Theoretical and experimental study on the penetration rate for roller cone bits based on the rock dynamic strength and drilling parameters

Roller cone bits have been used extensively in petroleum and natural gas mining, and the accurate prediction of their rate of penetration (ROP) is of crucial importance for improving drilling quality and reducing drilling cost. In this study, a new prediction model of the ROP considering the combined effect of the main drilling parameters and rock dynamic compressive strength is developed for roller cone bits. The model is derived based on the mechanism of rock fragmentation under a single tooth impact indentation. The newly introduced ROP model is different from others in that it replaces the rock static strength with rock dynamic compressive strength and can reflect the real process of rock dynamic crushing by a roller cone bit. The results of theoretical analysis show that the ROP linearly correlates with the bit rotary speed; the relationship between the ROP and weight on bit (WOB) is 3/2 power, and the relationship between the ROP and rock dynamic compressive strength is −3/2 power. Furthermore, lab drilling tests on sandstone and limestone are carried out with a full-scale bit drilling testing machine, and the influence laws of WOB, rotary speed and rock dynamic strength on the ROP are analyzed. In addition, the theoretical ROP values calculated based on the rock dynamic compressive strength and static compressive strength are compared with the experimental ROP values. The results from the tests indicate that the experimental ROP values are basically consistent with the theoretically derived values, and the comparative results show that the theoretical ROP values calculated according to the rock dynamic compressive strength are closer to the actual values with, an average relative error below 15%, signifying that the newly established ROP model in this paper is valid and can be used to predict the drilling rate with reasonable accuracy and can be used to provide useful guidance for optimizing drilling parameters.