Downhole or drilling motors, e.g., mud motors, are commonly used in the drilling industry to increase penetration rates and drill complex geometries such as directionally and horizontally. Mud motors work by removing energy from the drilling fluid using a Moineau pump in reverse. This energy is transferred from the drilling fluid to the rotation of a rotor inside the Moineau pump. The rotor is connected through a series of housings and bearings to the bit.
One of the most important parts of the downhole motor assembly is the lower end which houses the bearings. Bearings are required to support the inner rotating output shaft connected to the motor against the outer housing. Thrust bearings are used to support the drilling load. These bearings can be roller bearings with balls, tapered bearings with cylindrical rollers, or flat wear surfaces made of a hardened material such as a diamond surface. Mud motors also require radial bearings to support the side loads placed on the driveshaft. These bearings can be needle roller bearings or wear sleeves with hardened materials such as tungsten or diamond. There are two basic types of Moineau type motors in the industry: mud lubricated and sealed bearing. These two types differ by the mechanism to cool and lubricate the bearings.
Oil Sealed Mud Motors—Oil sealed mud motors contain seals around the bearing pack to maintain the bearings in an oil bath. This allows the bearings to remain lubricated and cooled with oil. Oil sealed motors also shield the bearings from the grit. The primary oil seals must compensate for the pressure difference between the surface and downhole conditions which may be many thousands of psi. As a result, these seals often slide on a piston to compensate. One of the major drawbacks of sealed bearing assemblies is the fact that they have a limited life once a seal loses its integrity.
Mud Lube Mud Motors—Mud lube motors have no sealing mechanism around the bearing assembly. These bearing assemblies bypass a fraction of the drilling fluid from the bit to lubricate and cool the bearings. Mud lube bearings commonly utilize hardened balls and races for thrust and tungsten coated sleeves to carry the radial load. Some designs incorporate manufactured diamond thrust and radial bearings. They are designed to withstand the grit and impurities in the mud system. The major drawback to mud lubed motors are limits to the service life due to the abrasive environment.
The drive shaft of a mud motor connects the rotor from the power section to an output shaft in the bearing section. This driveshaft is a complex device because it must compensate for the eccentric motion of the rotor as well as, in certain cases, bend through a bent housing in a small space. Commonly, mud motors utilize two bending joints in the driveshaft—one at the connection to the rotor, and one at the bend in the housing. These bending joints can be U-joints, jaw clutch type joints, or CV-joints.
Mud motors operate in extremely harsh, highly abrasive downhole environments. With the high costs associated with drilling wells, it is extremely advantageous to increase mud motor efficiency and life. Since mud motors are composed of many parts, their life expectancy is only as good as the weakest link. To illustrate the current complexity of mud motors, reference is made to U.S. Pat. No. 6,827,160, the disclosure of which is incorporated herein for all purposes.