Numerical Studies on bit-rock fragmentation mechanisms

The rock fragmentation process induced by a single button-bit, two neighboring button-bits, and multiple button-bits are numerically studied using the rock and tool interaction code (R-T2D). Through this study, a better understanding of the bit-rock fragmentation mechanisms is gained. It is found that side crack is initiated from the crushed zone or bifurcated from Hertzian crack to propagate approximately parallel to the free rock surface but in a curvilinear path driven by the tensile stress associated with the expansion of the crushed zone during the loading process. In the crushed zone, the mechanism of side crack is mixed tensile and shear failure, but outside the crushed zone, the dominant mechanism of side crack is tensile failure. A semiempirical and semitheoretical relationship among the side crack length, the drilled rock property, and the drilling force is formulated to approximately predict the side crack length. In the simultaneous loading, the interaction and coalescence of side cracks induced by the neighboring button-bits with an optimum line spacing enable formation of largest rock chips, control of the direction of subsurface cracks and a minimum total specific energy consumption. A formula is derived to determine the optimum line spacing on the basis of the drilled rock properties, the diameter and shape of the button-bit, and the drilling conditions. In the rock fragmentation by multiple button-bits, most of the rock between the neighboring button-bits is chipped as a result of the coalescence of side cracks. In the remaining rock, the intensely crushed zones and significant extensional cracks are observed adjacent to the sidewall and the inside of the borehole. Fragment side distribution shows more than 80% of the fragments are fines in the crushed zones as well as the cracked zones, the large fragments be indeed observed, which are the big chips caused by the coalescence of side cracks. © 2008 ASCE.