The 2nd method is the use of hydraulic downhole motors, which are mounted directly above the drill bit, as a drive. In this case the drill rod is arranged stationary. The 2nd method has a number of obvious advantages, it falls on the wall energy for rotation of the drill string path, the forces acting on the drill string loads are reduced and correspondingly, the number decreases of accident cases in the shaft.
Among all types of hydraulic downhole motors which are currently used for drilling of wells, screw downhole motors are widely used. These engines are characterized by a simple operation and easy operation as well as by small external dimensions, can be used with drilling fluids of different density and viscosity.
Such hydraulic equipment usually contain a housing, a drive shaft with radial and axial bearings and working members, which consist of 2 elements, as a rubber-coated sleeve, namely a stator having inner helical teeth and arranged within the stator shaft, namely a rotor having outer helical teeth.
The number of teeth of the bush exceeds a one, the number of teeth of the shaft, whereby a division of the inner space is guaranteed by the working members in high pressure and low pressure chambers in their interaction, when a liquid is pumped through the work organs. Under the action of the resulting the rotor begins to move relative to the stator, wherein the rotor axis describes a circle about the stator.
This rotation is transmitted to the drive shaft of the motor. As an energy source for the operation of the engine is usual, a liquid stream; the hydraulic machine can also saturated with air or liquid to be operated with compressed air.
When the power liquid flows into the screw motor, pressure difference will be produced between the inlet and outlet of the motor, then the pressure difference drives the rotor rotating of planetary motion round the stator, and provides running torque and speed to the drill bit while the drill pipe is not rotating during work process, thus the drilling process is being performed.
The advantages of downhole screw motors:
- Attrition of drilling pipe is declined obviously for downhole screw motors running torque is only to drive bit while the drilling is not rotating;
- Not only provide special drill assembly with high deflection rate, but also guide drill assembly with everlasting control;
- Out torque and pressure difference is nearly liner relationship within a certain range when the power liquid flows into the screw motor;
- Provide great delivery volume and improve speed of uphole velocity and carrying capability for rotor is hollow;
- Suitable for all kinds of bits.