Indentation experiments were performed on Berea sandstone, Indiana limestone and highly porous refractory brick specimens using axisymmetrical indenters. The process of rock compaction in the region beneath the indenter was found to be of primordial importance in the indentation of these porous materials. The extent of the compaction zone was found to be controlled largely by the porosity of the rock and the strength of its pore structure. The refractory brick specimens failed exclusively by pore collapse and consequently were used to visualize the development of the compacted zone isolated from influences of other modes of failure. We infer from our results that rock compaction under the teeth of roller cone bits suppresses chip generation and consequently reduces the rate of penetration. In addition, a considerable amount of indentation energy is used during compaction, thus reducing the efficiency of the overall process during drilling. This problem is thought to be specifically acute in the case of deep drilling of porous rocks for gas and oil using roller cone bits.
The generation of a compaction region during indentation is not a new phenomenon, it was observed in the past by virtually all investigators working with porous rocks; however, it has been consistently neglected as a relevant feature of the failure process. Paul and Sikarskie (1965) reported that experimental observations delineated the existence of two major types of rock behavior during indentation, some that merely deformed plastically under the indenter and others that cracked and produced chips. Their efforts were directed to study the type of rocks that failed by producing chips, and though a compaction or "crushing stage" was introduced in the physical description of failure, it was subsequently neglected during the mathematical idealization of the problem. Cheatham (1964) studied the indentation of rocks that behaved in a ductile manner using a rigid-plastic model. Although this model was later implemented by Miller and Cheatham (1972) by introducing a criterion allowing for compaction and work hatdenning of porous rocks, it was still restricted to rigid-plastic behavior and did not reflect the importance of the elastic deformation around the region of compaction. The relevance of the former on the indentation process was studied by Tabor (1970) and Johnson (1970), who pointed out that the effect of elastic deformation on the mode of failure was to reduce the constraint that the surrounding material imposed on the plastic zone. As a consequence, the characteristic displacement field was one that moved radially in front of the indenter, instead of around the indenter and upward towards the free surface, as in the rigidplastic case. Radial displacement fields were first proposed by Marsh (1964) for brittle glass, and latter by Tabor (1970) for rigidplastic materials that strain harden. The idea that the plastic zone expanded away from the indenter in a radial manner, constrained by the surrounding elastic field, proved to be a suitable mechanism to explain observations that conflicted with the mode of deformation postulated by the rigid-plastic model. Investigators working on indentation of brittle glasses and ceramics provided further evidence on the validity of the expanding cavity model, (Lawn and Wilshaw 1975; and Donovan 1989). Yoffe (1982) presented an analytical model in which plastic flow took place by either true plastic yielding, by compaction of the pore structure or by a combination of the two. The displacement field under the indenter was radial, not necessarily spherical as in the Johnson (1970) model, but one that could be elongated or shallow and change shapes as a function of the porosit