Downhole Motor

Downhole motor 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.

Downhole motor is composed of five assemblies of by-pass valve, anti-drop device, power section assembly, coupling assembly and bearing assembly.

1. By-pass Valve Assembly

It has two positions of by-pass and close (Fig.2)

It is in by-pass position during trip operation, circulates mud fluid in the drill string into the annular space by-passing the idle motor, so that no mud may spray out onto the platform during the trip operation. When mud flow rate and pressure reach the setting value, the valve stem moves down and closes the valve. Meanwhile, mud stream flows through the motor, and converts the pressure energy into mechanical energy. As mud flow rate is too low, or mud pump stops, and as the created pressure is not enough to overcome spring force and static friction force, the spring presses the stem upward, and by-pass is in open position. In general, the cross-over sub are used in deep well and large angle well horizontal well or hollow rotor selected (Fig.2-1).

2. Power Section Assembly

It consists of stator and rotor. Stator is made by squeezing rubber sleeve on the wall of steel tube. There forms spiral structure with a certain geometric parameter. Rotor is a crome-plated screw rod.

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. This is the basic principle of downhole motor.

Spiral seal line on rotor is divided into single end and multi-end (stator with one more end than rotor). The less ends the motor 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. When the rated working pressure of each stage lows down to 0.8 Mpa, the maximum pressure loss will be 1.3 times of the rated pressure. For an example, for 4-stage motor, the rated pressure loss shall be 3.2 Mpa, maximum pressure loss shall be 4.2 Mpa. As the pressure loss is over such value, motor may bring about leakage and speed will quickly slow down. More seriously, it may cause operation into complete stop, even cause the motor to be damaged. This is the caution for the customers.

In order to ensure rotor seal in good condition, the matching size between rotor and stator relates closely to depth temperature. Therefore, correct downhole temperature shall be provided to the manufacturer for reliable selection of appropriate motor. The actual mud flow rate is required in the recommended range. Otherwise incorrect flow rate may affect the motor efficiency, even cause speeding 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 shall be controlled by the pressure gauge and flow rate shall be controlled by the flow rate gauge on surface. This way may control both torque and speed of downhole screw drill.

2.1 Hollow Rotor Motor

To increase hydraulic horsepower and upward speed of mud stream, rotor is manufactured into hollow rotor with nozzles. Thus, motor total flow rate is equal to the sum of the flow rate through seal cavity and rotor. Each type of motor has its own recommended maximum and minimum flow rate value. lf flow rate is too high, rotor may be running at over speed, earlier damage would occur onto stator and rotor. lf it is too low, motor will stop operating. Therefore, the flow rate through seal cavity shall be ensured or even higher than the recommended maintenance value, while selecting nozzle size. Only in this way, can motor be kept in normal operation (Fig.4).

When mud density, nozzle size and total flow rate is a constant value, the flow rate through nozzle and seal cavity always changes with the load. When bit leaving hole bottom, the load is approximately zero. Whereas, the flow rate becomes minimum through nozzle on the rotor, but the flow rate becomes maximum through the seal cavity. While the bit dropping down, motor pressure loss will continuously increase, and the flow rate through rotor nozzle will become higher, but the flow rate through the seal cavity will become lower.

Flow rate through seal cavity is Qm, and flow rate through nozzle is Qz.

Then, total flow rate Q = Qm + Qz

Suppose motor speed n, calculate Qm

Based on the above recommended equation, the customers can timely change different diameter nozzles according to the actual demand, so as to reach ideal effect.

Allowed Maximal Clearance of Axial Bearing, please see Table-2

3. Universal Shaft Assembly

The function of cardan shaft is to convert planetary motion into fixed constant rotation of drive shaft, to transmit torque and speed from motor to the drive shaft, and to the bit. Cardan shaft mostly use flat shaft, but some are flexible shafts. Our flat type shaft used on our downhole motor is made by linear cutting technology. So the cut has high parallelism. lts roughness can reach 6.3 and it doesn't damage metal chemical composition. Thus, it has longer running life and less mechanical loss (Fig.5).

Downhole motor shall be disassemblied as quickly as possible after use to examine cardan shaft. lf it is found out that wear degree is over maintenance standard, relevant consumable components shall be changed promptly. lf it is not, downhole motor can not be running normally due to over run time of cardan shaft.

4. Drive Shaft Assembly

Drive shaft is used to transmit motor rotary dynamic force to the bit, meanwhile to withstand axial and radial load from drilling weight. We have two types of drive shafts, as follows:

1) Bit nozzle pressure loss is 7.0 Mpa, using hard alloy radial bearing and drive shaft assembly with thrust bearings (Fig.6);

2) Bit nozzle pressure loss is 14.0 Mpa, using hard alloy radial bearing and PDC cutter flat thrust bearing. lt has longer lasting life and higher bearing capacity.