Coupling model for carbon dioxide wellbore flow and heat transfer in coiled tubing drilling

• We propose a mathematical method to investigate the flow field with carbon dioxide as drilling fluid.
• The closed model fully couples the hydraulics, heat transfer and physical properties of carbon dioxide.
• The pressure profile, temperature profile and properties profiles are presented and analyzed.
• The feasibility of supercritical carbon dioxide as a drilling fluid is verified with earlier reports.

In order to drill with carbon dioxide as the circulation fluid, a mathematical model was proposed to investigate the flow field in both tubing and annulus. Based on finite volume method, the closed model fully couples the hydraulics, heat transfer and physical properties of carbon dioxide. According to field application, the model is solved and discussed with a case study. The results show that, the pressure is in positive correlation with well depth in both tubing and annulus. The fluid temperature increases fast after liquid carbon dioxide is pumped into tubing and then the increasing rate slows down with increasing depth. Carbon dioxide changes into supercritical state when the depth equals 780 m. The pressure drop of bit jet is 9.78 MPa and the temperature difference between carbon dioxide and formation rock is 12.11 K at bottom hole. In the annulus, the temperature decreases as carbon dioxide flows upward and it is higher than geothermal temperature when depth is less than 927 m. The changes in physical properties are mainly dominated by temperature change in the tubing and by pressure change in the annulus. The density, viscosity and thermal conductivity all witness a constant decrease along the flow route, and the changing trends develop faster at shallow well section in the tubing. At bottom hole, the density is large enough to drive down-hole motors. The heat capacity changes little in the tubing and then increases rapidly when flowing upward along the annulus. The capacity is much larger than that of air in wellbore. Carbon dioxide maintains in supercritical state in the annulus and provides advantages for reservoir exploitation. This study aims to lay theoretical foundation for practical application.