Analytical Modeling and Diagnosis of Penetration Rate Performance of PDC Bits

Abstract

The high cost of drilling deep (15000+ ft) wells, due to slow rate of penetration (ROP) at depth, has severely limited the utilization and economic significance of deep hydrocarbon resources. The overall objective of this study was to obtain a better understanding of the major cause(s) of slow ROP in deep drilling. An analysis of field data demonstrated the impact of the problem and identified shale and limestone as the primary lithologies where slow ROP occurs at depth. Previous researchers have concluded that interactions between rock, drilling fluid, cuttings, and the bit control ROP. More specifically, several researchers have concluded that bit balling is the major cause of slow ROP in deep shales drilled using water base mud (WBM). Lab and field examples of bit balling in oil base mud (OBM), although uncommon, were also found. This study developed analytical models to describe some of these interactions. The other questions addressed were whether some form of balling causes slow ROP in deep shales drilled with OBM and whether diagnostic symptoms could be defined to determine which form(s) of balling causes severely low ROP. An analytical model for single cutter drilling was adapted from a model of machining metal and then scaled up as a full scale polycrystalline diamond compact (PDC) bit model. The models compared well with laboratory test results by predicting penetration rate performance based on the strength and internal friction angle of the rock, confining pressure, and cutter and bit design. Multi-scale analyses of field cuttings were also performed. These provided knowledge of the cuttings structure and potential accumulation mechanisms that was used in developing models to explicitly account for the effects of cutter balling and global balling. The full scale bit model was applied to bit test data and shown to be useful in detecting drilling inefficiencies and for early diagnosis of global balling. Global balling when drilling both Mancos and Catoosa shales with OBM caused significant decreases in ROP and had essentially the same diagnostic symptoms as balling in WBM. This may indicate that the causal mechanisms of bit balling, and consequently, of slow ROP, in oil base mud are similar to those in water base mud.