Pergamon Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. Vol. 32, No. 3, pp. 277-283, 1995 Copyright © 1995 Elsevier Science Ltd 0148-9062(94)00043-3 Printed in Great Britain. All rights reserved 0148-9062/95 $9.50 + 0.00 Technical Note An Experimental Drill Cutters and E. KURU, A. K. WOJTANOWICZ~§ Study of Rocks Sliding Friction Between PDC INTRODUCTION It is well documented [1,2] that the shearing action of drilling with PDC bits equipped with drag cutters gener- ates friction and heat at the cutters/rock interface. Laboratory tests [3] and simulation studies [4] reveal that the rate at which friction induces heating exceeds the rate at which drilling fluids cool the cutters. This heat greatly reduces drill bits operational life. Drilling fluid flows vigorously around the cutters and reduces temperature by: (1) providing convective cool- ing; (2) lubricating the contact between wearflat and the rock; and (3) removing rock chips and obstructions that reduce cutting efficiency. Using laboratory tests and field experiences, bit designers have enhanced convective cooling of PDC cutters. Further improvement in this area is likely to be small because of two physical limits. First, at some point, high mud flow rates are counter productive because they cause unacceptable fluid erosion of the wellbores. Second, there is a limit to convective cooling, beyond which cutter temperature cannot be reduced by increasing the mud flow rate. For instance, at a certain level of friction the tungsten carbide cannot conduct heat away from the wearflat (to the convecting surfaces of the stud) fast enough to control the wearflat temperature. This often happens in hard formations where either a heavier weight on the bit or a higher rotary speed is required for efficient drilling. In these formations increasing the mud flow rate does not greatly improve convective cooling because thermal resistance of the PDC layer limits heat transfer [4]. An alternative method of controlling PDC cutter temperature is to decrease heat generation by reducing friction at the cutter/rock interface. Metal cutting prac- tices [5] show that higher frictional forces (six-to-seven fold) are required to produce visible hot spots when the surfaces are wetted with liquid. The principal factors that control the magnitude of the friction-induced heat tMiddle East Technical University, Turkey. :~Department of Petroleum Engineering, Louisiana State University, Baton Rouge, LA 70803-6417, U.S.A. §To whom all correspondence should be addressed. include cutter wear, cutting speed, and the friction coefficient for the diamond and stud materials that rub against the rock. The mechanism of PDC cutters' wear has been investigated using experiments with single-cutters [6-9], laboratory experiments with full-scale prototype bits [7,10,11] and field tests with full-scale bits [12-14]. Based on these extensive laboratory and field data, several models were presented that could predict the performance and wear of PDC drill bits [15, 16]. Glowka [15] measured drag forces and then reported values for the drag coefficient (defined as the ratio of drag force to normal force). He considered the drag force as a sum of the cutting force and the friction force. He then concluded that the reduction in friction forces would result in an equal reduction in the drag coefficient. He also concluded that for a given rock type the relation between penetrating force and drag force is relatively independent from the depth of cut. This implies a constant value of friction coefficient for all cutters across the bit face. Detournay and Defourny [16] developed a phe- nomenological model for the drilling action of PDC bits. They verified their model by using Glowka's laboratory data [15]. Their estimated friction coefficient was unusually high compared to the other reported values for various rocks [7]. They also stated that, "The contact friction angle underneath the cutter is reminis- cent of the internal friction angle of the rock" [16]. What they referred to as a friction coefficient appears to be a drag coefficient. This is supported by the fact that their estimated drag coefficient values compared well to drag coefficients measured by Glowka [15]. To date, Hibbs and Sogoian [7] have provided the only available data on measured sliding friction coefficients. They used a 1.372 m (54 in.) vertical turret lathe. In their tests, forces acting on the PDC cutter were measured as the cutter repeatedly moved across the same ridge on the rock surface. Such arrangement, however, did not simu- late the actual behavior of the cutter. In reality, the PDC cutter's drilling action includes cutting to the rock's new surface and then sliding across the wearflat area. Also, 277