Working mechanism and rock-breaking characteristics of coring drill bit

For improving the rate of penetration of the complex well structure and performance of the coring drill bit, this paper is to investigate a new type of coring drill bit that generates small-diameter core samples during the drilling process. A special jet channel is designed inside the drill bit body, and the high-pressure drilling fluid flows through the special jet channel to the coring channel, then the cored rock is taken out of the downhole from the drill bit under the negative pressure. Based on the hydrodynamics theory, rock breaking mechanics and actual drilling conditions, the rock breaking mechanism model of the coring drill bit is established. The difference of rock breaking performance between the new drill bit and the existing drill bit is analyzed based on the analysis of numerical example, laboratory and field test. The research results indicate that the special structure design in the new drill bit has the effect of 'absorbing the bottom rock cuttings and pushing the upper rock cuttings' during the process of rock breaking. The effect can reduce the repeated cutting of cores at the bottom hole and improve the ROP(rate of penetration). Field tests show that when the flow rate of drill pump is 25L/s, the result of ROP can be increased by 15%-40%. The new design drill bit can present innovative ideas for improving the efficiency of coring and the quality of geological logging during the drilling process. Moreover, the analysis models can provide references for the new research of improving ROP and drilling efficiency, especially under the conditions of complex structure or large displacement wells.