Analysis of Temperature Distribution and Performance of Polycrystalline Diamond Compact Bits Under Field Drilling Conditions

Abstract

ABSTRACT Analysis of laboratory tests on full-scale field-worn polycrystalline diamond compact (PDC) bits showed the frictional heat at the rock/bit interface to be largely generated at the diamond cutting edges of the PDCs. Inspection of the observed wear of the PDCs together with the analysis revealed that the diamond layer attacks the formation at a large negative rake angle and that rock flour accumulates under the cutting edge during drilling, forming a stable buildup edge. A thermal model has been developed on the basis of these results to predict the temperature distribution in the PDCs under quasi-static drilling conditions. The maximum temperature is found to occur at the PDC's cutting edge and is virtually independent of PDC position under normal rotary drilling conditions. At elevated rotary speeds however, the maximum cutting edge temperature occurs in PDCs at the gauge of the bit. This temperature can exceed the boiling point of water-based drilling fluids, resulting in higher cutting edge temperatures and wear rates of the PDCs which may lead to premature failure of the bit. Under normal rotary drilling conditions the maximum cutting edge temperature increases with the penetration rate. For the higher speed positive displacement motor drilling applications however, bits can be designed to give a virtually constant maximum cutting edge temperature over a wide range of bit penetration rates. This is achieved by selecting the optimum number of cutters whilst making use of the thermal insulating capacity of the buildup edge under the PDC. In addition a further reduction In the maximum cutting edge temperature can be achieved at these speeds by reducing the diamond layer thickness of the relevant PDCs. The results also showed that for effective cooling of the PDCs fluid velocities of at least 50 m/s are required along the diamond surfaces when drilling with oil-based fluids. With water-based drilling fluids, higher velocities should be used to prevent bit balling or boiling of the drilling fluid at the diamond surface of the PDCs.