There are many Factors & Variables when it comes to Drilling that contribute to the ‘drillability’ of different rock formations.
Rock UCS hardness (Unconfined Compressive Strength) is only one factor that contributes to establishing the optimum operating parameters for each Bit selection. They can include:
- Fracture toughness
- Dhear strength
- Young’s modulus of elasticity
- Poisson’s ratio of stress vs strain
- Internal angle of friction
These rock variables must be considered when attempting to determine the best bit for overall life at optimum penetration rates.
Weight On Bit (WOB) and Rotation are the key to achieving that optimum performance.
WOB (pull down) is required to adequately push teeth into the rock to break in efficiently. It is the primary factor in rate of penetration. The UCS of the rock determines how much pull down is to be applied. High UCS rocks may need time for the indenter to break it and Lower RPM is often required for efficient rock breakage.
Rotation is required to move the cutting teeth to the next rock cutting position. The faster you move the teeth to the next position, the faster you will drill. If the rock resists indentation by the teeth, there will be minimal rock breakage, and rate of penetration will not increase proportionally with higher rotation. For softer rock high RPM is required as Soft rock responds to frequency and for harder rock low RPM is required as Hard rock responds to time.
Other equally important factors affecting drilling performance are specific to the drilling equipment and operator. Notable items include:
- Driller experience and training
- Compressor size and condition
- Mechanical condition of the drill
- Pipe size and straightness
- Drill sub / stabilizer size and condition
- Deck bushing size and condition
- Surface condition and overburden
- Sub surface blast fracturing
- Angle drilling
- Revolutions per minutes utilized
- Weight applied to bit
- Hhole cleaning
- Water applied for dust control