Seeking help to solve ECO2M problems

Dear TOUGH community users,

      I have been struggling with the problem of not being able to successfully simulate running T2Well-ECO2M for too long and would like to seek help. If you could help me, I would greatly appreciate it!

      The model is established using Ansys apdl starting from 0m, with the first unit below the surface center serving as the wellhead unit. The aquifer reservoir is located at -750~-800m, and the wellbore extends to the bottom of the aquifer at -800m. The upper wellbore unit is uniformly dispersed as 65m, and the wellbore unit located in the reservoir is uniformly dispersed as 5m. The initial pressure distribution of the wellbore reservoir system adopts a hydrostatic pressure gradient distribution, with surface pressure set at a standard atmospheric pressure of 0.1MPa. The pressure distribution is calculated based on P=0.1 * 10 ^ 6+1000 * 9.8h (h is the burial depth of the formation); The geothermal gradient is set at 25 ℃/m, the surface temperature is set at 10 ℃, and the temperature distribution of the wellbore reservoir system is calculated based on T=10+0.025h (h is the burial depth of the formation); The lateral boundary of the model is the first type of boundary condition - constant pressure boundary (free boundary) condition; The initial reservoir water saturation is 1.0, the CO2 mass fraction is 0, and the NaCl mass fraction is 0. Inject CO2 into the aquifer at pressure and temperature of 8MPa, 25 ℃ (pressure value higher than the saturation pressure of CO2 at the selected temperature, corresponding to an enthalpy value of 0.263205e+06J/kg), with an injection rate of 10kg/s and a simulation time of 10 days. Now I have the following questions that I would like to ask you:
1.According to the pressure distribution of the wellbore reservoir system described above, the static pressure at the wellhead is about 0.4MPa. During multiple simulations, flash evaporation is often encountered at the wellhead, and the temperature at the wellhead rapidly drops to around -18 ℃.There seems to be a phase transition at the wellhead. Is the temperature at the wellhead dropping close to the critical value that ECO2M can simulate the main reason for the calculation not converging? If so, how should I adjust it?
2.Which should be chosen for the relative permeability function and capillary pressure function between the wellbore and reservoir in the simulation? Can we use IRP (and ICP)=7 (van Genuchten model), or do we need to choose a three-phase relative permeability model, such as IRP=9 (Parker model)?
3.Does the presence of units around the wellbore located in the upper part of the reservoir (such as cap rock areas, surrounding rock formations, etc.) have an impact on convergence, and the model can only be established as having only the wellbore and reservoir?
4.Do we need to add a separate new unit located about 1 meter above the surface as the injection wellhead? If so, what should be its initial state (such as whether its pressure and temperature values are equal to the initial pressure and temperature on the surface; does it need to be set to be surrounded by gas with a CO2 saturation of 1.0)?

    I earnestly request your help, thank you!