PDC bit with conical element targets rock cutting at bore center

New bit design drilled 9,631-ft lateral in single run in Bakken field test

By Steven Segal, Smith Bits, a Schlumberger company

Regardless of the type of well they are constructing, drillers the world over share a common goal: to drill a wellbore to target depth, safely, accurately and with maximum rate of penetration (ROP). However, as wells go deeper, farther and into more geologically complex formations, conventional drill bit technology may be compromising drilling efficiency. In particular, traditional polycrystalline diamond compact (PDC) bits may be creating unwanted dynamic artifacts that cause instability in the bit and bottomhole assembly. Both increase the time and cost of any drilling operation.

Smith Bits embarked on a multi-year project to develop a bit that would allow the rock at the center of the wellbore to be drilled more efficiently, with less damage to the cutters and with applicability in a wide range of formations. The result is the Stinger conical diamond element (Figure 1).

Due to space constraints, PDC bits might typically have only two or three cutters positioned to drill the center of the hole. Because rotational velocity of the cutters decreases with their proximity to the center of the cutting structure, rock removal by these center cutters is less efficient, particularly in harder rock formations (Figure 2). These few central cutters also tend to bear the greatest load, resulting in lower ROPs, destructive lateral vibrations and cutter damage that require more frequent trips out of hole to replace the bit.

The conical diamond element replaces these central cutters and drills the rock at the center of the wellbore in a different manner. Removing the central cutters creates a void space at the center of the bit, allowing a small rock column to form as the outer cutters remove the surrounding rock. The centrally positioned conical element then bears down on this rock column, which is now in an unconfined and lower stress state, and provides high-point loading to fracture the rock more easily and efficiently than the scraping action that is common with conventional PDC cutters. Finite element analysis modeling, which was run to determine the precise point at which the Stinger element tip indents a rock’s stress field, showed that significantly less applied load is required to fracture the rock when the load is concentrated at a single point.

The element is manufactured from a synthetic diamond material specifically engineered to provide improved impact strength and superior resistance to abrasive wear and has double the thickness of the diamond layer on conventional PDC bits. This, coupled with the element’s conical geometry, results in a unique cutting element that is suited to this task.

Simulation and design work were conducted to improve the cutting structure, including shortening the blades to provide sufficient space for the element and determining the element’s spatial position. Simulation modeling measured the effectiveness of adding the element to a reconfigured PDC bit’s cutting structure, and it indicated ROP increases between 10% to 15% in formations including shale, limestone and sandstone.

Detailed hydraulic analysis also were done via computational fluid dynamics software to simulate flow through the bit’s cutting structure and around the element. This provided insight into the best nozzle position to maximize fluid velocity for enhanced bit cleaning, cuttings removal and cooling of the element. Nozzle positions were also adjusted.

Comparative tests of a PDC with the new element and a conventional PDC bit were run in a hard-to-medium grained sandstone with an unconfined compressive strength of 9,000 psi. The borehole drilled by the new bit had less hole diameter variance, reflecting a more stable cutting structure less vulnerable to lateral and torsional shocks and vibrations.

The conical element also creates significantly larger cuttings, which allows for more accurate geological evaluation.

So far, the majority of the new bit’s field experience has been in the Bakken Shale. For one Bakken operator, conversations with Smith Bits field personnel focused on drilling a long lateral section – 9,000 ft or greater – with a bit that provided maximum ROP possible and stability to stay within the target reservoir zone and avoid multiple trips to replace worn out bits.

The new conical diamond element bit was selected for a field test; a 6-in., six-bladed bit containing 13-mm cutters with the new conical element at the center.

On the first day, a total of 2,903 ft were drilled in a single trip, which was the most footage that the operator had ever drilled in a lateral in a 24-hr period. Drilling continued with the same bit, and the operator was able to drill the entire lateral – 9,544 ft – with an average ROP of 97.4 ft/hr. The operator drilled the lateral in 4.25 days, setting a new company record.

The operator moved the rig 20 miles north to another test well and used the same type of 6-in. conical diamond element bit to drill the lateral. The bit again drilled the 9,631-ft lateral in a single run and with record-setting ROP. It achieved this with minimal sliding to stay within the formation’s sweet spot. The driller averaged only one sliding event every 850 ft, compared with one slide every 350 on average using conventional PDC bits.

When the bit was pulled out, a close examination showed signs of damage only to the regular cutters on the outside shoulder area. The conical element at the center was effectively unscathed.

Another Bakken operator interested in ROP improvement and reduced bit change-outs in their vertical well sections used the center-placed conical element on an 8 ¾-in., high-abrasion-resistance PDC bit with 16-mm cutters. The bit successfully drilled 8 ¾-in. vertical sections of between 6,209 ft and 6,477 ft, with an average ROP of 168 ft/hr and a maximum recorded ROP of 203 ft/hr. Compared with the average 115 ft/hr ROP reported using other bits in similar wells, the new element increased ROP by 46%.