Experimental study on rock-breaking performance of water jets generated by self-rotatory bit and rock failure mechanism

Abstract

Tree-type borehole drilling and volume fracturing in underground coal mines is a new method for improving the homogeneous permeability of coal seam. A self-rotatory bit is a potentially efficient bit, which can improve the drilling efficiency and stability of tree-type branch boreholes. In this paper, a new self-rotatory bit with a simple structure and small size was designed. The rock-breaking area and volume of the self-rotatory bit water jet were analyzed, and the effects of the nozzle distribution radii, jet pressure and rock strength on the rock-breaking and hole-forming performance of the bit were studied. Based on CT and SEM scanning, the rock damage properties and failure mechanism under the water jet impingement of the self-rotatory bit were investigated. The results indicated that compared with the existing typical water-jet bits, the proposed bit increased the rock breaking area and eliminated the rock-breaking blind zones, decreasing the specific energy consumption. The bit had the optimal nozzle distribution radii to break rock effectively and form a larger borehole. Nozzles with a close distribution could not achieve the joint rock-breaking efficiency, whereas when the nozzles were placed relatively far the rock broken pits could not connect with each other; thus, resulting in the formation of annular bosses and reduction in the drilling efficiency. The specific energy consumption of self-rotatory bit decreases with the pump pressure and increase with the increasing rock strength. The rock breakage behavior induced by the water jet of the self-rotatory bit was similar to that of an interrupted pulsed jet. The “water cushion” effect was effectively weakened. First, the rock failure patterns like sandstone were cementations and rock particles with weak strength were removed. Thereafter, the mineral grains were broken mainly in a brittle tensile failure caused by the tensile stress, generating micro-fractures and fissures such as transgranular and intergranular cracks. Annular broken pits with an approximate “V” profile and damage zone surrounding the pits were formed. This study could provide a reference for the design of self-rotatory bits and its application.