Numerical Simulation of Heat Transfer Characteristics of CO₂ Gas Mixture Injection into Wellbores

The flow behavior and heat transfer characteristics of CO2 in wellbores are crucial for its safe and efficient storage. Based on the classic Ramey wellbore heat transfer model, the prediction models of the temperature field and pressure field in the CO2 gas mixture injection wellbore were established in this paper. The model comprehensively considers the interactions between different gas molecules, Joule–Thomson effects, and heat generation due to wall friction and improves the energy and momentum equations. Methods for calculating the physical properties of the gas mixture under both high- and low-pressure (below 10 MPa) conditions are provided. The specific heat capacity at constant pressure, density, and Joule–Thomson coefficient of the gas mixture are calculated using the modified BWRS equation with mixing rules. The viscosity of the gas mixture is determined by using the Lucas correlation. The total heat transfer coefficient is iteratively calculated using a thermal resistance model. By using these physical parameters, the temperature and pressure fields of the wellbore can be calculated. Compared with the experimental data, the accuracy of the model is verified. The calculation error of temperature and pressure at each node is kept within 3%, indicating that the accuracy of the model is improved. Based on the improved model, the changes of the temperature field and pressure field in the wellbore under different working conditions are analyzed when different gas mixtures (CO2, N2, and CH4) are injected. The results indicate that a change in the composition of the gas mixture has a minor impact on the temperature field but significantly affects the pressure field; higher CO2 composition leads to increased bottom-hole pressure. The increase of the injection temperature and pressure both result in higher bottom-hole temperatures but have little effect on the pressure field. The pressure field reaches equilibrium faster than the temperature field. The research results can provide a reference for the migration law of gas mixture in the wellbore and the optimization of wellhead injection parameters.