How many types of Downhole motors in directional drilling do we have?

Today, downhole motors are used to turn the bit. The first turbo drill was invented in 1873. The components of a turbine motor are: a multistage vane-type rotor and stator section, a bearing section, a drive shaft and a bit rotating sub. The section that named as a “stage” is included a rotor and stator of identical profile. In this structure the stators are stable and the lock with turbine body. They cause the drilling mud to be diverted to rotors locked in the drive shaft. As the rotors rotate, the drive shaft also rotates, and finally resulting in rotating bits and sub-bits. 

Positive displacement motors

A hydraulically driven downhole motor that known as positive displacement motor or pdm is used to rotate the bit, independent of drill string rotation.  The pdm is included of four sections:

? By-pass valve or dump sub.

? Motor section.

? Universal joint or connecting rod section.

? bearing section with drive sub.

Image No.1: Differences between the turbine motor (left) and positive displacement motor (right) designs.

By-pass valve or dump sub

By using of the by-pass valve, the fluid fills the drill string while tripping in the hole and to drain while tripping out. When the mud is pumped, the valve closes to help move the liquid inside the tool. The spring valve in this part causes the piston spring to move upwards in the absence of pressure so that the liquid goes to the annulus.

Image No.2: Bypass Valve

Motor section

The reverse application of rene moineau’s pump principle is in this section. The motor section has two parts: a rubber stator and steel rotor. The pitch lengths in both rotor and stator is equal to the ratio of the number of rotor lobes to stator lobes. The cavities between the rotor and the stator are filled by the mud that being pumped. In this case, the rotor must rotate to eventually move to the bit.

Connecting rod assemblies

The shape of the rotor is spiral. So it tracks back and forth movements. To transmit these movements to the bit (via drive), the type of movement must be concentric. Connecting rod assembly can achieve this. So there are different types.

Universal-joint

U-connector sets are used in industry and in most positive displacement motors. There are two universal joints in this structure that both of them are grease filled and sealed with oil-resistant reinforced rubber sleeves for protecting the structure from drill fluid contamination. The lack of sufficient strength for higher torque applications, such as those encountered with recent generations of high torque pdm’s, particularly when used with PDC bits is one of disadvantages of the u-joint.

  Image No.3:Parts of Downhole Motors

Flex rod

The use of flexible steel or titanium “flex rods” is one of the recent development in connecting rod assembly technology. The degree of allowable lateral bending limits the flex rods and they don't need lubricants or rubber sleeves. Due to the increased length of flex rod, the overall rate of bend will decrease, because a long flex rod connected on the inside of the top end of the rotor and extending it through the rotor.

Bearing Section

There are three sets of bearings attached in positive displacement motor utilizes for driving shaft. also it have two sets of radial bearings (“upper” and “lower”) with one set of axial thrust bearings. The size of the tool can determines the number of these bearings. Diamond bearings can divert 20% of the flow in some motors and depending on the conditions.

Image No.4:Type of Connecting Rod Assemblies

Image No.5:Various bearings in PDMs.

Types of Positive Displacement Motors

There are different configurations of PDMs. and as mentioned before, the stator has one lobe more than the rotor. The simplest types of PDMs are 1/2 motors. The 1/2 motors can drill with with a PDC, diamond, or TSP-type bits. The other kinds of shorter models are made for directional purposes.