The positive displacement motor uses the Moineau pump principle. However, Downhole mud motors have limitations, This tool has found wide application in directional drilling and even straight hole drilling. The basic design of a positive displacement motor is shown in drilling mud motor diagrams # 1, 2,3 & 4. In this article, we shall cover the Drilling Mud Motor Components in more detail to help you in the downhole mud motor selection.
In General, The main Drilling Mud Motor Components are:
- Dump Sub
- Power Section
- Connecting Rod Assembly
- Bearing and Drive Shaft Assembly.
Drilling Mud Motor Diagrams
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Before going in deep for the main drilling mud motor components, we have gathered some drilling mud motors diagrams to help you better understand the rest of the article.
Drilling Mud Motor Components:
Mud Motor Dump Valve Assembly Sub:
This mud motor component allows the mud to fill or drain from the drill string while tripping pipe. When a minimum flow rate is established, the valve piston onthe below figure is forced down, closing the ports to the annulus. Thus, all the mud is directed through the motor. When the flow rate becomes less than this minimum value, a spring returns the valve piston to the “open” position, opening the ports to the annulus.
Dump Sub can be added to the top of the power section
To avoid the ingress of solids from the annulus when the mud pumps are off (especially in loose Sand), it’s normal to run a float sub as close to the motor as possible.
The Main Drilling Mud Motor Component: Power Section
This drilling mud motor component consists of mud motor housing, mud motor steel stator, and mud motor elastomeric rotator. The positive displacement motor is a reverse application of the Moineau pump. Fluid is pumped into the motor’s progressive cavities. The force of the fluid movement causes the shaft to rotate within the stator as below figure. Thus, it is a positive-displacement motor (commonly called a PDM). The rotational force is then transmitted through the connecting rod and driveshaft to the bit.
We can say that the power section converts the hydraulic fluid energy of the drilling fluid into mechanical horsepower. The mechanical horsepower of a multi-lobe rotor/stator power section is the product of high output torque and rotational speed.
The elastomeric stator is carefully bonded inside a steel tubular component which is an integral component of the motor casing. The number of lobes varies from 4 to 11 through the various models in the product range.
The steel rotor is produced with matching lobe profiles and a similar helical pitch to the stator, but with one less lobe. The rotor can therefore be matched to and inserted within the stator. The flow of the drilling fluid creates hydraulic pressure that causes the rotor to precess as well as rotate within the stator.
Drilling mud motors are divided into slow-speed, medium-speed, and high-speed types. This is done by changing the pitch of the motor stages and by the number of “lobes” and resultant cavities of the stator. In any way, the speed and torque of the resultant from the power section depend directly on the number of lobes on the rotor and stator as per the below figure.
The downhole motors will operate in either air/mist or mud. Some oil based muds are not compatible with the rubber stator and reduce the useful life of the positive displacement motor.
Drilling Mud Motor Connecting Rod Assembly:
This Drilling Mud Motor Component is attached to the lower end of the rotor. It transmits the torque and rotational speed from the rotor to the driveshaft and bit. Universal joints convert the eccentric motion of the rotor into concentric motion at the drive shaft. On some models of mud motor, reinforced rubber “boots” cover the u-joints. These prevent erosion by the mud.
The Adjustable Assembly connects the stator to the housings of the Bearing Assembly and houses the Drive Shaft Assembly. The Adjustable Assembly is easily set in the field from straight to 3 degrees of bend angle.
Mud Motor Bearing Assembly
The function of the bearing assembly in a positive displacement drilling motor is to support axial load (typically weight on bit) and centralize and support side loading on the motor’s driveshaft. The bearing assemblies used in mud motors have been developed for use in different applications with certain motor configurations offering a choice of bearing type options.
The downhole mud motor bearing assembly consists of the following:
- Multiple thrust-bearing cartridges.
- Radial bearings.
- Flow restrictor
Thrust Bearing Cartridge
The thrust bearings support the down thrust of the rotor, the hydraulic down thrust from drilling bit pressure loss, and the reactive upward thrust from the exerted weight on bit.
Metallic or nonmetallic radial bearings are used above and below the thrust bearings to protect & absorb lateral side loading of the drive shaft.
The flow restrictor allows from 5 to 8% of the circulating fluid to flow through the bearing section to cool and lubricate the mud motor bearing assembly
The common types of Bearing Assembly which you will find in most of motors manufacturers are:
- A drilling fluid-lubricated bearing, it is suitable for use on low- or medium-speed conventional motor applications where moderate weight on bit and drilling torque are required.
- High Specs Bearing, it features a larger internal diameter, allowing use of a larger diameter driveshaft and higher weight on bit. It is rapidly becoming the bearing of choice in an increasing number of applications as modern PDC bit designs demand higher torque,
- Diamond Bearing, the axial bearing surfaces in this design are constructed from specially designed PDCs. This design’s advantage allows the application of high-speed motors or a dramatic increase in weight on bit on low-speed motor applications. Typical applications include hard rock drilling, where either high bit speed or very high weight on bit is required to deliver optimal drilling performance.
The driveshaft is one of the mud motors components which is a rigidly-constructed hollow steel component. It is supported within the bearing housing by radial and axial thrust bearings.
This assembly connects the Power Section’s rotor & connecting rod to the rotating components of the Bearing Assembly. It can also compensate for any angle of the Adjustable Assembly or Fixed Housing as well as the bending subjected to the motor during directional control. The bit sub is attached to the drive shaft. When pumping through the motor, the drive shaft will turn the bit sub and the bit.
Drilling Mud Motor Rotor Catch
The Rotor Catch is a tool used in down-hole Drilling Motors to assure the extraction of the Rotor. On rare occasions, motors, mainly the power section, have parted below the Stator. When this occurs generally the stator comes out of the hole leaving the Rotor facing up the hole and resulting in a fishing job over an uneven surface.
Also, if the Drill bit is not stuck (check also: Pipe Sticking) it is hard to rotate over the Rotor to fish it, as there will be no resistance to it also turning. The Rotor catch sits at the top of the Power Section and screws directly into the Rotor. If the tool should part, the Rotor Catch will contact an internal shoulder and retrieve the Rotor.
Drilling Mud Motor Operation
Positive displacement motors were introduced in the 1960’s. The first positive displacement motor (PDM) was the DynaDrill, which was a high-speed low torque motor. After we have explained downhole drilling mud motor components, we will explain in this article the main drilling mud motor operation.
Testing & Making Up Drilling Mud Motor Operation
The motor should be tested for at least three flow rates within the operating range. The stand pipe pressure should be recorded at each flowrate. High vibration may occur at certain flow rates and is common to all motors.
With the motor hanging, the distance between the bottom of the motor housing and the top of the bit box should be measured. The motor should then be rested on the bit box and the same gap re-measured. This push-pull measurement aids in determining the condition of the thrust bearing assembly. Generally, if the difference between the two measurements is less than a 1/4” (6mm) [or 1/8” (3mm) on the smaller motors] the motor can be run.
Here are the General procedures for Making Up Drilling Mud Motor Operation:
- Pick up drilling mud motor, put in slips, install collar clamp, unscrew lifting sub.
- Test dump valve operation by hand (using water hose and e.g. handle of sledge hammer).
- If testing PDM, make up a bit first, use x/o and kelly/top drive. (PDM new from the base should be O.K.!).
- If testing PDM with a bent sub, keep pumping time to an absolute minimum. Pick up MWD collar. Make up float sub & bent sub (by hand) onto the collar. Stab bent sub pin into box of PDM.
- Align scribe line towards drawworks (or v-door).
- Pull MWD collar over with tugger so that bent sub-threads are in line with PDM box threads (i.e. horizontal).
- Put make-up tong on bent sub-body.
- Rotate rotary table (and hence PDM) slowly counter-clockwise.
- When all threads are engaged, torque connections to API value using rig tongs. Torque all connections up to MWD collar.
- Measure angular offset from slick pin to bent sub scribe line. Bring up bent sub scribe line mark to top of MWD collar
- P/U NMDC(s), UBHO (if used), and PRS. Stab into a box of MWD collar.
- Torque connections up to UBHO. Put UBHO in slips. Install collar clamp below UBHO screws.
- Back out NMDC(s).
- Align key of UBHO sleeve with bent sub scribe line. Lock in place using screws. Makeup NMDC(s) and RIH.
The motor should be tripped in slowly in areas of known restrictions to prevent drill bit and motor damage. It is a good practice to stage the motor into the hole, particularly in drilling environments with elevated temperatures. This is accomplished by circulating for several minutes after every ten stands of pipe or other predetermined depth. After running into the hole, flow pressure tests, similar to that performed at the surface should be carried out with the bit turning freely off-bottom.
The pressure reading at three or four flow rates around the operating flow rate should be recorded and used for comparison in similar tests following any drilling problems as Stuck Pipe, Lost Of Circulation, well control or any hole cleaning problem.
When tagging bottom, weight should at first be applied slowly and mud pump pressure changes noted. Initially, drilling should proceed carefully until a feel of the formation is acquired. It is necessary to feather the bit until a pattern has been cut. It normally is considered good drilling practice to break the drill bit in for ten to fifteen minutes.
The following procedures can be used as a guideline for downhole drilling mud motor running operation:
- Hole should be circulated clean before running PDM. Go slowly into the open hole. Driller must be careful!
- If high Bottom-Hole Temperature (BHT), break circulation periodically. If using float valve (normally), fill pipe at regular intervals.
- Be especially careful near the bottom (fill etc.).
- About 1 single off the bottom, P/U kelly. Circulate. Wash slowly to the bottom. Clean hole using expected flow rate. Rotate slowly only if required!
- If doing single-shot kickoff, rack back kelly & work torque out of string before orientation survey.
- If using MWD, work pipe with pumps on (rotary locked) before taking orientation survey.
- Orient tool face before drilling.
- For “blind” sidetrack (check also: sidetrack Drilling), orientation is not absolutely necessary. However, we must keep a tool face reference. The inclination is built along an arbitrary direction before being dropped back to vertical.
Drilling With Mud Motor
Increases in standpipe pressure are proportional to increases in drilling torque within the normal running range of the motors. Any change in torque will be due to a change in weight on bit or formation hardness. This will be reflected by a corresponding change in standpipe pressure.
For continuous operation, the operational pressure differential quoted for each motor, in the performance specifications tables, should not be exceeded. Although all motors are capable of delivering higher output torques than the continuous (operational) torque rating, loading of the stator above the continuous rating decreases its downhole life.
When the optimum drilling rate has been found, maintaining constant standpipe pressure will give steady torque at the bit and prevent erratic bit walk.
The rate of penetration (ROP) may be optimized with the rotary table if hole conditions are suitable. This will keep an even weight at the bit and prevent the drill string from hanging up. The rate of penetration may be optimized by small incremental adjustments to weight-on-bit. Sting rotation should be limited to 80 rpm to reduce stator or thread connection damage.
A rise in the standpipe pressure will occur when tagging the bottom with a diamond bit without delivering torque due to a reduction in the cross-sectional area of the flow path around the face of the bit.
Below are the general procedures for downhole drilling mud motor drilling operation:
- Record off bottom circulating pressure.
- If the standpipe pressure (SPP) is calculated and does not increase as WOB is applied, it is an indication that the dump valve has not closed. Pick up off the bottom, surge the pumps. Try to force the piston to close.
- As the bit is lowered to the bottom and drilling begins, torque demand on PDM increases. The pressure differential across motor (P motor) increases in proportion. We must limit Pmotor to the value recommended for the particular motor (e.g. 360 psi for D500 Dynadrill).
- Drill with pressure gauge (WOB gauge normally not accurate in sliding mode). Maintain a constant SPP while drilling. This ensures a steady DWOB in homogeneous formation. It should also ensure a steady tool-face (provided no change in formation).
- For troubleshooting with a PDM in the hole, please refer to the DD UOP or the ANADRILL PowerPak PDM Manual.
Downhole Drilling Mud Motor Pulling & L/D Operation
When the tool comes to the surface, the thrust bearing and stabilizer wear should be checked. Performance over the last run, total circulating hours on the tool, and bearing condition must be taken into account before the motor is re-run. Before the tool is laid down, mud should be flushed out of the power section by turning the output shaft to the right with the rotary table while holding the motor body with the break-out-tongs.
You can consider the following downhole drilling mud motor pulling & L/d operation:
- Use pipe spinner in open hole.
- Rotate string slowly if necessary.
- If dump valve ports clear, we should pull “dry”.
- Dump valve less likely to “work” in soft formation (ports plugged with formation).
- Slug must be kept well above the dump valve when pumped.
If you don’t look after your mud motor, it won’t work for you next time! It’s a good idea to paint on the body of PDM the number of D+C (Drilling + Circulating) hours done so far with this tool. This is especially useful for your relief DD!