This paper was selded for presentation by an SPE Program Committee followng rwiw of tnfornubon contained in an abstract submitted by the author(s) Contents of the paper as t~. ham n'Jt~n rw-by the SccIety of Petroleum Engineers and are subject to w-on by the author(s) me material, as presented, does not necessarily reflect any Posmonof the =!ety of Petroleum Engineem, its officers, of membem Papers presented at SPE meetings are subject to publication fww by Edftorjal Committees of the~iety of Petroleum Engineers~ecbo"Ic reprodudon, distribution, or storage of any pa~of this paper for commercial pu~es without the w,tten tn"sent of the Soclefy of Petroleum Engineers is prohlbted PerrmssIon b reprcduce In PM is res~cted to an abstract of not more than 300 words, dtusbations may not be copied The abstract must mntaln conspicuous acknowledge. ment of Mere and by whom the paper ws presented Write Librarian, SPE, P O Box , Richardson, TX 7S0S3.%=, U S A, f~Of-972.952.9435 AbstractThe paper deals with an experimental method to assess the strength of sedimentary rocks. The testing procedure (a "scratch" test, or cutting test performed at a "small" depth of cut of typically 1 mm or less), the method of interpretation, and some experimental results are described, Experimental observations of rock cutting indicate that a "plastic" mechanism of failure takes place at small depth of cut d. This mode of cutting is also referred to as "scratching", The results of scratch experiments on various sedimentar ocks have indicated the possibility of estimating the rock strength from such tests. These experiments have shown that the force on the cutting face is proportional to the crosssectional area ,f of the groove (which is equal to u'd for a rectangular cutter of width IV)and that the intrinsic specific ener~&(the cocfflcicnt of proportionality between the cutting force component in the direction of cutter motion and A ) appears to be WC1lcorrelated with the uniaxial compressive strength q. Moreover, tests have shown that logs of rock strength arc possible; the force measurements have to be averaged typically over 1 cm. in order to produce logs ofs that are consistent and repeatable.The paper finally discusses the influcncc of the wear flat and the depth of cut on the results and also the limitations of the method,

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper describes a new approach to understand the causes of stick-slip vibrations experienced by PDC bits. This new model takes into consideration not only the axial and torsional modes of vibration, but also the coupling between these two modes through bit-rock interaction laws, which are consistent with laboratory results from single cutter experiments. These interaction laws, which account for both frictional contact and cutting processes at the bit-rock interface, are formulated in terms of the depth of cut, a variable that brings into the equations the position of the bit at a previous a priori unknown time. They also account for potential loss of frictional contact between the wear-flats and the rock. The delayed and coupled nature of this interaction is ultimately responsible for the occurrence of self-excited vibrations, which can degenerate into stick-slip oscillations under certain conditions. The features of the torsional vibrations that are predicted with this model are well in accordance with field observations. Furthermore, the results reveal that the model predicts apparent rate effects as an inherent outcome of the nature of the bit-rock interface.