Numerical study of non-isothermal flow and wellbore heat transfer characteristics in CO2 fracturing

A novel 1D+1D model for wellbore temperature and pressure prediction during CO2 fracturing is established with the consideration of CO2 property changes, pressure work and viscous dissipation. The model is verified by the field data from a CO2 injection well, analytical solution and grid independence validation. Improvement of the model is presented by a comparison with the previous work. Then a case study of CO2 fracturing is performed to analyze the influence of various treating parameters based on the novel model. The simulation results indicates that when the injection rate increases from 0.5 m3/min to 4 m3/min, the bottom-hole temperature decreases first then increases, while the wellbore friction increases by 30.14 MPa. However, the high friction drop caused by a high injection rate can be lowered by the tubing and casing injection significantly. Additionally, the time derivative of temperature is still obvious at the end of the fracturing, which makes the steady state assumption not suitable. A lower injection temperature can reduce CO2 temperature profile significantly and increase CO2 pressure profile slightly, which is beneficial for sand carrying. Increasing injection pressure can raise CO2 pressure and the temperature as well, although the pressure gradient is almost unchanged.