Experimental study of layouts of PDC cutters in core bit drilling

Over the past two decades, various single cutter force models have been developed that consider formation properties and bit characteristics. These models have been extensively used to describe the interaction of a Polycrystalline Diamond Compact (PDC) bit with formation rocks; however, their use to predict bit drilling efficiency is usually inadequate. There is an ample of field evidence that the density and layouts of PDC cutters affect drilling efficiency. Still, however, there is no reliable model of the PDC bit performance allowing determination of these bit design features for a specific bit's applications. In order to better understand the relationship between rock properties and PDC cutter layouts for different bit applications, laboratory drilling tests were performed with PDC core bits. The concept of an eight-blade PDC core bit was used to build a bit with a common body and replaceable heads. Core-bit design features such as: bit, cutter size, and cutter geometry were constant. In this study, the full-scale test rig facility at the University of Tulsa was used to obtain consistent data for six different layouts of PDC cutters. Operating parameters, such as the rate per minute (RPM) and weight on bit (WOB), were varied and performance parameters (rate of penetration (ROP) and bit torque (TQB)), were measured at each time step with the frequency of 10 Hz. The ROP were calculated as a function of bit displacement over time. All operating parameters were recorded as part of the data acquisition platform. Depth of Cut (DOC) was calculated from ROP, and the average WOB and TQB were estimated at each depth of cut. Tests were performed on two different rocks: Bedford and Carthage Limestones. For the purpose of comparing performance among different cutter layouts, we experiment with a maximum of four different depths of cut, which corresponded to four different weights on bit. The tested layouts allowed a constant maximum DOC per cutter of 0.25 in. Water was the drilling fluid for the entire experimental phase. The results showed that under the same drilling parameters, the cutter layout plays a key role in drilling efficiency. Strong linear relationships were found between DOC, WOB and TQB at different conditions that were characteristic of the specific bit design. Also, the PDC cutter layouts affected the relative drilling performance differently for different rock properties.