What are Positive Displacement Mud Motors in Drilling for Oil and Gas?

Positive-Displacement Motors (PDM) make use of a power generation section which is made up of a rotor/stator combination. In order to move a rotor part,  a PDM requires hydraulic power from drilling fluid flowing through the power generation part. With a PDM, the stator and rotor work in tandem in the same way that gears do. The stator acts as the outer gear, and is made from a moulded elastomer featuring at least two lobes. The OD of the elastomer is protected by a secure metal casing. The rotor is positioned within the stator, and acts as an internal gear. This rotor is made of metal, and will have one less gear or lobe than the stator. Because of this difference, a cavity is created which is filled with drilling fluid when the PDM is downhole. This cavity acts as a wedge when it is put under pressure, and because the drilling fluid itself can’t be compressed, the force applied to the top of the wedge causes the rotor to move.

Mud Motor

Figure 1 – Mud Motor

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Deviating the Wellbore by Positive Displacement Motor (Directional Drilling)

A positive displacement motor (PDM) is one of the most popular tool for drilling a directional well. It works by boring downwards and pumping mud through the motor itself. As shown in figure 1, the bottom section of the motor has an adjustable bend housing.

Figure 1 – Positive Displacement Motor (Courtesy of Schlumberger)

Before the motor is run into the hole, a set-up process needs to be carried out

  1. The bend will be adjusted according to the directional performance that the motor needs to achieve. This bend is only very slight, usually being under 2°.
  1. The motor is hooked up to navigational tools, which are then calibrated, in order for the driller to see where the bend is pointing when drilling. These tools are known as measurement while drilling, or MWD, and are described in detail later in this document.
  1. The other parts of the system will be adjusted to account for the required directional performance- the severity of this will depend on the drill design.

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