Bits about Bits


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Auger Drill Bit
A drill bit is the sharp component of a drill tool that penetrates materials such as rock or wood to create holes. Powered by axial force, the bits drill cylindrical and non-cylindrical holes into materials, dependent on the type used. Inserted into the core of a rotary drill, bits are used in a variety of applications from mining to construction.
Prior to the invention of drill bits, materials were put together using rivets and bolts. Made from carbon steel or high-speed steel, the bit was developed around 1845. Some bits are made from tungsten carbide, a material capable of drilling into some of the hardest materials found on sites. Polycrystalline diamond (PCD) is another type of hard material capable of cutting through even the most challenging matter.[1]
There are many types of drill bits, each designed for cutting holes in different types of materials, such as brick, glass, rock, plastic, and wood. Bits are available in different types: flat-faced, convex, concave, and kavex. Bit sizes for the cones are typically between four and seven inches (10.2 and 16.5 cm).
Flat-faced bits are used in heavy-duty drilling applications where the drilling is deep. As described, the face of the bit is flat as it drills.
The convex bit is used in applications where the rock is somewhat worn away and drills with two buttons as opposed to one.
The concave bit is one of the most common. It is used in medium to hard rock applications, and drills a straight hole.
The kavex is a combination of both the concave and the convex with a bit that works best on abrasive materials, drilling straight holes.
Bits have different types of faces, which are suitable for varying applications.
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Close Up of a Polycrystalline Diamond Drill Bit
Dome bits have spherical faces and are mostly utilized for DTH drills. Well-suited for a variety of applications from medium to hard rock as well as broken formations, the dome shaped face is less likely to break than the other types.
The double-domed face is similar to the dome shape, but with an additional dome positioned on top of the other. The additional bit adds life to the bit while providing a sharper penetration.
Ballistic bits are capable of drilling in more rapid motions, but are used less because they tend to break when used in certain formations they were not designed for.[2]


Learn to enjoy dull reading

Bits, that is. Smith produces an excellent handbook on dull bit characteristics. Get a copy (a .pdf file is available on their website
http://smith.com). A careful study of a worn bit can help identify drilling problems and suggest solutions. Being able to identify exotic characteristics like heat checking and tracking has in the past enabled me to modify BHAs and reap huge benefit in ROP and/or extended bit runs. We tend to be fairly rushed into grading a bit as soon as we get it out of the ground.


Understandable, since we want to be heading the other was as soon as possible. Make a point of going back and having a more careful study of the dull when you get the chance. Be methodical; start in the middle, work to the outside along each rib or cone, then down each gauge surface. Turn it over, examine the pin (or box) and finally look inside. A second careful look, especially in daylight, can reveal useful information and on rare occasions turn you into a hero if you spot something important.

Occasionally a bit company will send out a specialist to nurse a particular bit or application. He sees more dulls in a week than you see in a year. Take advantage. Get with him and pick his brains.

Un-balling Bits
First problem is recognizing that you have a balled up bit. If the ROP suddenly slows and the torque settles at a low value it could be a hard stringer. But if there is no expected change in formation then start thinking balled bit, especially in sticky clay with simple water based mud.

One frequent but often unrecognized indication of balling problems is a sudden increase in ROP after a connection. I don’t believe in coincidences and formation changes rarely happen at connections. While back-reaming and surveying at the connection you have probably accidentally un-balled the bit.

PDC bits are especially prone to balling, usually by running too much WOB. To clean it, pick up till you are just off bottom, circulate and rotate at max rates. The idea is to wash off the plug of clay using the backwash of the jets hitting bottom. The heat from drilling can bake clay surprisingly hard. How hard depends on how long you have been drilling on it. I have had to use a chisel to chip clay out of water courses that had been baked almost to a ceramic. So it can be difficult to wash off. After say five minutes re-start drilling very gently looking for torque and being careful not to run too much WOB again.

Hycalog put a lot of good information in their bit boxes, much more than just makeup torques. Several pages of “how-to” drilling information and well worth making a copy. They have a section on un-balling and more importantly on preventing balling.

Rock bits are usually less likely to ball and easier to free up. If you are not running a motor then pick up off bottom, stop rotating and spud the bit firmly into bottom; the idea is to push the teeth through the ball and engage the formation beneath. Start rocking or slowly rotating the bit and if you have broken through the ball the weight should drill off quickly and with a single bound you are
free.

It gets a little trickier with a motor. Try the PDC method first but if no success then stop rotating, grit your teeth and pile on a lot of weight till the motor stalls; you may have to cut back on the flow rate to do this. Again the idea is to push the teeth through the ball. With the motor stalled, pick up slowly till it spins free, increase flow rate at this point and hopefully the weight will drill off and you can drill ahead.

A dose of nut plug pumped as a slug is also a prescription for at least temporary relief of chronic bit balling. I have not seen it done; others swear by it.

Balled up bits don’t steer well. If you have a sudden loss of dogleg capability (I have seen this with the GeoPilot) then a balled bit may be the problem.

PDC Bits – As mentioned in the section on bit balling Hycalog put a lot of good information in their bit boxes, much more than just makeup torques. Several pages of “how-to” drilling information and well worth making a copy.

Handling PDCs on deck you don't have to treat them with kid gloves any more. The early cutters used to be brittle and only brazed in place, but they are a lot tougher now and more securely fixed. Simply sitting the bit down on a steel deck will not damage it unless you drop it from a height. Put a wiper rubber under the bit if you want to lower the BHA down over the pin prior to making it up by hand, not just to protect it but also to make it easier to spin up.

Tripping in watch out for all the usual hang up points, ram pockets, wear bushings, wellheads, liner laps, casing shoes etc. In open hole, especially if following a hole drilled by a rock bit, be aware that a PDC bit is more solid than a rock bit and does not have the advantage of rolling cones. Hit a ledge with a rock bit and you will most likely deflect off but with a PDC you will stand up. A rock bit can squeeze through restrictions but a PDC, especially if it has a tight junk slot area will get wedged into any slight restriction. If you anticipate tight spots, consider precautionary reaming rather than trying to force your way through (otherwise see Jarring).

PDC bits are not designed for reaming so if you have a long section of undergauge hole then consider not even using a PDC.
Reaming with a PDC use light WOB and full flow. Light weight because it is all being supported on the gauge cutters. Rule of Thumb for weight on PDCs = 500lbs per cutter so if you only have 6 gauge cutters then 3000 lbs max WOB. Full flow is required to keep the cutters cool.
Heat is death to PDC cutters. Some people like to ream with low RPM, which is fine in soft, easily displaced formations, but if you have to ream through tight limestone ledges then spin the rotary up - for two reasons.
One - PDC cutters cut faster the more often you present them to the formation.
Two - if you are reaming through restrictions then a higher RPM will give you more inertia. Should the bit take a bite, you get more warning, you can react, slow down, even pick up, and therefore the string is less likely to stall, less likely to screw into the restriction and get stuck. (See Jarring)

Always wash the last stand down, tag bottom, pick back up a foot and circulate for a few minutes to blow away anything that might be lying there. Tag bottom with 50/60 RPM and start drilling a new profile. The watercourses on any fixed head bit (PDC, diamond, TSD, impreg) only provide the necessary cooling when the bit is fully engaged with the formation. Drill at least a foot, the length of the bit, with light WOB. This can take time in hard formation but this is exactly when it is most important to take that time.
Remember you are trying to minimize point loading and heat concentrations.

Once you have a pattern you are free to go. PDC ROP is almost always in direct proportion with RPM. Some studies suggest a linear increase up to about 200 RPM, then a plateau to ±250 RPM followed by a slow decrease at higher revs. Even hard formations often respond best to higher RPM but it depends on what the formation is and particularly it's abrasiveness. You hear people worrying about burning a bit up but as long as you keep the circulation going and don't pile too much weight on it you won't be far wrong going with max RPM. However a very hard calcite cemented sandstone is usually best tackled with a lower RPM and higher WOB. On the other hand very hard limestone usually responds best to higher RPM.

Speak to geologists. Find a sonic log. Look at the cuttings. Gather information.

The PDC golden rule is "play with it". You rarely get a PDC bit run where one set of parameters works best top to bottom.
Formations change and you should vary your parameters to suit. You can usually push the WOB till you see some torque surging, then back off till it settles again and this is usually the optimum for this bit/formation. Avoid the surging if possible since this is "stick/slip", bad for both bit and drill string and usually not the best ROP either. "Stick/slip" and "Whirl" can create new patterns on bottom that can excite the drill string, essentially a classic feedback problem. Usually all that is needed is to vary either WOB or RPM by about 15% but sometimes the only escape is to pick up off bottom and drill a new, clean pattern. Very frustrating in hard formations but these excitations can destroy an MWD, so it is worth taking the time.

Most Top Drives are good up to ±160 RPM but watch out for drill string harmonics.

After heat, the next worst thing for PDCs is shock. To minimise shock it is good drilling practice to land a PDC bit on bottom with light WOB and 50-60 RPM then bring the rotary up as weight is added. This habit got lost while we drilled mostly with motors since flow creates RPM and cooling is more important as noted above. Now that we are drilling less with motors and more with rotary steerables like GeoPilot and AGS/AGM it is time to revive this habit.

Bit grade 0-0 – Some company men object but why not? – Modern PDC bits are so durable that it is not unusual to pull a bit that is almost completely un-marked. It is therefore misleading to call it 0-1 or the like, especially since a PDC bit tooth is seriously worn and almost useless at grade 4.
The fact that you have graded the bit tells the story that it has been run. If it were new it would be
graded NEW. Next time you pull a tough FM2743 and can’t see a mark on it then grade it 0:0:NO:A:X:0:NO:TD.

In gauge? Grade it “0”. – I am conducting a one-man campaign to stop people grading in gauge bits as “I”. (Arthur says it is a two-man campaign) Everything else is graded 0-8 so why not bit gauge. With most hand writing and some computer fonts it is difficult to separate a capital “I” from a “1”. For example
“ Lucida Sans -1I1I”, “Minion Web -1I1I” , “Arial Narrow -1I1I”
Nottalottadifference. But it is not a caligraphical web site...So back in our plan;-)

Bit Breaker
– No standardisation between bit companies so check you have the right one. Should come with the bit but often does not. ( Some compagnies like to desingne some beautifull plastic, and you look stupid in a midle of the night when you discover this)
There are a lot of different sizes, even from the same bit company, so either slip it on the bit beforehand or calliper it.
Check the depth of the slots versus the thickness of the plate. When you torque up make sure the bit is being pulled into the back of the breaker. This stops the bit being pulled against the slip bowl, damaging both bit and bowl, and minimises the forces trying to warp the breaker plate.

Jets
– Screw type jets (Smith and all PDC bits) do not have to be hammered tight. Hand tight is Smith's recommendation for both the two-piece rock bit jets and especially the brittle one-piece tungsten carbide jets for PDCs. Make sure the o-ring is clean and intact. Quite often you find a little swarf in the jet pockets, so watch those fingers. With all tungsten jets be careful that you don't get the threads crossed.
The tungsten is so hard you can cross cut the thread in the bit. If you feel resistance in the first few turns then back it out till the thread jumps then try to run it in straight this time. You should only feel increased resistance as the base of the jet squeezes into the o-ring and then seats on the base of the jet pocket.

Security nail-type fixing. Security used to provide a little claw hammer for the nails but I haven't seen one for a while. Check you have the right length of nail for the jet size (four different lengths). Check the o-rings and nail grooves are clean. Make sure the o-ring is not in the nail groove (has happened).
Push the jet in and if necessary tap it gently home with the wooden handle of a hammer. Jets have sharp edges when new so don't try to push one in with your thumb, you can cut yourself (done that, felt stupid). Push the nail in and start it round the locking track; you should be able to feel if there is a clear passage. Tap the nail the rest of the way in; it should go easily. Leave the head a little proud so that the nail can be easily removed when required (there is a recess in the bit to allow for this).

BHI / Reed circlip fitting. Get a good set of internal circlip pliers, often rare as hen's teeth. Check o-rings and circlip grooves are clean. These jets are push in same as Security. Take the same precautions. Compress the circlip making sure it doesn't twang
free and either smack you in the eye or disappear into a corner, or drop somewhere where you can’t grap it... Pop the circlip into place then give it a twist. You can't twist it if it isn't seated.

Gauge Rings
– In an ideal world each rig should have four gauge rings at each size – a "GO" and "NO-GO" ring for both roller cone bits and fixed head bits. Fat chance. So appraise yourself of the type and actual size of gauge ring available and the permitted tolerances of the type of bit you are using. IADC tolerances as follows –

Roller Cone Fixed Head
Nominal Size – in Tolerance - in Nominal Size – in Tolerance - in

3
3/8 – 13 3/4 -0 : +1/32 up to 6 3/4 -0.015 : +0
14 – 17
1/2 -0 : +1/16 6 25/32 - 9 -0.020 : +0
17
5/8 and larger -0 : +3/32 9 1/32 - 13 3/4 -0.030 : +0
13
25/32 and larger -0.045 : +0

The point is that if you use an 8½" rock bit GO ring to gauge a new 8½" PDC bit you might grade it
1/16" undergauge when it is not.

A One-Cone Bit
– Sounds like a bad joke but they did exist. Zublin was (is?) a Californian company and they made these bits with an almost spherical single cone set on a 45° cranked shaft. They came in big sizes and were designed for spudding on land rigs.
The theory was they kept the hole vertical, didn't skid and didn't ball up. My old boss (a Californian) claimed they worked, at least in Los Angeles. (Hi Gene ;-)

I have now learned that one cone bits are being used for small hole drilling in Alaska. The point is that with only one cone you can maximise the size of the bearings, making them as robust as possible for any given hole size.