Purpose. The study of pulse flushing impact on weakening of rock formation in the course of diamond core drilling using diamond bits. Methodology. Theoretical research of rock formation softening processes under transient thermal impact is based on the theory of thermoelasticity. Comparative analysis of thermal stress condition of rock formation in the course of diamond core drilling using both the pulse and continuous flushing modes was carried out in the course of the research. Findings. The process of rock formation breaking in the course of diamond core drilling using stationary and transient (pulse) flushing with drilling fluid was studied herein. Conditions for transition of rock formation fractures to movable state and strength reduction have been defined. The estimated relation between the fracture development rate and delay of rock formation decomposition are obtained herein. The study revealed that under the pulse mode of drilling fluid supply conditions appear for thermal decomposition of rock formation. Owing to a greater, as compared to the continuous flushing mode, amplitude of temperatures in the borehole working area, strength reduction of the rock formation surface occurs. When boring granitic rocks, its strength reduction is 12 % on average. Originality. The analysis of thermal stress condition of the rock formation was carried out for the first time along with grounding of the possibility to employ the thermal cycle effect to enhance the efficiency of rock decomposition in the course of the core drilling using diamond bits. For the first time ever the characteristics of thermal decomposition of rock formation were determined as follows: minimum dimensions of fractures and minimum delay time of decomposition commencing; the evaluation of rock formation strength reduction for the diamond core drilling process. The study revealed that the technique of pulse supply of the drilling fluid stipulates for notable reduction of energy consumption of the rock formation decomposition process. Practical value. The outcomes of the study can be applied to develop pulse techniques of wells boring, as well as to establish efficient and cost saving parameters of the process.

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