Penetration-rate performance of roller-cone bits

Abstract

The penetration rate obtained with a roller cone bit is controlled by either the cuttings generation process or cuttings removal process. Most previously reported penetration rate models have not explicitly included the cuttings removal effects. This paper presents a penetration rate model that includes the effect of both the initial chip formation and cuttings removal processes. The model is an extension of a previously published model that neglected the effects of cuttings removal. The cuttings removal effects must be included in a valid penetration rate model because they can often limit the ROP to less than 20% of the expected ROP for perfect cleaning. The model is used to show that the reduction in penetration rate at high borehole pressure is the result of both local cratering effects and global cleaning effects. Increased hydraulics will increase the penetration rate when it is limited by global cleaning effects. The penetration rate reduction due to local cratering effects is largely a function of mud properties and borehole pressure and is not helped much by increases in hydraulics.