We present results of 3D numerical modeling of free thermal convection in liquid filled square pipe, which simulated a borehole. It is shown that convective currents form a system of upward and downward spiral jets, concentrated in the central part of the channel, and a number of smaller gyTes near the walls. The spatial structure of currents and temperature perturbations are permanently changing, which leads to temperature variations over a wide temporal range. Obtained results are in good agreement with the experimental data.