The drill machine turns the steel, the steel turns the bit, the bit turns the cones. Weight is applied by the drill + the weight of the drill string. The total weight pushes the carbide buttons in the 3 cones, into the rock. The bit carbides must stress the rock to make the rock fail to create chips. Air (pressure) flushes the chips away from the buttons and air volume pushes them up and out of the hole.
1. WOB = Weight on bit
2. RPM = Revolutions per minute
3. Air = Pressure (psi) and Volume (cfm)
4. ROP = Rate of penetration
More on RPM:
RPM is required to move the carbide buttons to the next cutting position.The faster the carbides move to the next position, the faster you will drill. However: If the formation resists penetration of the carbides, there will be minimal rock breakage and rate of penetration (ROP) will not increase proportionally with higher RPM.
Unconfined Compressive Strength or UCS is a comparative measurement system for the strength of different types of rock. Low UCS – Low strength rock. Lower weight on bit is required to break the rock. A higher speed or RPM is required. High UCS – High strength rock. Higher weight on bit is required to break the rock. A Lower speed or RPM is required.
Soft Rock Failure:
Soft rock will typically fail or fracture faster than hard rock. This action will create more cuttings or chips in a shorter time thus the bit will penetrate further for each revolution or turn.Penetrating too quickly will begin to create issues with bailing all of the cuttings out of the hole.Cuttings that do not exit the hole will fall back into the hole and be re-cut. This will reduce the penetration rate and prematurely wear the bit and drill string.
To keep the rate of penetration in the sweet spot it will be necessary to maintain the RPM on the high side of the recommended speed and cut back on the down pressure or weight applied to the bit. In some soft conditions when the bits wants to penetrate rapidly it may even be necessary to apply some hold back pressure. Again, always attempting to maintain that “Sweet spot”
The ability to drill faster or increase the rate of penetration is both good and not so good. Over penetrating the rock will result in several adverse conditions.
1. Plugging the bit with potential stalling
2. Wear on the cone steel
3. Not flushing out (bailing) the hole
4. Re-cutting rock chips
5. Loss of production
Soft Formations = Higher RPM (Soft rock responds to frequency)
Hard Rock Failure:
Requires more weight on the bit to adequately push the carbides into the rock to fracture it efficiently. WOB is the main factor in ROP. High strength rock needs time for the carbides to penetrate the rock sufficiently to break it.
Hard formations = Lower RPM -Hard rock responds to time.
Bailing and Cutting Exit Velocity:
1. Bailing Velocity= the speed of the air travelling up the drilled hole.
2. Settling Velocity= the speed that the cuttings want to fall back down the drilled hole.
3. Exit Velocity= the difference between the two. It is how fast the cutting exit out of the drilled hole
For light, dry material the recommended bailing velocity is 5,000 to 7,000 ft/min. For heavy and/or wet material the recommended bailing velocity is 7,000 to 9,000 ft/min. The recommended bailing velocity increases when the material is wet because the water makes the cuttings stick together, making larger and heavier pieces to lift.
Bit Air Pressure:
We recommend, 35 to 45 psi at the bit. Cab pressure will be higher depending on system pressure losses. Bit pressure should not cause thecompressor to modulate. Make sure that the compressor is adjusted correctly to give the rated volume at the rated pressure. Proper air pressure at the bit will give you:
1. Adequate bottom hole cleaning pressure.
2. Adequate air to cool the bearings.
3. Optimum penetration rates.
A cleaner hole bottom means that more energy is used to break the rock and less energy is wasted re-grinding the cuttings.
Water Injection - Water is sometimes required to suppress dust. Water is very hard on the bit bearings and can cause bearing to corrode from the inside. Mud and cuttings can contaminate bearings and could ultimately reduce bearing life as much as 50%.
Recommended Water Injection Rate:
1. Use only enough water to suppress the dust.
2. Do not use so much that it creates wet cuttings.
3. Do use 2 to 8 liters/minute depending on conditions
4. Large or faster drilling – use more
5. Small or slower drilling – use less
6. 12-1/4” hole, 75 – 100 ft/hr. ROP – 2.5 to 7 L/Min.
Oil injection will increase bit life by coating and lubricating the bearings. The oil seals the surface of the bearings, keeping out water and slowing corrosion of the bearings. Oil Injection can also increase ROP by reducing friction and torque.Make sure to use a quality, environmentally friendly, Lubricant.