natural gas composition and operating pressure. Flash calculations using Peng-Robinson (PR) cubic equations of state can accurately determine the water dew point; By combining thermodynamic equations to analyze the dual dew point phenomenon in natural gas, it is found that the critical point of many natural gases does not coincide with the highest point of the phase envelope. This results in the existence of dew points or bubble points in certain supercritical regions, ultimately leading to the formation of dual dew points or dual bubble points; Numerical methods are employed to avoid convergence to false solutions. These false solutions arise from the non-monotonic behavior of vapor-liquid compressibility factors beyond the critical pressure. Slope restriction is used to prevent the numerical calculation from converging to these false solutions; The water dew point curve is calculated using a single-variable iteration method, where the slope of the curve is used to automatically select either temperature or pressure as the independent variable.

Result Using the calculation method developed in this study, the supercritical water dew point curve of natural gas can be accurately plotted. The results show that the error between the calculated natural gas water dew point and actual production data is within 2%.

Conclusion For water dew point calculations beyond the critical pressure, by analyzing the trend of compressibility factor changes and adopting slope restriction, convergence of the numerical calculation to a lower-temperature false solution can be avoided.