This table shows run details from five offset wells in the West Africa case study, comparing performance of Smith bits with ONYX PDC cutters with bits from two other manufacturers. (Image courtesy of Smith Bits)

Drilling hard, abrasive, and interbedded formations with polycrystalline diamond compact (PDC) bits remains difficult. To develop new cutter technology, Smith Bit’s engineering team started by analyzing the frictional heat generated at the rock/cutter interface, a critical factor that makes drilling difficult. Smith also analyzed thermal degradation and micro-chipping commonly experienced during long bit runs in deep, high-temperature boreholes.

The study revealed that different applications require different cutter properties. Generally, wear resistance and thermal stability are required to efficiently drill abrasive formations, while a more impact-resistant cutter is best suited for interbedded sections and formations with higher rock strength. This differentiation was the basis of a research and development effort that focused on raw PDC material selection and a new high pressure-high temperature manufacturing process.

Cutter technology

ONYX cutters are the first PDC shearing elements to successfully address all three critical longevity issues. ONYX features improved thermal properties with greater wear resistance and fatigue life than other PDC cutters.

The new cutters are capable of maintaining a sharp cutting edge in abrasive sands and hard carbonates. The improved endurance translates into more footage drilled at maximum rate of penetration (ROP). Field trials with ONYX cutters have established new performance benchmarks in fields in West Africa, East Texas, and the North Sea.

Case study, West Africa

Drilling a challenging 121?4-in. hole section offshore West Africa with PDC bits was producing unacceptable results. The section includes hard, abrasive interbedded sands and shales with compressive strengths in excess of 20,000 psi. Typically, the section requires up to four eight-bit runs to complete. In most cases, PDC bits were pulled in poor, dull condition, exhibiting ring-out and worn cutters.

The goal was to drill the 121?4-in. section in one run or eliminate trips and engineers devised a two-pronged approach. The first was to design cutters that would remain sharp in the abrasive formations.

However, even the most sophisticated cutters cannot drill efficiently if exposed to vibration-induced impact damage. Therefore, the second deliverable was to design a bit body that would remain dynamically stable to minimize adverse cutter loading. This was accomplished using Smith’s integrated dynamic engineering analysis system (IDEAS) create a computer-generated picture of the bottomhole pattern and adjusting the design to ensure vibration-free drilling.

Results

Using the modeling program in combination with ONYX cutter technology, Smith engineers designed and built the eight bladed 121?4-in. MDSi816 bit, fitted with back-up cutters and optimized blade and nozzle geometry. The bit was run in a highly abrasive formation on a rotary steerable bottomhole assembly on well #2 and well #5 with outstanding results.

On well #2 the bit drilled the entire 121?4-in. hole section, from shoe to total depth, for the first time in field history. The reduction in trip time saved the operator six days of non-productive time, reducing costs by US $2 million.

On well #5, the total footage and penetration rate performance of the MDSi816 bit increased — more than doubling the ROP. It also had good LWD data capture, eliminating time needed for post-well logging. Compared to the three-well offset average (six bit runs), the new style PDC bit drilled 165% more footage (5,584 ft=1,702 m) with an ROP (69.5 ft/hr = 21.2 m/hr) increase of 122%, in addition to completing the hole section in one run. The bit was pulled green with a dull grade of 1-2-WT-A-XXX-IN-DL-TD.

Comments