Also known as a drilling motor, a mud motor is a device used in geological drilling procedures. Technically, a mud motor is a progressive cavity positive displacement pump, which transfers fluid through the rotation of a helical drive. Such a drive is very similar to what one would find on an Archimedes screw, where the "screw" shape of the device can push both liquids and solids when rotated. The mud motor is slightly different, because drilling fluid or compressed gases are used to create the eccentric motion found within mud motors. The eccentric motion is then converted into the concentric motion that turns a drill bit.
Typically, mud motors rotate at anywhere from 60 to 100 revolutions per minute (RPM). Higher speeds of rotation are often avoided, because torque will decrease as rotation speed increases. For this reason, mud motors are very rarely rotated at more than 100 RPM. Loss of torque is not the only variable when it comes to efficient drilling operations; lower RPMs also are desirable because of the decreased wear on the mud motor and drill bit, which helps to increase drilling efficiency and longevity simultaneously.
Drilling mud motors consist of a variety of components that make up their overall assembly. The top sub connects the drill string to the mud motor itself. Next is the power section, where the rotor and stator are optimized for maximum power output. The power section is connected to the transmission, where the eccentric rotation from the mud motor is converted into concentric motion for the drill bit. After the transmission, a pressure-resilient bearing housing connects to the bottom sub and drill bit.
For the most part, mud motor drilling is employed in directional drilling. Directional drilling applies to all non-vertical drilling processes, which are suited best for mud motors. This is because directional drilling often requires the hole to bend toward the desired area at certain points during the boring process. Doing this without a mud motor can be costly and inefficient because drilling motors have the ability to bend and flex at limited angles. In vertical drilling, there is no need for a bend in the hole being drilled.
One of the major disadvantages of mud motors is the lining of the stator, which is an elastomer. Ideally, such a lining would be able to withstand rough and extreme environments for longer amounts of time, but an elastomer that can withstand such conditions for long periods of time has yet to be developed. For this reason, most drilling motor failures result from elastomer problems.