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Downhole motor(PDM) is a kind of downhole dynamic drilling tool upon the power of drilling mud. Mud stream from the outlet of mud pump flows through a by-pass valve into the motor. This stream produces pressure loss at both inlet and outlet of the pump to push the rotor into rotating,and to transmit the torque and speed onto the bit. The downhole motor property mainly depends upon its property parameters.
Constitution
Downhole motor is composed of five assemblies:by-pass valve, motor, anti-drop, cardan shaft and drive shaft(see Fig. 1).
Power Section Assembly
It consists of stator and rotor. Stator is made by squeezing rubber sleeve on the wall of the steel tube. There forms spiral structure with a certain geometric parameter. Rotor is a crome-plated screw rod.
The basic principle of downhole motor goes like this: stator and rotor matches with each other, to form spiral line and seal cavity through their guide rail difference.With rotor running in the stator, the seal cavity is moving along its axial direction, continuously forms and disappears to complete its energy conversion.
Spiral sealline on rotor is divided into single end and ( multi-end stator with one more end than rotor). The less ends the rotor has, the higher speed and the lower torque is. The more ends it has, the lower speed and the higher torque is figure 3 shows the sectional profile of several typical motors.
A guide rail forms a seal cavity in motor stator. This is called one stage. The rated working pressure loss of each stage is 0.8 MPa, while the maximum pressure loss is 1.3 times of the rated pressure. Take a 4-stage motor as an cxample, the rated pressure loss shall be 3. 2MPa, and the maximum pressure loss shall be 4. 2MPa. The users are cautioned:if the pressure loss is over such value, motor may bring about leakage and speed will quickly slow down.More seriously. it may bring the motor into a complete stop, even cause the motor to be damaged.
The matching size between rotor and stator relates closely to depth temperature. To ensure motor seal in good condition,specific downhole temperature shall be provided to the manufacturer for reliable selection of appropriate motor. The actual mud flow rate is required to be within the recommended range. Otherwise incorrect flow rate may affect the motor efficiency, even speed up motor wear.
Motor torque is proportional to its pressure loss, but its outlet speed is inversely proportional to mud flow rate.With load increasing. screw drill speed decreases. Thus,pressure can be controlled by the pressure gauge and flow rate can be controlled by the flow rate gauge on surface.Thus, both torque and speed of downhole motor may be controlled.
