Thermal regime in borehole is simulated by water-filled vertical channel passing through the rock massif. A constant temperature gradient is maintained at the external borders of the massif. Results of numerical simulation of natural thermal convection arising in the channel are presented. Analysis of the results shows the existence of two types of convective thermal effects disturbing the natural temperature field: transient effect, which manifests itself as temperature oscillations around the mean temperature value at a given depth, and quasi-stationary effect causing distortion of natural temperatures and temperature gradient. The obtained statistical relations for estimation of the characteristics of thermal effects and convection flow velocities agree with the data recorded in real boreholes.