Conical Diamond Element Bit on High-Torque RSS Mitigates Vibration Maximizing ROP in Hard Carbonates Saving 90 Hours of Rig Time, Offshore Mexico
Authors: Luna O. , Salinas G. , Lozano C. , White A. 
Published: November 2015

An operator is drilling exploration wells near Xanab field in Mexico's Southwest Marine region. The 8½-in. hole section is difficult to drill, with damaging shocks and vibrations occurring in the hard/interbedded Cretaceous carbonates. These formations, with relatively high unconfined compressive strength (UCS) of 15,000 to 28,000 psi combined with alternating lithologies with chert nodules, have been drilled mostly with roller cone IADC-527X bits. Although the roller cone produced vibration-free operations, suboptimal rates of penetration (ROP) and short run lengths resulted in multiple trips to total depth (TD) of the section. Attempts were made to design polycrystalline diamond compact bits (PDCs) that could endure the difficult downhole environment by increasing diamond volume per blade. However, when encountering the hard formations, high vibration levels were still causing broken cutters in the nose/shoulder areas.

In a new exploration well, a new fixed cutter bit was used. The bit is equipped with stinger conical diamond elements (CDE) with an ultrathick diamond layer that improves impact resistance, along with PDC cutters. A modeling system based on finite-element analysis (FEA) was used to design a new-type eight-bladed hybrid bit by positioning CDEs and PDCs from bit center to gauge on all eight blades. The unique cutter pattern would fail the rock with a combination of plowing and shearing action. To ensure all CDEs engage the formation at the beginning of the run, the elements were positioned at a height equal to the top edge of the conventional PDC cutters. This strengthens the bit by leveraging the superior impact strength of the CDEs. To deliver sufficient torque and to keep a vertical trajectory, a rotary steerable system (RSS) was selected, with an integrated positive displacement motor (PDM) to supply additional power.

The bottomhole assembly (BHA) was run on the Keluk-1 well and drilled to TD in two runs, 30% faster (2.59 m/hr) than the average ROP of the nine 8½-in bit runs (seven with PDC and two with roller cone bits) in Xanab field (2.0 m/hr). The first bit run made 206 m and could have finished the entire section but was pulled for downhole tool failure. The second bit drilled 65 m to TD. This compares favorably to the nine-well offset average of 176 m. The RSS/motor combination worked as planned, providing the CDE bit with the necessary energy to maximize ROP while reducing stick/slip occurrence to less than 20% during the run. The reduced vibrations enabled the operator to maintain RPM (150 to 170) and weight on bit (WOB) (10 to 16 ton) when drilling the critical middle/lower Cretaceous mudstone with chert nodules. Previous runs in this section required reduced parameters to mitigate vibration, thus slowing ROP.

Both bits were pulled out in excellent dull condition (0-1/0-0) verifying that the CDEs provided the added durability needed to drill through the challenging application. The increase in drilling performance saved 90 hours of rig time equaling USD 320,000. Field testing of other CDE bits are producing favorable performance results combined with larger drill cuttings for improved formation evaluation.