Research of the Influencing Factors on Thermal Insulation Effect of Casing with Parasitic Pipes and Central Tubing

In the process of drilling in permafrost, the frozen soil is susceptible to melt under the influence of drilling fluid. To solve this problem, casing with parasitic pipes and central tubing was introduced to insulate the pipe from the permafrost. Then, a set of test system for the thermal insulation of casing with parasitic pipes and central tubing was designed to analyze its performance and influencing factors. The experiment reveals that the central part of the casing can effectively suppress heat transfer in the radial and axial direction and keep the outer wall at a low temperature. However, thermal insulation of the couplings on both sides are poor, which raise the outer wall temperature, resulting in a U-shape distribution of it. In addition, the sensitivity analysis and single-factor fitting of environmental and operational parameters indicate that the temperature rise from the environment and fluids will increase the outer wall temperature linearly, and the increase of drilling fluid displacement will reduce the outer wall temperature following a logarithmic relationship. From the coefficient of the multivariate fitting equation, it is clear that the most influential factor on outer casing wall temperature is the environmental temperature followed by cold fluid and hot fluid temperature. The research results show that using casing with parasitic pipes and central tubing to protect the permafrost is very feasible, and engineering technical measures such as lengthening the intermediate casing section, improving the coupling structure, maintaining low temperature and displacement of cold fluid can significantly improve the protective effect.