Deviation Forces From the Wedge Penetration Failure of Anisotropic Rock

Deviation of boreholes is caused by drill string flexibility and the interaction between the drill bit and the formation being drilled. While work has been devoted to the analysis of drill string mechanics, less work has been directed toward the understanding of bit rock interaction. This paper presents the results from some experimental work on the deviation forces and chip volumes generated by the wedge penetration of anisotropic rock. The single blow wedge penetration tests were conducted on blocks of Green River shale cut to simulate formation dip angles from 0 to 90 deg. During the tests, the time history of the force required to penetrate the rock, together with the deviation forces perpendicular and parallel to the strike of the formation were recorded. Tests were conducted using wedges with 30 and 60 deg included wedge angles at atmospheric and 10,000 psi confining pressures. In conjunction with these tests, triaxial tests were performed to determine the failure characteristics of this particular rock. Results from these tests showed that appreciable deviation forces were generated by the 30 and the 60 deg wedges at both confining pressures. Maximum deviation forces were generated in the 30 deg dip region and in the 60 deg dip region, with the deviation forces tending to zero at 0 deg, 90 deg, and in the region of 45 deg. The results are in general agreement with field observations. The volumes generated correlate with the measured forces as well as field experience. Predominant chip volumes were created on the updip side of the tooth at low dip angles and on the downdip side of the tooth at high dip angles for both tooth geometries. Published predictions on the advantages of 60 deg teeth for deviation control were found to be in disagreement with experimental results. When combined with the measured rock failure properties, the theoretical work upon which the above predictions were based did predict deviation forces in qualitative agreement with the experimentally determined values.