0 Comments | Dec 21, 2013

Bit Weight: A Key Factor in Drill String Design and Drill Pipe Performance

A very important factor in your drill string design & drill pipe performance is bit weight.

Every bit is designed to run with a given amount of down pressure to work properly. It takes a certain amount of push.

When drilling vertically, this is bit weight.

Whether drilling with a tricone bit or hammer bit, the manufacturer sets an ideal weight to get the best performance from the bit. Different manufacturers will have their own chart or formula.

For DTH hammer bits, I have seen a set weight of 6500 pounds to 7700 pounds total bit weight and also a “rule of thumb” that aid 500 pounds for every inch of bit diameter. Here a 6” hammer bit would require 3,000 pounds of bit weight.

With tricone bits, the recommended weight is a lot more and will depend upon the size or formation. For tooth bits, the weight can range between 1,000 to 4,000 pounds per inch of bit diameter. For button bits (TCI), this can increase to 6,000 pounds per bit inch. So when you stick on that 12 3/4” button tricone bit in a medium hard formation, you may need 36,000 to 72,000 pounds of bit weight for the bit to perform as designed.

In traditional oilfield applications, this requirement is why the big rigs run strings of drill collars. To get 36,000 pounds of bit weight, they would run 8 joints of a 8” x 31’ drill collars. These would sit above the stabilizer, or bit sub & bit. Once they have attained the desired bit weight and drill pipe added, the rig will actually go into a hold back mode when the weight of their collars & drill pipe meet the recommended bit weight. This is how drill pipe is designed to operate. – in tension.

That’s oilfield. In the waterwell or shallow oil & gas, we use rigs that have pulldown. When we drill the top hole, we make up for the lack of weight by pushing with the rig’s hydraulics. However, as the hole gets deeper & more rods are added, we start pushing on more sections of drill pipe. At some point, the drill rods will bow out or “snake” in the hole.

This is bad news for the drill pipe, as the pipe and especially the connections rotate through the bends, the surface continually stretches & compresses creating fatigue stress. Since fatigue is accumulative, eventually, this will cause a failure. Once the hole is started, it is good drilling practice not to reply on pulldown for bit weight.

Unfortunately, when that first couple pins break & you get your drill rods inspected, you might find nearly every pin cracked and the entire string ruined. I have seen this where operators were drilling 12 or 15” holes down a thousand feet without any regard for bit weight other than a short stabilizer.

Relying on your rig’s pulldown for bit weight can lead to drill rod disasters. Such as breaking pins, cracked boxes and tube breaks. Instead, design your string to put the required weight nearest to the bit ideally holding back and running your drill pipe in tension and your hole straighter.

Keep in mind also that you might want to watch the weight on your DTH hammers. Ask your hammer supplier what the bit weight should be and when is too much weight have an bad effect on hammer performance. After all, you exceed 3000 pounds with 200 feet of 4 ½” drill rods.

Bit weight. Just another factor in drilling performance and the life of your drilling tools.

This 12 1/4" button tricone bit may have a recommended bit weight more that your rig's pullback rating.

This 12 1/4″ button tricone bit may have a recommended bit weight more that your rig’s pullback rating.

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