Assessment of flow with phase transition of geothermal fluids containing multi-component non-condensable gases and flash depth in medium-high temperature geothermal production wells

The flow with phase transition of geothermal fluids containing multi-component non-condensable gases in the wellbore is an important basis for evaluating the productivity, flash depth, and scale inhibition/removal of medium-to-high temperature geothermal production wells. Based on fugacity models for the gas mixtures and activity models for the liquid mixtures, a mutual solubility model and property calculation models (including density, viscosity, and enthalpy) for geothermal fluid containing non-condensable gases of CO₂ and CH₄ were established for temperatures up to 300 ℃ and pressures up to 60 MPa, and were validated against previous results. Then, according to the conservation laws of mass, energy, and momentum, combined with the two-phase thermodynamic equilibrium model and the drift-flux model for gas-liquid velocity, a one-dimensional steady-state wellbore flow model with phase change for the CO₂–CH₄–H₂O system is developed. A robust “bottom-up” stepwise iterative solution method was proposed to accurately predict the phase behavior, flow process with phase change, and flash depth of geothermal fluids containing multi-component non-condensable gases during the production of medium-to-high temperature geothermal wells. Based on the conditions of three typical geothermal wells in Hungary, Turkey, and China, the phase-change flow processes in the wellbore were evaluated. The results show that non-condensable gases have a significant influence on the phase-change location and phase-change flow of geothermal fluids in the wellbore. For the same content of CO₂ and CH₄, due to the low solubility of CH₄, CH₄ is more likely to exsolve from the liquid phase to form a gas phase, thereby significantly increasing the flash depth. The wellbore pressure and temperature distributions are strongly affected by phase change: the deeper the phase-change location and the higher the degree of phase change, the greater the average wellbore pressure and temperature. The bottom-hole temperature, pressure, and the total and relative contents of CO₂ and CH₄ determine the characteristics of the phase-change flow throughout the wellbore. The wellbore phase-change flow model can accurately predict the flash depth, while the commonly used pressure-gradient identification method can approximately estimate the depth range of two-phase flow. Only in geothermal wells with a high degree of vaporization can the flash depth be identified more accurately.