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.
Before the motor is run into the hole, a set-up process needs to be carried out
- 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°.
- 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.
- 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.
In the illustration , figure 1, the main parts of the system are shown, moving from the bottom upwards:
- Drill bit – The motor is turned by mud which is pumped down the drillstring. The bit sits on a bent housing, and therefore does not point straight ahead. This causes a side force, which allows the bit to drill a curved hole.
- Near bit stabilizer – Smaller than the bit itself, this stabilizer forms a fulcrum, within which the motor acts as a lever, so that side force may be generated at the site of the bit.
- Mud motor – The bottom part of the motor itself has an adjustable bend. It converts hydraulic power from drilling fluid into mechanical power (rotating a bit)
- Dump valve – Above the motor, this valve can divert mud to the top of the motor if needed.
- Stabililzer – This piece acts as the end of the lever, and exerts an opposing force on the drill bit.
During the actual drilling process, the mud motor will be oriented in the desired direction so that the drill bit will be able to drill a directional well. This allows a change in inclination and azimuth, or a combination of the two. It is also possible to rotate the entire drillstring, to allow the bit to drill in a straight line at certain points. This explains why this system is also known as a steerable motor, since it can carve out a complicated path towards the desired target.
Mud motors are often used to begin the well from a vertical position, as well as to continue to control the well path at later points in the drill. Once the well inclination exceeds 60°, it becomes increasingly difficult to make an “assembly slide” (a drill without rotating the drillstring) while in the process of drilling.
Mud motors also cause issues with keeping the hole clean. Rotation of the drillstring vastly improves the transport of cuttings out of the hole, but at high inclinations it can cause cutting beds to build up and stick to the drillstring, preventing it from working properly. A preferable approach would be to use a steerable tool which works while the drillstring rotates. While such a device does already exist, it is unfortunately a highly expensive tool.
The following video will help you get more understanding about a mud motor and its configuration.
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