Indentation Experiments on Dry Rocks Under Pressure
Authors: Gnirk P.F. 
Published: December 2007

An experimental investigation has been made to study the erects of tooth angle and confining pressure on the force required for indentation of dry rock samples under confining pressure. In these experiments static single-tooth indentations were made on two sandstones, a limestone, a schist and a slate under confining pressures of 5,000 to 15,000 psi. Sharp wedge-shaped teeth with included angles ranging from 30 deg. to 105 deg. were used.In all cases the force-displacement curves are approximately linear for rocks tested under confining pressure, as contrasted with the discontinuous curves obtained for similar rocks at atmospheric conditions. Photomicrographs of thin sections through the craters indicate very little evidence of chip formation during indentation. In general the experimental data lie between calculated values for perfectly rough and perfectly smooth tooth-rock interfaces.

Introduction

The variables associated with drilling at depth are numerous and, in many instances, are not easily isolated and controlled. Also, many of these variables are interdependent; however, if the variables which are sufficiently independent of each other are held constant and are appropriately introduced, the dependent variables can be more easily studied and evaluated. It is the purpose of the present investigation to study experimentally the simplest case of bit tooth-rock interaction. In essence, the experiments have involved the static loading of sharp wedge-shaped tools on the surfaces of effective semi-infinite dry rock samples subjected to confining pressures. During this investigation the following quantities have been maintained constant:Pore pressure: atmospheric (i.e., the rock is dry).Permeability: although the rocks may be permeable, they are "jacketed" in such a manner that the fluidis not permitted to flow through them.Temperature: 75 deg F.Rate of loading: essentially static (approximately0.002 in./second).Bit tooth: a sharp wedge-shaped tooth loadednormal to the rock surface.Rock surface: smooth and flat.Drilling fluid: hydraulic oil.

In addition to the above conditions, the rock surface was free of a layer of cuttings. There was no indexing to previously indented surfaces, and the influence of corners of the borehole was neglected because the rock samples were effectively semi-infinite. The following quantities have been based, and the resulting effects have been studied:RockType (e.g., limestone, sandstone etc.).Mechanical properties Necessarily dependentPorosity on type of rock.Bit tooth (included angle of wedge-shaped tooth).Depth of penetration.Confining pressure.

On the basis of laboratory experiments which have shown that many rocks exhibit ductile behavior at confining pressures corresponding to depths usually reached during drilling, a theory has previously been advanced which predicts the force required for penetration by a single sharp wedge tooth into a ductile material. The material is assumed to be isotropic, homogeneous and rigidly plastic (on elastic deformation) and to obey the Coulomb yield criterion (Figs. 1 and 2).

JPT

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