I’ve got an email asking me about how to figure out the total bit revolution (RPM) at down hole when the mud motor is used in the drill string and the rotating at surface is also conducted.
There are two components contributing in the total revolution of downhole drilling bit using mud motor.
1. The rotor RPM of drilling mud motor– each drilling mud motor has its specific performance data which will tell you how many revolutions per flow rate flowing through it.
For example (see the image below), it shows Rev/Gal which means the rotor will turn 0.147 turn/min per one gallon/min of drilling mud passing thought.
2. The rotary speed from top drive or rotary table.
The total drilling bit revolution downhole:
The total drilling bit revolution is equal to summation of the rotor RPM at specific flow rate and the rotary speed on surface.