Numerical simulation of coaxial heat transfer capacity of deep vertical wells

Aiming at the scientific evaluation of coaxial casing heat transfer of geothermal resources with medium burial depth(1 500-4 000 m), a rock-water coupled heat transfer model of a vertical single well was built based on the simulation platform of COMSOL Multiphysics software. Taking the stratigraphic and geothermal characteristics of Xi'an area as the simulation background, the heat transfer capacity of single well(3 500 m)casing under two different geothermal gradients(0.027℃/m and 0.030℃/m) and four flow velocities(0.25, 0.50, 0.75, 1.00 m/s) was calculated and analyzed. Meanwhile, the numerical models of group wells with five kinds of well spacing were established, and the variation law of outlet temperature of the central group wells with different well spacing was analyzed. The simulation results show that the greater the geothermal gradient, the bigger the temperature difference between the inlet and the outlet, and the larger the heat transfer per unit time and the heat transfer per unit length of borehole. The larger the inlet injection velocity, the smaller the temperature difference between the inlet and the outlet, and the larger the heat transfer per unit time and per unit length of the borehole, which indicating that the higher the geothermal gradient and the flow velocity, the higher the heat transfer efficiency is. Under the deep working condition of 3 500 m in Xi'an area, when the distance between adjacent geothermal wells is 30 m, it would ensure that the formation temperature will not interfere with each other. The conclusion in this paper would provide scientific reference for the development and utilization of the deep geothermal resources.