Directional Drilling Mud Motor Hydraulics And Selection
What Is The Drilling Mud Motor:The Drilling Mud Motor is a progressive cavity positive displacement pump (PCPD) placed in the Drill String to provide additional power to the bit while drilling. The PCPD pump uses drilling fluid (commonly referred to as drilling mud, or just mud) to create eccentric motion in the power section of the motor which is transferred as concentric power to the Drilling Bit .
The drilling mud motor uses different rotor and stator configurations to provide optimum performance for the desired drilling operation, typically increasing the number of lobes and length of power assembly for greater horsepower. In certain applications, compressed air, or other gas, can be used for mud motor input power. Normal rotation of the Drilling Bit while using a mud motor can be from 60 rpm to over 100 rpm
The use of a drilling mud motor in the Drill String changes the hydraulic calculation significantly. So understanding the factors affecting drilling mud motors selection is a must.
Factors Affecting Drilling Mud Motors Selection:
Range of flow rates allowable:
- Each size and type of PDM is designed to take a certain range of volumes of fluid.
- The proper drilling mud motor selection for hole cleaning issues will be the multilobe motors, as they have a broader flow rate range and a much higher maximum allowable flow rate than 1:2 lobe PDMs of the same O.D. This gives better hole-cleaning capability - useful when ROP is high.
No-load Pressure Loss:
- Is a certain pressure loss is needed to overcome the rotor/stator friction forces and cause the drilling mud motor to turn off btm.
- This pressure loss and motor RPM are proportional to flow rate.
- Their values are known for each size and type of PDM. The no-load Pressure Loss is usually no greater than 100 psi.
Pressure Drop across the Drilling Mud Motor (Pmotor):
- Is the increase in pressure as the bit touches bottom and effective WOB is applied, (P on bottom - P off bottom).
- Motor torque increases in direct proportion to the increase in differential pressure.
- For a multilobe motor, it can be 500 psi or even more.
- There is a maximum recommended value of motor differential pressure.
- At this point, the optimum torque is produced by the motor.
- If the effective WOB is increased beyond this point, pump pressure increases further. P motor increases to a point where the lining of the stator is deformed. The rotor/stator seal is broken and the mud flows straight through without turning the bit. The pump pressure reading jumps sharply and does not vary as additional WOB is applied. This is known as stallout condition.
- Recent studies have shown If the PDM is operated at 50%-60% of the maximum allowable motor differential pressure, the same performance should be achieved as when operating at 90% of differential.
Pressure Drop Across The Drilling Bit (Pbit):
- For a given mud weight and flow rate, the TFA of the bit nozzles determines the pressure drop across the bit. The smaller the TFA, the greater the bit pressure drop. This effects the volume of mud diverted to cool the bearings.
- The greater the percentage of mud diverted, the greater the wear on the bearings. For every make and type of PDM, there is a certain recommended value of Pbit. This should not be exceeded. For the ANADRILL multilobe motors, Pbit must be in the range 500-1500 psi.
Drilling Mud Motor Rotor NozzleMost multilobe motors have a hollow rotor. This can be blanked off or fitted with a jet nozzle. When the standard performance range for the motor matches the drilling requirements, a blanking plug is normally fitted. The rotor nozzle can be easily replaced on the rig. (It normally entails removing the dump valve first). Use of this rotor nozzle can:
- Increase the total mud flow through the motor (e.g. in performance drilling, for better hole cleaning).
- Reduce the Drilling Bit RPM at high flow rates. This is a useful option when using a conventional tricone bit. It helps to prolong bit life by reducing wear on the bit bearings etc.