Abstract: To investigate the effect of coal stress on drilling energy during coal drilling, the coal rock drilling experiment was carried out by using the self-made drilling experimental device with constant drilling pressure, the drilling parameters were measured, and the relationship between drilling energy and coal stress was studied. The surface energy of drill cuttings was calculated by using drill cuttings parameters, and the relationship between coal stress and surface energy was examined. By comparing the drilling energy and the surface energy of drill cuttings, the energy conversion during drilling was analyzed. It was shown that the torque work accounted for more than 95% of drilling energy, and the drilling energy increased with the increase of coal stress, which is approximately in linear correlation with coal stress. The amount of drill cuttings and the surface energy of drill cuttings were both increased with increasing of coal stress. There was 8%−33% energy loss during the conversion of drilling energy to the surface energy of drill cuttings. Energy loss was mainly contributed by acoustical energy and frictional heat loss.

Key words: drilling energy, surface energy of drill cuttings, transformation of energy, energy transfer efficiency, coal stress