Drilling advances

	Vol. 229 No.11
LES SKINNER, PE, CONTRIBUTING EDITOR, LSKINNER@SBCGLOBAL.NET

Bit technology

Ah, the bit: the pointy end of the spear where irrestible force meets immovable object. To many people, bit technology is a black art complete with mysterious incantations, smoke, rattling bones, jungle drums and the reading of tea leaves. But it is much more complicated than these “priestly” devices.

Several new technologies have emerged that deserve discussion. These show that bit construction is advancing to increase penetration rate and reduce overall well cost. One involves a new bit configuration that allows access to the open hole below the bit. When I first mentioned this to a colleague, he immediately asked what I had been smoking. Nope, folks; it’s true.

One company has perfected a casing bit with a disconnectable center. Once the well is drilled through the pay zone using casing, the drillstring is pulled to the top of the pay and a wireline logging tool with a special bottom sub is run to the bit. The sub latches onto the bit’s center, attaching it to the logging tool while disconnecting it from the bit. Then, the well below the bit can be logged with the openhole tool. Once logging is complete, the tool is pulled back up inside the bottom of the casing and the bit’s center is reattached to the casing bit, allowing additional hole to be drilled.

Amazing!

When I first saw a model of the tool at a conference in Denver, I thought that someone must have been smoking something funny to come up with the concept, much less the design. After several field trials, the through-bit concept is now proven. What I’d like to see now is expandable through-bit design.

Another new concept involves an adjustable reamer with cutters that look like a PDC bit blade. This reamer extends in a controlled fashion, allowing consistent-diameter cutting under a wide variety of hole conditions. Most mechanical under-reamers and hole openers are limited by their arm length. They are not truly adjustable, but the expandable under-reamer is. Recent field trials in an Oklahoma research facility drilled over 26,000 ft of hole through a variety of formations on both rotary and rotary steerable BHAs. Then, the tool went to the field and had several successful runs. The tool is more aggressive than previous designs and can match the ROP of even the best PDC pilot bits available.

The reamer is activated by a dropped ball, and the cutters move up or down a slide track, allowing the reamer blade to contact the hole at a variety of positions. The cutting structure is variable with respect to hole ID depending on where it is positioned along the track. The cutters have gauge pads to ensure a slick, smooth hole behind the bit, whether in a straight or curved hole segment.

Astounding!

The obvious trick is to select a pilot bit that is matched to the under-reamer. An overly aggressive bit means higher drillstring torque as the under-reamer fights to maintain a gauge hole. A slower bit means the under-reamer spins without working to its full extent, resulting in prematurely dull cutters.

Another new development in PDC bits involves nanotechnology. A new material, developed by the Los Alamos National Laboratory and licensed to a manufacturer, involves thermally stable, diamond-impregnated, nanostructured silicon carbide. Cutters are manufactured from this material using a proprietary liquid-phase sintering process. The nanostructure prevents cracking and checkering that normally plagues PDC cutters at high temperatures. The embedded diamonds cut longer without matrix degradation, meaning that the cutters are very tough, wear-resistant and stable at high temperatures.

Fascinating!

Now, even the highest-temperature formations are susceptible to cutting with long-lived PDC bits. Since most of these formations are also at considerable depths, the savings in trip time to replace a dull, expensive PDC bit will pay for the nanotechnology. Clearly, this field is open for additional advancements. What’s next-the mythical undullable bit?

Another large-diameter PDC bit design has emerged. In this design, the cutters are placed on the outside of a cage-like structure that extends the bit OD and allows fluid to return through the inside of the bit. The center of the bit and its base resemble a conventional four- or five-blade PDC design. Coming off the bit’s bottom is a structure that resembles a bridge with voids in the sides. This cage allows PDC cutters to reach the edge of the extended hole without a solid body carrying the cutters. The entire lattice is available for cuttings returns, providing better hole cleaning.

Unbelievable!

This design is both innovative and practical. Who would have considered putting cutters on anything but a solid core? Now, all designers need to do is make the cage adjustable, so one can drill a variable-diameter hole depending on formation (e.g., salt, gumbo, shale) to eliminate reaming. Coupled with an adjustable under-reamer, this could revolutionize drilling. I’m sure the bright guys that came up with this new cage-like bit are already considering this idea.

Work is progressing on another design for hard-rock PDC drillbits. When a PDC bit is used in hard rock and high rotary speed is employed to increase ROP, the cutters wear unevenly. The angular velocity of the outside cutters is much higher than that of the inside cutters, so the outside cutters wear faster. Once they dull, the bit is junk, even though the inside cutters may have no appreciable wear.

One manufacturer is field testing a bit with a second row of outside cutters. These are set just behind and just below the top of the first cutter row. Thus, when the first cutter set dulls at high rotary speed, the second set is exposed. Both sets then cut until the entire bit is dulled uniformly, improving bit life and saving trip time.

Incredible!

The tests are not yet complete, but the concept is sound: Put more cutters where the wear occurs.

What a difference technology makes. I can’t wait to see what’s on the horizon to keep the pointy end of the spear sharp.