Heat Transfer in Tubing-Casing Annulus during Production Process of Geothermal Systems

In geothermal systems, the temperature distribution of heat flow in the wellbore is dependent on the well structure and the geological conditions of the surrounding formation. Understanding of heat transfer in the tubing-casing annulus can reduce the heat losses of wellbore fluid during the production process. The present study discusses the possible means of heat transfer in the annulus, and develops a piecewise equation for estimating the convective heat transfer coefficient with a wider valid condition of 0 < Ra < 7.17×10⁸. By converting the radiation and natural convection into equivalent thermal conduction, their sum is defined as a total thermal conductivity to describe the heat transfer in the annulus. The results indicate that the annulus filled with gas can be utilized as a good thermal barrier for the fluid in the wellbore. Additionally, the contribution of radiation will increase to occupy a majority proportion in the total thermal conductivity when the annular size increases and the materials have high emissivity. Otherwise, thermal radiation is just the second factor.