The trend in drilling for oil and gas is hard rock and deeper formations. Rate Of Penetration (ROP) is the single most important factor impacting drilling costs and performance, as well as the economic development of certain difficult to reach hydrocarbon resources. A fundamental understanding of the micromechanisms of rock fracture and rock microstructure-fracture relationships is essential in optimizing drilling parameters for high ROP and drill bit performance. Although indentation damage is fundamental to rock cutting in roller cone and even in drag bit drilling, rock fracture and damage even under simple indention are poorly understood, especially as they relate to rock microstructure. Advanced analytical electron microscopic characterization techniques have been utilized to study rock microstructure, damage, and fracture under simple indention in a hard marine sandstone from the Piceance Basin. Advanced Electron Backscattered Diffraction in a scanning electron microscope have revealed that many of the sandstone grits are characterized by polycrystalline structures and that the propagation of indention induced micro-cracks is along various interfaces but not via trans-granular cleavage. Numerical simulations using a discrete element code are being developed to model rock damage processes under the complex stress paths developed under the indenter.


Rock fragmentation via indentation of a bit is fundamental to most rock drilling processes. Rock cutting by roller cone cutters and rippers is primarily an indentation-type process. The movement of a single cutting tooth can thus be considered as indentation loading (Nishimatsu 1972, Lindqvist 1982, Cook et al. 1984, Kou 1995, Liu 2003) of the rock. Even though the principle of rock fragmentation by indentation loading has been widely used from ancient times, the details of the rock fragmentation process adjacent to an indenter remain poorly understood.

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