MOTOR HANDBOOK
Edition 7

Motor Handbook
Motor Handbook

Seventh Edition, May 2011
Version 7.4
This handbook is intended to be an aid to the operator and is solely
provided for information and illustration purposes.
P...
Introduction

CONTENTS
SECTION I: NOV Motors ..............................................1
1.	Introduction ................
Motor Handbook, 7th Edition

5.	 Operation........................................................................... 25
	...
Introduction

APPENDIX: REFERENCE INFORMATION...................... 239

11.	Engineering Data................................
Motor Handbook, 7th Edition

List of Figures
Figure 1: Motor Components......................................................
SECTION I:
NOV MOTORS
NOV Downhole is the largest independent downhole
tool and equipment provider in the world. We have the
expertise to optimi...
Introduction

1. Introduction
The increased reliability of drilling motors, in combination with improved
drill bit designs...
Motor Handbook, 7th Edition

2. Motor Sub-Assemblies

POWER SECTION
Options:
• HEMIDRIL®
• PowerPLUS™
• Vector™
ADJUSTABLE...
Motor Sub-Assemblies

Top Sub Options
(Not shown in Figure)

Top Sub

The top sub is simply a crossover housing at the top...
Motor Handbook, 7th Edition
The stator will have one more lobe than the rotor. This difference in lobes
creates a fluid in...
Motor Sub-Assemblies
U
	 • 	 se a HEMIDRIL even wall power section. HEMIDRIL Power Sections
		 have a contoured stator tub...
Motor Handbook, 7th Edition
The NOV radial bearings and thrust bearings are sealed in an oil chamber
balanced to the inter...
Motor Sub-Assemblies

Other Options
Stabilizers
NOV motors are available with a thread on the OD of the bearing assembly t...
Motor Handbook, 7th Edition

3. Motor Technology
‘ST3’ Sealed Bearing Technology
(HEMIDRIL®-ST3, PowerPLUS™-ST3, Vector™-S...
Motor Technology

‘ML3’ Mud Lubricated Technology
(HEMIDRIL®-ML3, PowerPLUS™-ML3, Vector™-ML3, DuraPower™-ML3)

NOV ‘ML3’ ...
Motor Handbook, 7th Edition

DuraPower™
(DuraPower™-ST3/-ST2/-ST1 DuraPower™-ML3, DuraPower™-SR3)

Each of the motor techn...
Motor Technology

PowerPLUS™ Drilling Motor
(PowerPLUS™-ST3, PowerPLUS™-ML3, PowerPLUS™-SR3, PowerPLUS™-ST2H)

The PowerPL...
Motor Handbook, 7th Edition

CT HEMIDRIL® Motor
(HEMIDRIL®-CT3)

The HEMIDRIL® Coiled Tubing motor has been developed to m...
Motor Technology

Coiled Tubing Motor
(Vector™-CT1)

Designed specifically for coiled tubing and slim hole applications, t...
Motor Handbook, 7th Edition

Other NOV Technologies
BlackBox® Data Recorder
BlackBox® analysis is a full downhole dynamics...
SECTION II:
MOTOR OPERATING DATA

17
Operation

4. Applications
The following information aids in selection of the proper motor for an
application. First, the ...
Motor Handbook, 7th Edition
	 • Mud Chlorides Level
		 High levels of chlorides in mud system can significantly limit moto...
Operation

Directional Drilling
Most NOV motors are used with adjustable housings to provide a method
of drilling directio...
Motor Handbook, 7th Edition
The critical issue with using a motor in an air or two-phase drilling
application is minimizin...
Operation

Volume Requirements
In a standard air drilling application, the required air flow rate (in SCFM) for
proper mot...
Motor Handbook, 7th Edition

Performance Curves
The performance of a mud motor when run in an air drilling application is
...
Operation

5. Operation
Run Preparation & Rig Site Testing
NOV motors are shipped from the service facility with all compo...
Motor Handbook, 7th Edition

Warming a Motor for High BHT
When running into a hot hole, it is important to gradually warm ...
Operation
The range of NOV motors permits selection of the correct motor to provide
the optimum combination of bit speed, ...
Motor Handbook, 7th Edition

Stalling
If too much WOB is applied, the torque required to keep the bit turning
creates a hi...
Operation
In excessive quantity, these vibrations can considerably reduce drilling
performance and in extreme cases, can c...
Motor Handbook, 7th Edition

Solutions
	 • Reduce WOB and increase RPM
	 • Increase weight and stiffness of BHA (Inclinati...
Operation

Solutions
	 • 	Pick up off bottom and hold string stationary until all energy is released 	
		 (typically a cou...
Motor Handbook, 7th Edition

Solutions
	 •	Change RPM and WOB combinations to try and get a stable drilling
		 situation a...
Operation
	 • 	The pressure drops across the top/dump sub, adjustable assembly,
		 driveshaft assembly and bearing mandrel...
Motor Handbook, 7th Edition
LOBE
HEIGHT

R
TO Ø
RO JOR
A
M

R
MA OTO
JO R
R
Ø

ROTOR MAJOR Ø
ROTOR MAJOR Ø
- LOBE HEIGHT
-...
Operation
Drilling fluids with a pH below 4 or above 10 can cause damage to the
stator elastomer and seriously attack the ...
Motor Handbook, 7th Edition
NOV has seen higher frequencies of stator chunking incidents when run in
drilling mud with ani...
Operation
The drilling environment may be controlled using one or more of the
following (Reference NACE MR0175):
	1.	Maint...
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2013 NOV Mud Motor Handbook and technical reference for all applications. Includes all specifications and performance data for drilling / directional drilling operations.

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Nov mud motor handbook

  1. 1. MOTOR HANDBOOK Edition 7 Motor Handbook
  2. 2. Motor Handbook Seventh Edition, May 2011 Version 7.4
  3. 3. This handbook is intended to be an aid to the operator and is solely provided for information and illustration purposes. Please feel free to contact any of our locations with questions not answered in this handbook. The technical data and text in this handbook is subject to change without notice. Seventh Edition (May 2011) [Version 7.4] Handbook Designer & Copy Editor: Tyler Dyck Editor: Ryan Gee Graphic Designers: Trevor Alner Tyler Dyck Technical Contributors: Steve Barton Tyler Dyck Kim Gates Ryan Gee Blair Guidroz Daniel Perez Tony Pink Mike Poirier Huzaifah Razali Lorna Snoek Peter Shwets Mark Voghell ISO 9001: 2000 Licensed Manufacturer Under: API Spec. 7 - License Number 7-1-0063 www.nov.com 09696:801197 09696:802078 DHPSales@nov.com
  4. 4. Introduction CONTENTS SECTION I: NOV Motors ..............................................1 1. Introduction ........................................................................ 3 2. Motor Sub-Assemblies ....................................................... 4 Top Sub Options .................................................................. 5 Top Sub ..................................................................................... 5 Dump Sub ................................................................................. 5 Motor Catch & Rotor Catch Top Subs ........................................ 5 Adjustable Assembly ................................................................. 7 Fixed Housing ........................................................................... 7 Oil Sealed Bearing Assembly ..................................................... 7 Mud Lubricated Bearing Assembly ............................................ 8 Power Section ..................................................................... 5 Adjustable Assembly & Fixed Housings ................................ 7 Driveshaft Assembly ............................................................ 7 Bearing Assembly ................................................................ 7 Drill Bits ............................................................................... 8 Other Options ....................................................................... 9 Stabilizers ................................................................................. 9 BlackBox® Top Sub and Bearing Mandrel ................................... 9 3. Motor Technology ................................................................ 10 ‘ST3’ Sealed Bearing Technology ........................................ 10 ‘ST2’ Sealed Bearing Technology ........................................ 10 ‘ST1’ Sealed Bearing Technology ........................................ 10 ‘ML3’ Mud Lubricated Technology ...................................... 11 ‘SR3’ Short Radius Technology .......................................... 11 DuraPower™ ..................................................................... 12 HEMIDRIL® Drilling Motor .................................................. 12 PowerPLUS™ Drilling Motor ............................................... 13 Vector™ Drilling Motor ....................................................... 13 CT HEMIDRIL® Motor ......................................................... 14 CT PowerPLUS™ Motor ..................................................... 14 Coiled Tubing Motor ........................................................... 15 Power Section Elastomers .................................................. 15 Other NOV Technologies ..................................................... 16 BlackBox®Data Recorder ......................................................... 16 VibraSCOPE™ Dynamic Analysis ............................................. 16 SECTION II: MOTOR OPERATING DATA ......................17 4. Applications ........................................................................ 19 Performance Drilling .......................................................... 20 Directional Drilling .............................................................. 21 Air or 2-Phase Drilling ........................................................ 21 Selection / Setup ..................................................................... 21 Pressures .................................................................................. 2 Temperature .............................................................................. 2 Volume Requirements ............................................................. 23 Fluid / Lubricant Requirements ................................................ 23 Drilling with Nitrogen (N2) ........................................................ 23 Operation ................................................................................ 23 Performance Curves ............................................................... 24 Milling ............................................................................... 24 Coring ............................................................................... 24 Rat holes, Mouseholes and Spudding ................................. 24 Workover ........................................................................... 24 iii
  5. 5. Motor Handbook, 7th Edition 5. Operation........................................................................... 25 Run Preparation & Rig Site Testing...................................... 25 Running In.......................................................................... 25 Starting the Motor............................................................... 26 Drilling................................................................................ 26 Reactive Torque.................................................................. 27 Stalling............................................................................... 28 Over-Running the Bit........................................................... 28 Vibration............................................................................. 28 Torsional Vibration (Stick Slip).................................................. 29 Lateral Vibration (Bit/BHA Whirl)............................................... 30 Axial Vibration (Bit Bounce)...................................................... 31 Rotary RPM........................................................................ 32 Motor Pressure Drop........................................................... 32 Bit Pressure Drop................................................................ 33 Power Section Fits.............................................................. 33 Drilling Fluids...................................................................... 34 Oil Based Drilling Fluids............................................................ 35 OBM Run Practices.................................................................. 36 H2S (Sour) Service................................................................... 36 The Effects of Chlorides on Power Sections.............................. 37 Drilling Fluid Additions.............................................................. 38 Downhole Temperature....................................................... 39 Plugging Off........................................................................ 40 Hydraulic Extension............................................................ 40 Back Reaming.................................................................... 40 Partial Rotor Bypass........................................................... 41 Running Additional Tools Below the Motor........................... 41 Rotor Nozzling.................................................................... 42 Tripping Out & Checking the Motor...................................... 42 Rig Site Maintenance.......................................................... 42 Motor Storage..................................................................... 43 Servicing............................................................................ 44 SECTION III: SUPPORTING INFORMATION.................... 45 6. 7. 8. Nozzle Selection................................................................ 47 Adjustable Assembly Operating Instructions..................... 49 2°, 3° and 4° Adjustable Housings....................................... 49 2.38° Adjustable Housing.................................................... 50 Lock Housing Make-up Torques.......................................... 51 Legacy Black Max™ Adjustable Housing Setting Procedure. 52 Troubleshooting................................................................. 54 SECTION IV: MOTOR PERFORMANCE DATA.................. 55 9. Motor Performance Summary........................................... 57 Motor Performance Summary............................................. 58 Weight on Bit Limits – Imperial............................................ 62 Weight on Bit Limits – Metric.............................................. 63 Motor Comparison Tables – Imperial................................... 64 Motor Comparison Tables – Metric...................................... 65 10. Motor Performance Data................................................... 66 How to Use Spec Pages...................................................... 67 iv
  6. 6. Introduction APPENDIX: REFERENCE INFORMATION...................... 239 11. Engineering Data............................................................. 241 Formulas.......................................................................... 241 Imperial Conversions........................................................ 242 Metric Conversions........................................................... 243 Millimeter Equivalents....................................................... 244 Millimeter and Decimal Equivalents................................... 245 12. Drill Bit Sizes................................................................... 246 13. Heavy Walled Drillpipe.................................................... 247 14. Buoyancy Factor for Drill Collars..................................... 248 15. Drill Collar Weight........................................................... 249 16. Drill Collar Make-up Torque............................................. 250 17. Drillpipe & Tool Joint Properties...................................... 258 18. Mechanical Properties of Drillpipe.................................. 264 19. Hole Curvature................................................................. 268 Alphabetical Subject Index.................................................... 269 v
  7. 7. Motor Handbook, 7th Edition List of Figures Figure 1: Motor Components..................................................................... 4 Figure 2: Rotor & Stator Cross Section...................................................... 5 Figure 3: Lobe Profiles.............................................................................. 6 Figure 4: Bearing Mandrel with BlackBox Pocket........................................ 9 Figure 5: Downhole Vibration Modes....................................................... 28 Figure 6: Power Section Fit...................................................................... 34 Figure 7: Max Pressure De-rating for Power Sections............................... 39 Figure 8: Nozzle sizes and equivalent flow rates for water......................... 47 Figure 9: 2.38° Adjusting Ring................................................................. 50 Figure 10: Performance Curve Example................................................... List of Tables Table 1: Make-up Torque for Stabilizers, Thread Protectors & Offset Pads... 9 Table 2: Available HEMIDRIL® Sizes....................................................... 12 Table 3: Torsional Deflection Angle Values for Drillpipe............................. 27 Table 4: Basic Chemical Compatibility Chart............................................ 36 Table 5: Chloride Content......................................................................... 38 Table 6: Available Nozzle Sizes................................................................ 42 Table 7: Max Allowable Endplay to Re-run............................................... 42 Table 8: Make-up Torque, Top Sub, or Dump Sub to Stator....................... 43 Table 9: Nozzle Torque Values.................................................................. 48 Table 10: 2°, 3° and 4° Conventional Lock Housing Make-up Torque........ 51 Table 11: 2.38° and 3° HEMIDRIL-rated Lock Housing Make-up Torque.... 51 Table 12: Adjustable Housing Bend Settings............................................ 51 Table 13: Black Max™ Adjustable Make-up Torque.................................. 53 vi
  8. 8. SECTION I: NOV MOTORS
  9. 9. NOV Downhole is the largest independent downhole tool and equipment provider in the world. We have the expertise to optimize BHA selection and performance, supporting over 150 locations in more than 80 countries. Our complete range of solutions for the bottom hole assembly and related equipment includes: • Drill Bits • Drilling Motors • Borehole Enlargement • Drilling Tools and Products • Coring Services • Fishing Tools • Intervention and Completion Tools • Service Equipment • Advanced Drilling Solutions We take pride in delivering superior performance and reliability. Our objective is to exceed our customers’ expectations, improve their economics, and be an integral part of their strategies. NOV Downhole is an ISO 9001 Registered Firm, and is licensed to monogram API Products under Spec 7 License No. 7-1-0063.
  10. 10. Introduction 1. Introduction The increased reliability of drilling motors, in combination with improved drill bit designs and Measurement While Drilling (MWD) systems, presents operators with enhancements to rotary drilling in an increasing number of applications. It is generally acknowledged that an effective drilling motor, matched to the drill bit and formation, provides a better rate of penetration than rotary drilling alone. In addition to increasing rate of penetration, a drilling motor allows a reduction in drillstring rotary RPM which reduces wear and tear on the casing, the drillstring, and other rotating topside components. A properly configured bottom hole assembly (BHA) also allows for effective directional control of the wellbore. The BHA is the lower portion of the drillstring, comprised of a drill bit, the drilling motor, and other specialized drilling or directional tools. The main sub assemblies of a drilling motor include the power section, adjustable or fixed housing, driveshaft, and the bearing assembly. • The power section offers a wide range of bit speeds and torque outputs. With recent improvements in elastomer technology and manufacturing techniques, the power section can be tailored to function in high bottom hole temperatures and invert mud systems. • The adjustable or fixed housing and driveshaft are located between the power section and the bearing assembly, and are engineered to withstand the increased torque rating of power sections. • As drilling programs become more complex and drilling distances push further, NOV Downhole engineers its bearing assemblies to withstand higher loads and torque, while increasing its reliability and longevity. NOV Downhole offers both oil and mud lubricated bearing assemblies. Your NOV Downhole representative will gladly assist you in selecting the most appropriate motor for your drilling application. Please consider the following questions when ordering your equipment: What is the drilling APPLICATION? Milling, Coiled Tubing, Coring, Borehole Enlargement, Other What is the general TYPE OF DRILLING? Straight, Directional, Performance (Vertical), Horizontal, Extended Reach Drilling What is the FORMATION comprised of? Soft Rock, Hard Rock, Abrasive Formations, Thinly Laminated Formations What is the estimated WELL DEPTH? Shallow Wells (up to 3,000ft), Intermediate (5,000 – 7,500ft), Deep (7,500 – 12,000ft), Ultra Deep (+16,000ft), Offshore Shelf, Offshore Deepwater What is the anticipated downhole ENVIRONMENT? High Pressure, High Temperature, Corrosive, High Chlorides, Oil-based Mud Are there any ADDITIONAL CONSIDERATIONS? Rate of Penetration (ROP) Requirements, Directional Controls, Stabilization Requirements, Dogleg Severity 3
  11. 11. Motor Handbook, 7th Edition 2. Motor Sub-Assemblies POWER SECTION Options: • HEMIDRIL® • PowerPLUS™ • Vector™ ADJUSTABLE ASSEMBLY Options: • .38°, 3°, 4° adjustable 2 F • ixed straight and bent housings DRILL BIT Options: • Roller cone P • DC • i-center B D • iamond impregnated DRIVESHAFT ASSEMBLY Options: • Ultra Duty • Conventional BEARING ASSEMBLY Options: • Oil Sealed • Mud Lube • Short Radius • High Speed Figure 1: Motor Components 4
  12. 12. Motor Sub-Assemblies Top Sub Options (Not shown in Figure) Top Sub The top sub is simply a crossover housing at the top end of the motor. The top connection is an API tool joint box, and is available with an optional “float bore” to accommodate API float valves. The lower connection uses a proprietary thread that connects to the upper box of the stator housing. Dump Sub (bypass valve, dump valve or bypass sub) The dump sub allows fluid to bypass the motor and fill the bore of the drillstring when tripping into the hole. It also allows the drillstring to drain when tripping out of the hole. When no dump sub is used, a wet trip out of the hole will occur. When the rig pumps are off, a spring holds a ported piston in the upper position, exposing ports in the dump sub body which allow drilling fluid to flow into or out of the drillstring while tripping. When the rig pumps are started, the flow through the piston bore causes a pressure drop across the ports in the piston. This forces the piston down, overcoming the spring force and closing the ports which in turn directs the drilling fluid through the motor. When the rig pumps are stopped, the spring forces the piston up, opening the ports. NOV dump subs are preset to close at less than the minimum recommended circulation rate of the motor assembly. There is little pressure loss through the dump sub when operating. Motor Catch & Rotor Catch Top Subs The rotor catch system is designed to retrieve the motor in case of a housing fracture. It will retrieve the motor from the upper stator box connection down to the drill bit. The motor catch system has the additional feature of an integral catch flange within the top sub. It will retrieve the motor from the top sub down to the drill bit. Power Section (motor section, motor assembly, multistage motor, positive displacement motor (PDM) drive, or rotor and stator) NOV Monoflo® defines a power section nomenclature by its outer tube diameter, rotor/stator lobe configuration and number of stages. Tube sizes range from 1 11/16” to 11 ¼” tube OD. The rotor and stator are designed as helical elements with a major and minor diameter. The lobe is the curved spiral shape formed by the difference in the major and minor dimension. Stator Elastomer Stator Major (valley) Stator Minor (peak) Stator Tube Rotor Rotor Major (peak) Rotor Minor (valley) Figure 2: Rotor & Stator Cross Section 5
  13. 13. Motor Handbook, 7th Edition The stator will have one more lobe than the rotor. This difference in lobes creates a fluid inlet area (cavity) where fluid can be pumped through to create rotation. A stage is the distance measured parallel to the axis between two corresponding points of the same spiral lobe. This distance is commonly referred to as the lead of the stator. The lobe configuration selection is dependent on the application need. As a general rule, a high rotational speed power section will produce lower torque; inversely a low speed power section generates higher torque. Below are common lobe configurations with a generic summation of performance: 1:2 3:4 4:5 5:6 LOW TORQUE 7:8 9:10 HIGH TORQUE HIGH SPEED LOW SPEED Figure 3: Lobe Profiles The rotational speed generated by the power section is proportional to the rate of fluid flow through the power section, i.e. increasing the flow rate through a given power section directly increases the output speed. To increase the output speed of a power section without changing the flow rate, the cavity size is changed. A high speed power section will require a larger fluid inlet area (cavity) to allow more fluid throughput into the cavity. The torque generated by the power section is proportional to the differential pressure applied across the power section and is independent of fluid flow. Generally, the more weight applied to the bit, the higher the torque needed to keep the bit turning, so the higher the differential pressure across the Power Section. The maximum recommended differential pressure is limited by the stator elastomer. If pressure increases beyond the limits of the elastomer, the stator elastomer will deform, breaking the cavity seal so the mud flow leaks past the rotor and rotation stops – this is commonly known as a stalled motor. Refer to Stalling on page 28 for the recommended procedure to restart a stalled motor and limit potential motor damage. An increase in torque output can be achieved by three methods: U • se a power section with more stages. As torque is proportional to the applied differential pressure, a power section of similar tube diameter, lobe configuration and profile construction will generate more torque as the number of stages increases. For example, the 500-67-60 and 500 67-80 power sections will rotate at similar speeds but the 500-67-80 power section allows a higher maximum differential pressure, thus generating higher torque output due to more stages. • Use a high performance elastomer. PowerPLUS™ allows 50% higher differential pressure across each stage, generating 50% more torque with the increase in differential pressure. For example, the 500-67-60 power section’s maximum recommended differential pressure is 860 psi (5,930 kPa) and generates 2,370 ft-lbs (3,220 Nm) of torque using standard elastomer. The same model in PowerPLUS™ has a maximum recommended differential pressure of 1,290 psi (8,895 kPa) and generates 3,560 ft-lbs (4,820 Nm) of torque. 6
  14. 14. Motor Sub-Assemblies U • se a HEMIDRIL even wall power section. HEMIDRIL Power Sections have a contoured stator tube ID and a thin elastomeric liner of even thickness. Backed by the contoured tube, the thinner elastomer liner maintains its sealing ability up to 75% higher differential pressure across each stage. HEMIDRIL® is the ultimate solution for increased torque output and reliability. ® Adjustable Assembly & Fixed Housings Adjustable Assembly (bent coupling housing, adjustable housing, AKO) The NOV adjustable assembly connects the stator to the sealed bearing assembly and encloses the driveshaft assembly. The angle setting is field adjustable to produce a wide range of build rates. Operation of the 2.38°, 3° or 4° adjustable housings, is outlined in Section 7: Adjustable Assembly Operating Instructions on page 49. The adjustable assembly is made up of only four components and is thus simple to service and maintain. The design allows for a large internal diameter and thus enables a larger, stronger driveshaft to be assembled. Fixed Housing Fixed, non-adjustable housings are available (special order) in straight or fixed bend configurations. Driveshaft Assembly (transmission, flex coupling, universal joint, coupling assembly) The driveshaft assembly converts the eccentric motion of the rotor into concentric rotation for the bearing assembly. It also accommodates any angle set on the adjustable bent housing (or fixed bend housing) and carries the thrust load from the rotor caused by the pressure drop across the power section. The assembly consists of a driveshaft and two sealed and lubricated universal joints connecting to the rotor and the sealed bearing assembly. NOV Downhole offers both conventional and “Ultra Duty” high torque driveshaft assemblies, designed to handle the highest torque power sections, with a proven record of reliability. Bearing Assembly (output shaft assembly, bearing pack, bearing stack, bearing section) Oil Sealed Bearing Assembly The sealed bearing assembly transmits the rotation of the driveshaft assembly to the drill bit. It transmits the compressive thrust load created by the weight of the collars and drill string to the rotating bit box, and supports the radial and bending loads developed while directional or steerable drilling. It also carries the tensile “off-bottom” thrust load produced by the pressure drops across the rotor and the drill bit, as well as any load caused during back reaming. 7
  15. 15. Motor Handbook, 7th Edition The NOV radial bearings and thrust bearings are sealed in an oil chamber balanced to the internal tool pressure. The thrust bearings are high capacity and there is no need to balance hydraulic thrust load to bit load with the NOV sealed bearing motor. The high capacity radial bearings readily withstand side loads caused by drilling with a deflection device or uneven cutting action along the drill bit periphery. The lower connection is typically an API regular bit box. Custom connections and pin-down options can be special ordered. Mud Lubricated Bearing Assembly The mud lubricated bearing assembly is interchangeable with the sealed bearing assembly and performs the same basic function. In a mud lubricated assembly, a small percentage of the drilling mud is allowed to pass through the bearing chamber, to lubricate the bearings. The tungsten carbide radial bearings and angular contact bearing section supports the radial loads along the full length of the bearing assembly, creating a very stiff, strong assembly. Mud lubricated bearing assemblies can be used in the hottest holes with the lowest aniline point drilling fluids, as there are no elastomeric seals. The lower connection is typically an API regular bit box. Custom connections and pin-down connections can be special ordered. Drill Bits The broad range of circulation rates, bit speeds and bit torques available with NOV motors make them suitable for use with journal bearing and roller cone bits, PDC bits, coring bits, bi-center bits, and diamond impregnated bits. Significant work has been conducted by NOV® ReedHycalog® on developing specific Fixed Cutter (FC) designs for directional motor assemblies, with intent on providing high ROP when in rotating mode, along with a smooth torque response for efficient steering when in sliding mode. Several publications already exist on these developments, but in brief summary, the four prime FC design characteristics that have been engineered for steerable motors are: • SmoothTorque™ Torque Control Components (TCC): Secondary components specifically positioned in the cone of the bit to prevent cutter over-engagement. These provide a reduction in torque fluctuations for superior toolface control. • Optimized cutter backrakes: Relatively aggressive backrakes utilized to provide high penetration rates in rotating mode, while TCC’s are optimized (in terms of tip offset) to control torque when sliding. • Lateral torque control: Gauge inserts for lateral control and to provide a low friction bearing surface • SmoothTorque gauge design: Features laterally exposed gauge cutters to clean up the hole in rotating mode, and a tapered upper section to reduce gauge pad interference while in sliding mode A number of these features will also be critical to success with motors in vertical applications. Although there is no requirement for the torque control features to assist in sliding, they will still deliver a smooth torque response, thus improving the overall efficiency of the design. They also allow the use of more aggressive cutting structures in challenging applications as TCC will dramatically reduce the risk of Stick-Slip. This has been proven in a number of field examples using the BlackBox® data recorder in near bit subs. NOV can provide recommendations for matching ReedHycalog bits to motor configurations. 8
  16. 16. Motor Sub-Assemblies Other Options Stabilizers NOV motors are available with a thread on the OD of the bearing assembly to accept straight or spiral blade screw-on stabilizers or screw-on offset pads. A protector is installed over this thread when stabilizers or offset pads are not being used. Stabilizers, thread protectors, and offset pads should be torqued to the values shown below. Table 1: Make-up Torque for Stabilizers, Thread Protectors & Offset Pads Torque Tool Size (lb-ft) (N-m) 1 7/16” N/A N/A 1 11/16” N/A N/A 2 1/8” N/A N/A 2 3/8” N/A N/A 2 7/8” 2,000 2,700 N/A 3 1/8” N/A 3 3/8” N/A N/A 3 1/2” 3,800 5,200 5,700 3 3/4” 4,200 4 3/4” / 5” 6,400 8,700 6 1/4” 10,000 13,600 6 1/2” / 6 3/4” / 7” 12,000 16,300 7 3/4” 20,000 27,100 8” 21,000 28,500 9 5/8” 35,000 47,500 11 1/4” 32,000 43,400 BlackBox® Top Sub and Bearing Mandrel The NOV BlackBox® can be used in several places in the BHA. Special sub and bearing mandrel options are available to house the Blackbox. See BlackBox Data Recorder on page 16. Figure 4: Bearing Mandrel with BlackBox Pocket 9
  17. 17. Motor Handbook, 7th Edition 3. Motor Technology ‘ST3’ Sealed Bearing Technology (HEMIDRIL®-ST3, PowerPLUS™-ST3, Vector™-S3, DuraPower™-ST3) NOV ‘ST3’ Sealed Bearing Technology (previously known as the Series 36 bearing assembly) uses the strongest and most versatile sealed bearing assemblies in the NOV fleet. This is a fully sealed bearing assembly – 100% of the drilling fluid passes directly to the bit with no loss via flow restrictors, so bit hydraulics are maximized. Features: • Sealed, oil-lubricated bearing assembly • No-bypass design: 100% flow to bit • Ultra-Duty bearing mandrel for high torque power sections • Pin-down bearing mandrels available • Optional BlackBox™ pocket on bearing mandrel • Ball-type driveshaft assembly Benefits: • 100% of the fluid exits at the bearing mandrel bit box • Full flow to bit yields: - Better bit and hole cleaning - Increased ROP - Better bit life • Ideal for rotary steerable applications: - Full flow to power rotary steerable - No signal loss when running MWD pulser below motor • Handles abrasive drilling fluids better – no flow restrictor wear • Best option for air drilling – not relying on mud to lubricate bearings ‘ST2’ Sealed Bearing Technology (HEMIDRIL®-ST2H, PowerPLUS™-ST2H, Vector™-ST2, DURAPOWER™-ST2) NOV ‘ST2’ Sealed Bearing Technology (previously known as Series 24 / 24X bearing assemblies) and ‘ST2H’ (previously known as Series 24XH bearing assemblies) are capable of withstanding radial and axial loads similar to the ‘ST3’ assemblies. This design incorporates the use of flow restrictors to balance the lower rotary seals and therefore bypasses a small percentage of drilling fluid away from the bit. They have logged over 4 million hours of successful drilling since year 2000. ‘ST1’ Sealed Bearing Technology (Vector™-ST1, DuraPower™-ST1) NOV ‘ST1’ Sealed Bearing Technology (previously known as Series 14 / 17 bearing assemblies) also incorporates the use of flow restrictors to balance the lower rotary seals and therefore bypasses a small percentage of drilling fluid away from the bit. This technology has also been used successfully for many years and smaller sizes are still regularly used. 10
  18. 18. Motor Technology ‘ML3’ Mud Lubricated Technology (HEMIDRIL®-ML3, PowerPLUS™-ML3, Vector™-ML3, DuraPower™-ML3) NOV ‘ML3’ Mud Lubricated Technology (previously known as the Series 40 bearing assembly) is designed with a mud lubricated bearing assembly featuring a bi-directional angular contact bearing pack that gives a uniform load distribution and high weight on bit capacity. In addition, the mud lubricated bearing assembly is radially supported over its entire length and can, therefore, tolerate very high side loads. The additional feature that separates the ML3 mud lubricated motor from competitors’ tools is the “Twin Torque Nut” design. This patent-pending feature separates the power section output torque from the compression of the bearings, so the torque rating of the bearing assembly is not limited by the bearings. Features: • Twin Torque Nut feature allows use with high torque power sections • Bi-Directional Angular Contact Bearing Pack - Tensile or compression load is supported by multiple rows of bearings • Bearing Assembly is radially supported over its entire length • Maximum operating temperature is limited only by power section • Ball-type driveshaft assembly Benefits: • Higher operating torques • Higher bottom hole temperatures • Extended operating hours • Easy to service; Beneficial in remote locations • Versatile; Operates in virtually any drilling fluid • Higher weight-on-bit (WOB) capacity ‘SR3’ Short Radius Technology (HEMIDRIL®-SR3, PowerPLUS™-SR3, Vector™-SR3, DuraPower™-SR3) NOV ‘SR3’ Short Radius Drilling Technology (previously known as the Series 39 bearing assembly) provides the ability to drill higher build rates, for short radius drilling. This uses a sealed bearing assembly – a shorter version of the ST3 design. Another use of a short bit-to-bend motor is to reduce the stresses in the adjustable assembly by drilling a standard radius curve but using a lower bend setting than a standard motor. This reduces the bending stresses experienced by the adjustable assembly, decreasing the risk of a housing fracture. Features: • Short bit-to-bend length • Sealed, oil-lubricated bearing assembly • No-bypass design: 100% flow to bit • Ball-type driveshaft assembly Benefits: • Higher build rates and motor steerability • Lower bend settings reduce stress on components • Minimizes True Vertical Depth (TVD) to reach horizontal 11
  19. 19. Motor Handbook, 7th Edition DuraPower™ (DuraPower™-ST3/-ST2/-ST1 DuraPower™-ML3, DuraPower™-SR3) Each of the motor technologies (-ST3, -ML3, SR3, etc.) are available for order without a power section. This configuration of a motor bottom end without a power section is named DuraPower. HEMIDRIL® Drilling Motor (HEMIDRIL®-ST3, HEMIDRIL®-ML3, HEMIDRIL®-SR3, HEMIDRIL®-ST2H) The HEMIDRIL® Drilling Motor offers the highest power and torque output by utilizing even wall technology. The power section consists of a steel supported core with flexible multi-body construction, an even thickness elastomer and Power Rib™ technology. These features combine to deliver increased torque, power, and efficiency that are required in challenging drilling environments. The HEMIDRIL’s steel supported core allows for increased differential pressure across each stage over the length of the power section. The constant elastomer thickness reduces fluid leakage and heat buildup. The Power Rib technology further increases the seal efficiency which improves the conversion of pressure to torque and power. Delivering an increased rate of penetration at a consistent RPM, the HEMIDRIL is ideally suited for challenging drilling environments including high temperatures and invert mud systems. Features: • Even wall design • Flexible multi-body construction • Power Rib technology • Available with Sealed or Mud-Lubricated Bearing Assembly Benefits: • Higher torque and power output – increased RPM • Consistent RPM and directional control – well suited for use with rotary steerable systems • Increases rate of penetration and drilling efficiency • Withstands aggressive drilling applications and conditions • Extends drilling distances Table 2: Available HEMIDRIL® Sizes Size Stages Type 5/6 3.5 HEMIDRIL 5” 4/5 6.3 HEMIDRIL 5” 7/8 3.8 HEMIDRIL 6-1/4” 7/8 2.9 HEMIDRIL 6-1/4” 7/8 4.8 HEMIDRIL 6-7/8” 7/8 2.9 HEMIDRIL 6-7/8” 7/8 3.0 HEMIDRIL 6-7/8” 7/8 5.0 HEMIDRIL 8” 7/8 4.0 HEMIDRIL 9-5/8” 7/8 4.8 HEMIDRIL 11-1/4” 12 Lobes 2-7/8” 7/8 4.8 HEMIDRIL
  20. 20. Motor Technology PowerPLUS™ Drilling Motor (PowerPLUS™-ST3, PowerPLUS™-ML3, PowerPLUS™-SR3, PowerPLUS™-ST2H) The PowerPLUS™ Drilling Motor features an advanced elastomer power section delivering increased torque output capacity, power, and efficiency. The increased mechanical properties of the power section elastomer provide the ability to drill through tough formations with high reliability. Delivering increased rate of penetration at a consistent RPM, the PowerPLUS Drilling Motor is suited for all drilling applications where increased torque is desired, or when increased reliability over standard elastomer is needed. Features: • Advanced elastomer stator • Available with any bearing assembly option Benefits: • Capable of 50% higher torque and power output • Increases rate of penetration (ROP) and drilling efficiency All sizes available: 3 1/2” to 11 1/4” Vector™ Drilling Motor (Vector™-ST3, Vector™-ML3, Vector™-SR3, Vector™-ST2, Vector™-ST1) The Vector Drilling Motor is considered a standard within the industry and is the motor of choice for conventional drilling applications. The motor is available in an array of power section configurations that deliver the desired speeds and torque. Furthermore, it is configured with an adjustable or fixed housing and an oil or mud lubricated bearing assembly. The Vector Drilling Motor is readily available and cost effective. It is a reliable tool that has a minimum number of components and is simple and efficient to service. Features: • Wide range of speeds and torques • Adjustable or fixed housing option • Available with Oil Sealed or Mud-Lubricated bearing assembly • Stabilization option Benefits: • Reliable and consistent performance • Increases ROP and drilling efficiency • Adaptable to a variety of drilling environments • Cost effective – simple to maintain and service Standard Sizes: 3 1/2” to 11 1/4” 13
  21. 21. Motor Handbook, 7th Edition CT HEMIDRIL® Motor (HEMIDRIL®-CT3) The HEMIDRIL® Coiled Tubing motor has been developed to meet industry demand for aggressive, high torque, low speed requirements. This powerful motor features even wall technology which is the highest torque and power-output class of power sections available to the market. The HEMIDRIL power section has a steel supported core with flexible multibody construction, even elastomer thickness, and Power Rib technology. These features combine to deliver increased torque, power, and efficiency that are required in today’s aggressive environments. Features: • HEMIDRIL power section provides up to 75% higher drilling torque than traditional power section • Power Rib technology further increases sealing efficiency and therefore improves the conversion of pressure to torque and power • Steel supported core allows for increased differential pressure across each stage over the length of the power section • Titanium flexshaft w/ straight housing Benefits: • Experiences less swell, so rubber last longer… increased reliability • Constant elastomer thickness reduces fluid leakage and heat build up • Sealed bearing assembly – best choice for air/nitrogen 2 7/8” 5/6 3.5 Stage configuration available CT PowerPLUS™ Motor (PowerPLUS™-CT3) The PowerPLUS™ Coiled Tubing motor utilizes the high performance PowerPLUS elastomer that delivers 50% more power and torque over standard NBR elastomer power sections. PowerPLUS has proven to perform remarkably in both water based fluids and nitrogen making it an ideal power section for CT milling operations. Features: • High performance PowerPLUS elastomer • 50% more power & torque than conventional CT power section Benefits: • Improved CT milling/drilling performance • Increased milling speed • Excellent performance in water based fluids and nitrogen • Less stress on coiled tubing • Reduced operational cost Standard Sizes: 1 11/16” to 3 1/8” 14
  22. 22. Motor Technology Coiled Tubing Motor (Vector™-CT1) Designed specifically for coiled tubing and slim hole applications, the Coiled Tubing Motor provides all the benefits of the Performance Drilling Motor. This motor is used with coil tubing units for continuous drilling and workover applications such as vertical deepening, milling, de-scaling or dewaxing of production tubing, windows, and cement plugs. Features: • Small diameter applications • Wide range of speeds and torques • Smooth outer diameter – Slick Benefits: • Coiled Tubing and Slim Hole applications • Consistent performance • Cost effective and simple to maintain Standard Sizes: 1 11/16” to 3 1/8” Power Section Elastomers Standard NBR (Monoflo® Reference code: RR) • Commonly known as Nitrile, Buna Nitrile • Most widely used elastomer in oilfield applications because of general chemical compatibility and good mechanical properties HNBR (Monoflo® Reference code: OC) • Commonly known as Hydrogenated Nitrile, HSN • Hydrogenation process offers better resistance to oils and high temperatures PowerPLUS™ (Monoflo® Reference code: PRR) • High performance version of standard NBR • Rated for 50% higher differential pressure than standard NBR, providing 50% higher torque output capacity See Power Section Fits on page 33 for more information. 15
  23. 23. Motor Handbook, 7th Edition Other NOV Technologies BlackBox® Data Recorder BlackBox® analysis is a full downhole dynamics analysis service which combines multiple and flexible positions of measurement combined with high frequency data capture to provide a full picture of the downhole dynamic conditions. Through in-depth analysis and expert interpretation of both downhole and surface data, effective vibration mitigation and reduction techniques can be identified leading to increased drilling efficiency. BlackBox Data Recorders provide: • Downhole Temperature • Maximum Lateral Accelerations • RMS Lateral Accelerations • Torsional Vibration Indicator (TVI) • Downhole RPM Features: • No non-magnetic requirements • One-piece with self contained power supply • No interference with other electronic devices • Compact, flexible reliable design • High frequency data capture rate Benefits: • Can be run below motors – Gives true bit dynamics capture • Can be run in its own sub or fitted in existing drillstring components (See BlackBox® Top Sub and Bearing Mandrel on page 9) • Can use multiple recorders at various places in the BHA • Enables complete system dynamics analysis VibraSCOPE™ Dynamic Analysis VibraSCOPE™ is a drillstring dynamics modeling software that enables prewell analysis of the BHA and drillstring. It utilizes finite element analysis to model the dynamics of the entire string from bit to the rig floor. It predicts parameters that initiate vibration and high impact loading that can lead to premature bit and/or downhole tool failures, identifying combinations of conditions and parameters that are most likely to results in vibration modes while drilling. The proprietary software delivers a recommended set of drilling parameters, based on a proven scientific approach that minimizes risk of vibration. 16
  24. 24. SECTION II: MOTOR OPERATING DATA 17
  25. 25. Operation 4. Applications The following information aids in selection of the proper motor for an application. First, the intended application must be known, whether it is Vertical Drilling, Directional/Horizontal Drilling, Performance Drilling, Milling, Coiled Tubing, Coring, Borehole Enlargement, or other. • Hole Size Used to determine motor size. Sufficient space between the hole ID and motor OD must be left to allow fishing using common catch tools. NOV Downhole offers a full range of overshots for fishing applications. Note: Motor sizes are nominal sizes – there may be upsets on the OD that are larger or grooves that are smaller than the nominal motor size. • Bit Type Used to determine power section configuration and motor setup. Aggressive bits typically require more torque output. Diamond impregnated and milling bits typically perform best at high speeds. Bicenter bits typically require no motor upsets or stabilizers until 30 ft / 10 m above the bit, for passing through casing. • Desired Bit Speed Used to determine power section configuration. Bit speed is the sum of the Motor output speed and the top drive/rotary table rotary speed. The rotary speed must be subtracted from the desired bit speed to obtain the desired motor output speed. Alternatively, this choice is often based on a need for a configuration “faster than” or “slower than” a configuration that was previously used in that application. • Desired Output Torque Used to determine power section configuration. The motor must generate sufficient torque to keep the bit turning under the highest anticipated WOB for the application. Generally roller cone bits require less output torque than PDC bits, but more WOB is applied to roller cone bits. Aggressive bits typically require more torque output. Alternatively, this choice is often based on a need for a configuration “stronger than” a configuration that was previously used in that application. • Mud Type Used to determine the power section fit, stator elastomer, coating on the rotor, and possibly the type of bearing assembly used – sealed or mud lubricated. See Drilling Fluids section on page 34. Air drilling applications benefit from the use of a slower speed power section, typically with looser fits to extend the life of the elastomer. • Expected Bottom Hole Temperature – Static Used to determine power section fit. The elastomeric liner of the stator swells as the temperature increases, which tightens the fit between the rotor and stator. As the fit tightens, the possibility of chunking increases. Conversely, a loose fit intended for higher temperatures will be prone to stalling if used in a cooler hole. Note: if the BHT varies by more than 40°F (20°C) over the course of the motor run planned, provide starting and finish temperatures. • Mud Weight, Solids and Sand content Higher mud weights and abrasive solids/sand content are abrasive to motor components. In these applications, mud-lubricated bearing assemblies and tungsten carbide coated rotors may be preferable. 19
  26. 26. Motor Handbook, 7th Edition • Mud Chlorides Level High levels of chlorides in mud system can significantly limit motor life, by causing corrosion of the rotor or weakening of the bond between the stator elastomer and stator tube. (See The Effects of Chlorides on Power Sections on page 37.) An alternative coating on the rotor, such as tungsten carbide, would be beneficial in these situations, to prevent corrosion. • Flow Rate Range Used to determine power section configuration and possible need for bypass nozzle. Flow rates higher than the Max Flow Rate specification for the power section will cause accelerated deterioration of the stator elastomer liner, and may cause excessive washing of the internal motor components. A rotor bored for bypass flow may be required, to bypass the extra flow. • Desired Build Rate Used to determine bend angle recommendation. Each power section has a chart of “Predicted Build Rates” with the latest available series of sealed bearing assembly in SECTION IV: MOTOR PERFORMANCE DATA. • Stabilization Choose either of: True Slick, Slick with Thread Protector, Straight Blade stabilizer, Spiral Blade stabilizer, Integral Blade stabilizer, or Offset Pad stabilizer. Stabilizers will affect the Predicted Build Rate in a directional application, and will also reduce lateral vibrations and improve hole quality in many applications. • Top Sub / Dump Sub options Various top connection options are available. Float bore is optional, with two or more float bore choices in some motor sizes. • Catch Stem Optional accessory, included as a standard feature in most regions. • Bit Box options Certain motor sizes have various bit box thread options, including pin threads (known as pin-down mandrels) which are typically used for running other tools below the motor, such as Rotary Steerable tools. • Other Tools Run Below Motor Certain tools run below the motor benefit from the use of no-bypass motor bearing assemblies (e.g. ST3 and SR3 motors), to ensure all flow goes to the tool below the motor, without any loss of flow or pressure. Performance Drilling NOV motors have been used successfully with higher output power sections for straight hole performance drilling. These power sections have more stages and consequently, provide more uniform torque output since the pressure per stage required to produce a given torque is less. The result is optimum rates of penetration. The slow speed PowerPLUS™ and HEMIDRIL® motors are most commonly used for these applications, as they allow higher WOB to be applied, to increase rates of penetration while maintaining reliability. For vertical drilling through formations with high dip angles, which tend to push the motor away from vertical, NOV offers stabilized motor products to improve performance in vertical drilling applications. See Vector™ Drilling Motor under Section 3: Motor Technology on page 13 for more information. 20
  27. 27. Operation Directional Drilling Most NOV motors are used with adjustable housings to provide a method of drilling directionally downhole. The desired angle is set in the adjustable housing sufficient to alter hole course with the drillstring not rotating and the tool face oriented. Charts showing Predicted Build Rates are shown for each configuration in SECTION IV: MOTOR PERFORMANCE DATA. When the drillstring is rotated with the motor operating, the system drills straight ahead. Refer to SECTION IV: MOTOR PERFORMANCE DATA for the “Maximum Adjustable Bend Setting For Rotary Drilling”. Note: NOV recommends a maximum bend setting of 1.83° for rotary drilling. NOV adjustable housings are easily adjustable on the rig floor. (See Section 7 for Adjustable Assembly Operating Instructions.) Adjustable assemblies are available in the following bend settings depending on motor size: • 0° to 2° in 12 increments • 0° to 2.38° in 8 increments • 0° to 3° in 12 increments • 0° to 4° in 18 increments The NOV adjustable housing eliminates the need to select a bent housing angle before a motor can be assembled and shipped. NOV motors contain a screw-on stabilizer thread on the lower bearing assembly to enable the motor to be run slick with the NOV adjustable housing, or with field installable stabilization as required. Review of NOV customer requests over the past few years showed that motors are used at bend settings of 2.38° or less in 96% of applications. This led to the development of NOV’s newest adjustable assembly, which has a maximum bend setting of 2.38° in 8 increments. The benefit is that a significantly larger diameter driveshaft can be used within this assembly, greatly increasing its torque capacity. NOV Downhole concentrates on the ongoing development of improved motors and accessories to compliment the overall steerable system. Air or 2-Phase Drilling The NOV motors may often be used for drilling with air or two-phase drilling fluids as the circulating medium. Two-phase drilling fluids can be defined as follows: • Mist: occurs when the liquid fraction is less than 2.5% at downhole conditions. In this case the liquid stays as droplets within the gas. • Foam: occurs when the liquid fraction is between 2.5% and 25% at downhole conditions. Foams are typically specified as “% foam quality”. Foam quality is the volume fraction of the gas (i.e. 75% foam quality is 75% gas and 25% liquid, by volume). • Aerated mud: occurs when the liquid fraction is greater than 25% at downhole conditions. In this case the gas stays as bubbles within the liquid. Since there are large volumes of oxygen present in air drilling, corrosion of the drillstring can be a concern. Passivating (oxidizing) inhibitors should be used to minimize this corrosion. Selection / Setup Sealed Bearing motors are generally preferable to Mud Lubricated motors, as air does not conduct heat as well as liquid (mud), so the bearings in the Mud Lubricated motor are prone to lock-up due to over-heating. This may not be an issue in aerated muds. 21
  28. 28. Motor Handbook, 7th Edition The critical issue with using a motor in an air or two-phase drilling application is minimizing the temperature generated within the stator elastomer liner. This can be accomplished as follows: • Run stators with a looser power section fit than would be run in a similar hole temperature with drilling mud. • Use the lowest foam quality possible (i.e. the highest amount of liquid). • Minimize the RPM • Minimize times with no circulation All of the above, in addition to the downhole temperature, interact to determine the life of the stator. Failure of the stator elastomer liner is accelerated by internal heat build-up. As the liner heats up, it expands, causing an increase in the interference fit that in turn causes more friction and further heat build-up. Hard spots then develop in the liner and eventually chunking occurs. The problem is further compounded by the poor thermal conductivity of air compared to a drilling mud. There are rotors and stators available that are designed specifically for air or two-phase drilling. These accommodate high flow rates and run at lower pressure drops and should be selected if available. Please contact NOV Downhole for availability and application of these power sections. When air drilling power sections are not available, use the slowest available power sections that have the same target flow rate. Ideally, use high performance power sections (more stages) because the differential pressure per stage required to produce a given torque is less. This lower differential pressure will be less damaging to the stator elastomer liner. Although the motor will produce the same torque at a given differential pressure when using air, the maximum obtainable differential pressure will be less due to the extra slippage that occurs with a gas. Consequently, the maximum obtainable torque (and stall torque) will be lower. A top sub should be used above the power section rather than the dump sub because airflow will not cause the dump sub to close. Alternatively the dump sub ports can be blanked off. Use a stabilized motor to reduce vibrations if possible – air does not provide as much vibration dampening as drilling mud. While drilling with air or two-phase fluid, the fluid density in the annulus may be higher than in the bore due to the cuttings in the annulus. When a connection is being made, the reduced pressure in the bore can result in cuttings entering the bore. This can damage the motor or plug the bit. A float valve should therefore be used when drilling with air or two-phase fluids to prevent this. A small hole is sometimes drilled in the flapper to allow pressure to equalize if the drillstring does become plugged between the float valve and the bit. Pressures The pressure at the outlet of the motor (i.e. the motor exit pressure) has a pronounced effect on the response of the motor. The motor exit pressure is the sum of the hydrostatic pressure and the bit pressure drop. This is the pressure that determines the volume flow rate through the motor. As the motor exit pressure increases, the motor becomes less susceptible to runaway and acts more like a motor running on liquid only. Temperature The 7/8 2.0 power section configurations are designed specifically for Air Drilling, as they have a longer lead length and looser fit. The standard fit in this configuration can be used in static bottom hole temperature up to 220°F (104°C). 22
  29. 29. Operation Volume Requirements In a standard air drilling application, the required air flow rate (in SCFM) for proper motor operation is typically three to four times the maximum motor liquid flow rate (in GPM). This rule applies for motor exit pressures up to approximately 300 psi. As the motor exit pressure increases, the airflow rate required to achieve the same motor operating speed (rpm) also increases. This is common in some under-balanced applications. In these cases, the required air flow rate increases. Consult NOV Downhole engineering in these cases. If higher flow rates are desired, the rotor can be fitted with a nozzle to bypass a portion of the flow. However, the motor becomes even more sensitive to stall if the rotor bypass is used. Fluid / Lubricant Requirements A lubricant is required to lubricate the stator elastomer liner. Soap or gel thoroughly mixed with water and injected at an approximate rate of 5% by volume at downhole conditions is adequate for most applications. In formations that could swell and be damaged by water, vegetable based oil is used to lubricate the power section. This is referred to as ‘dusting’ a well. Typically, the flow rate for this case is very low – only 1 to 2 gallons of oil per hour – to avoid making the formation wet and prevent getting stuck. Other drilling conditions may dictate a higher flow rate (i.e. avoiding formation of mud rings). Drilling with Nitrogen (N2) Air itself consists of approximately 78% nitrogen. The density of nitrogen is approximately 3% less than that of air at standard temperature and pressure. Functionally, the motors will run the same on nitrogen as air. However, the explosive decompression, as discussed below, is more severe with nitrogen. Since nitrogen is an inert gas, the Nitrile sealing components within the motor, including the stator, are not affected chemically by nitrogen. However, any sealing compound will absorb nitrogen (as well as other gases) to some extent while under pressure. If the pressure has been applied long enough and the pressure is released too quickly, the gas does not have sufficient time to be expelled from the Nitrile and explosive decompression can occur, resulting in blistering. This is typically not a problem with continuous pressure drops across the motor of 400 psi (2,800 kPa) and less. After drilling for extended periods with nitrogen, stator damage can be expected. For more detailed recommendations, consult an NOV Downhole representative. Operation In general, a motor driven by air, mist, or foam should be started while on bottom. It should not be allowed to run freely before tagging bottom because this can cause high shock loads as the bit tags bottom which may damage the motor. Ideally, the motor should be started with fluid first. A motor is much more torque sensitive when using air, mists, or foams, than with liquids and consequently is more susceptible to stalling. When drilling operations are to be stopped, let the motor drill off as the compressors and boosters are being shut down. Picking up “off bottom” prior to equalizing the pressure can permit the air compressed in the drillstring to expand, which would lead to excessive motor speeds and possibly damaging the motor. It can also cause the internal connections to back off. 23
  30. 30. Motor Handbook, 7th Edition Performance Curves The performance of a mud motor when run in an air drilling application is not as consistent across temperature and pressure ranges as when run with drilling mud due to the compressibility of the air. NOV Downhole Engineering can provide an air drilling performance chart if the following information is provided: • Downhole pressure • Downhole temperature • Anticipated power section differential pressure • Flow rate planned (scfm / m3/s) • Foam quality – percentage of air to liquid (approx) • Outdoor temperature at rig Milling NOV low speed motors provide the high torque required for milling and cutting with tungsten carbide dressed mills and cutter blades. This is an alternative to rotating the drillstring, especially in deep deviated holes where milling can be slow and can cause extensive drillstring and casing wear. Applications include packer milling, drilling cement plugs, casing cutting and junk milling. Coring NOV high torque, low speed motors are ideally suited to coring operations. The motor provides increased penetration rates at lower bit weights achieving better core recovery rates. Lower drillstring and casing wear is experienced, particularly in high angle holes. As a rule no more than 30 feet (9 m) of core barrel should be run at a time. Rat holes, Mouseholes and Spudding NOV motors have applications for drilling rat holes, mouseholes, and spudding. The motor is either made up to the kelly, or made up with two drill collars and a circulating head above it, to add weight and accomplish the tasks faster. The reactive torque of the motor is restrained with the rig tongs. Workover NOV offers 1 11/16” to 4 3/4” diameter workover motors for cost-effective use in severe workover applications inside production tubing strings, for use with coiled tubing units. 24
  31. 31. Operation 5. Operation Run Preparation & Rig Site Testing NOV motors are shipped from the service facility with all components inspected, all new seals and oil (in sealed bearing motors), and all tool connections made up to the proper torque. Motor dynamometer testing is also available at select service facilities. Motors are shipped with the rotor bore plugged unless otherwise specified. All motors are shipped with a protector covering the screw-on stabilizer thread unless a screw-on stabilizer is ordered. When ordered in advance, the screw-on stabilizer is installed at the service center before shipment. If a dump sub is used with the motor, the following check is recommended before running into the hole: • Set the motor in the slips and install a safety clamp. Remove the lift sub and make up the Kelly/top drive. Remove the safety clamp and slips and lower the motor until the dump sub is below the drilling nipple, but visible. • Start the rig pumps slowly; fluid should flow out of the dump sub ports. • Increase the pump rate slowly until the dump sub closes. Leave the pumps running and make note of the circulation rate and stand pipe pressure when the dump sub closes. With the pump running and the dump sub closed, check to ensure that there is no drill fluid leakage through the ports. It is advisable to increase the pump speed in two or three steps, to the maximum circulation rate expected downhole, and note the circulation rate and standpipe pressure in each case. • Shut down the pump. The dump sub may not open due to a pressure lock in the short hydraulic test circuit. If this occurs, bleed off the pressure to permit the dump sub to open. • Make up the drill bit to the proper torque with a bit breaker and the rig tong placed on the output shaft directly above the bit. Do not put rig tongs on the sealed bearing assembly housings. Inspect the output shaft seal area for any indication of an oil leak. NOV motors are shipped from the service center with the adjustable housing set at zero degrees, unless otherwise requested. To set the adjustable housing to a different angle, refer to Section 7 Adjustable Assembly Operating Instructions on page 49. Running In The drillstring with a straight NOV motor installed can be run into the hole normally. When using a bent sub, or a non-zero angle in the adjustable housing, be careful passing the motor through the blowout preventers, casing shoes, liner hangers, ledges, or key seats to ensure that the motor or drill bit does not hang up. Do not run into bottom, or “bottom fill”, as it could plug the bit or damage the motor. 25
  32. 32. Motor Handbook, 7th Edition Warming a Motor for High BHT When running into a hot hole, it is important to gradually warm the motor during its descent. 1. Run in hole and stop at the depth where the expected downhole temperatures are in the range of 240°F to 260°F (115°C to 125°C). 2. and pump drilling fluid to cool the motor. Stop 3. Pump for about three minutes every 400-500ft until you reach bottom. 4. bottom start at around half the de-rated pressure and work up to At the max de-rated pressure over thirty minutes. Refer to the NOV Downhole Temperature Max Pressure De-rating guidelines on page 39 to determine max de-rated pressure. Note: Avoid long periods without circulation if possible. Warming a Motor in Cold Climates Warming up a motor in colder climates may be completed to aid in the makeup of connections to the drill string and bit, and is recommended for increasing reliability of the motor. When performing this process using steam, the motor should be evenly heated to the point it is warm to touch. One option is to run the motor down into the BOP and put the steam hose down in the BOP next to it, and leave it for 10-15 minutes to warm up. Spot heating or heating the motor until it is too hot to touch can cause damage to seals, the stator elastomer, or other components. Circulation should be started slowly and increased gradually to avoid damaging the cold elastomeric liner. Starting the Motor Begin circulating “off bottom” with the bit turning freely. Perform circulation and pressure tests at the same circulation rates as the surface test, and note the readings. The pressure will be higher due to the restrictions of the drillstring components added. The “off bottom” pressures noted may be higher than calculated. This is caused by bit drag on the side of the hole due to the bent sub, adjustable housing angle, and stabilization. Drilling After a short hole-cleaning circulation period, slowly lower the bit to bottom. When bottom is tagged, the standpipe pressure gauge will show an immediate increase. Increase the bit weight slowly to achieve the desired build up rate and/or rate of penetration. Do not exceed the recommended maximum differential pressure across the motor. The “off bottom” pressure is the total system pressure (read on the stand pipe gauge), from the standpipe, through the drillstring, the annulus, and back to the drilling nipple, while circulating with the bit “off bottom” (i.e. zero weight on bit). Periodically recheck the “off bottom” pressure. The standpipe pressure will slowly increase after hole cleaning due to the hydraulic energy required to lift the cuttings. The torque applied to the bit while “on bottom” is directly proportional to the difference between the “on bottom” and “off bottom” pressures (i.e. there are no friction losses through the rotating drillstring). An increase in the weight on bit produces an increase in torque. As the bit drills off, the weight on bit decreases and correspondingly the pressure and torque decrease. The standpipe pressure gauge can therefore be used as a torque indicator. 26
  33. 33. Operation The range of NOV motors permits selection of the correct motor to provide the optimum combination of bit speed, bit torque, and circulation rate for maximum rates of penetration. When the drilling conditions permit, the rotary can be engaged. Running into bottom can damage thrust bearings, and excessive overpull on a stuck bit can damage “off bottom” bearings in the sealed bearing assembly. NOV motors are designed for extended intervals of “on bottom” drilling. Sealed bearing motors should be serviced after 150 operating hours, in standard drilling conditions. Drilling conditions other than standard, such as high WOB, high solids/sand content, corrosive drilling fluids or rotating with high bend settings will reduce this interval accordingly. Reactive Torque The drill bit attached to the mud motor at the bit box turns in a right hand, or clockwise direction, if viewed from the drill floor. There is a reactive, counterclockwise (or left-hand) torque produced as a result of the torque applied to the bit. This counterclockwise torque must be considered when establishing tool face orientation and can be calculated by relating the difference between “off bottom” free spinning pressure; and actual “on bottom” operating pressure. An estimate of the angle of twist of the drillstring can be performed as follows: 1. Measure and record standpipe pressure “on bottom” with the desired bit weight and circulation rate and “off bottom” with the same circulation rate. 2. the Motor Specifications and Performance Sheets, obtain the From torque corresponding to the difference between the “on bottom” and “off bottom” pressures. 3. The torsional angle can be determined by multiplying the above torque, the length of drillpipe in the hole and the angle values from the table below. Values are based on torsional deflection of drillpipe. Angle of Twist= Reactive Torque × DP Length × Angle Value Table 3: Torsional Deflection Angle Values for Drillpipe Drillpipe Torsional Angle Values US Metric 8.0° / 100 lb-ft / 1000 ft 19.3° / 100 N-m / 1000 m 3 1/2” – 15.5 lb/ft 7.0° / 100 lb-ft / 1000 ft 17.0° / 100 N-m / 1000 m 4 1/2” – 16.6 lb/ft 3.7° / 100 lb-ft / 1000 ft 9.0° / 100 N-m / 1000 m 5” – 19.5 lb/ft 2.5° / 100 lb-ft / 1000 ft 6.1° / 100 N-m / 1000 m 3 1/2” – 13.3 lb/ft Directional drillers can get an indication of the reactive torque from the measurement while drilling (MWD) equipment and can adjust and lock the rotary table in order to accommodate the desired tool face direction. In most applications the use of measurement while drilling or steering tools to provide real time surface readout of azimuth, inclination and tool face is much more reliable than the single shot method of orienting. 27
  34. 34. Motor Handbook, 7th Edition Stalling If too much WOB is applied, the torque required to keep the bit turning creates a higher differential pressure than the seal between the rotor and stator elastomer can maintain. The drilling fluid breaks the seal and leaks through the power section without turning the rotor, so bit ceases rotation, or ‘stalls’. An increase in standpipe pressure will occur and penetration will cease. As the fluid leaks past, it erodes the elastomeric liner, which makes further stalling more likely and damages the liner, eventually leading to chunking. Also, stalling generates large pressure pulses, creating torque spikes that can cause chunking, connection back-off, or fracture of driveline components. Motor stall should be avoided, but when it occurs, it should be quickly remedied. If the bit is picked up off-bottom while drilling, the “trapped” torque within the drillstring will be released uncontrollably, potentially causing damage to down-hole components or causing connections to back-off. This is especially true when a stall has occurred. If a stall condition occurs the following procedure should be followed as soon as possible: 1. Stop rotation of the drillstring immediately 2. Shut down the pumps if possible; if not, slow down the pumps as much as possible 3. Release trapped torque slowly using the rotary table brake. 4. Lift the bit off bottom Over-Running the Bit Rotating the drillstring with any positive displacement motor in a stalled condition may cause the upper portion of the motor (and drillstring) to over-run the bit. This condition can damage the stator elastomer liner and cause connection back-offs within the motor. Vibration Although BHA and motor component fatigue can occur over an extended period, NOV analysis has confirmed that many motor fractures occur as the result of vibration induced fatigue over a single run. As such, it is important to mitigate vibration in order to avoid related downhole motor failures and unscheduled downtime or costly fishing trips. There are 3 modes of vibration that can occur when drilling. • Torsional Vibration – Stick Slip • Lateral Vibration – Bit and BHA Whirl • Axial vibration – Bit Bounce Figure 5: Downhole Vibration Modes 28
  35. 35. Operation In excessive quantity, these vibrations can considerably reduce drilling performance and in extreme cases, can cause significant damage to the bit, motor and other downhole components. There is a complex combination of factors that can have an effect on downhole vibration. NOV provides tools such as the BlackBox® Dynamic Data Recorder, MWD systems, and VibraSCOPE™ Drillstring Dynamics Modeling to help reduce vibration by optimizing the drilling system. Torsional Vibration (Stick Slip) Torsional vibration is the cyclical rotational acceleration and deceleration of the bit, BHA, or drill string. If the torsional vibration is severe, stick slip can occur. Stick slip is the momentary stop in rotation of the bit or BHA that can cause the drill string to instantly torque up then release, accelerating the BHA to dangerously high speeds (often 2-3 times rotary speed, but can go as high as 15 times). This is also known as microstalling. Heat checking on the bit is usually an indication that stick slip has occurred. Torsional vibration and stick slip may occur downhole even when a constant RPM is input at the surface. Causes • Incorrect bit selection – overly aggressive PDC bit • Improper BHA stabilization – Undersize stabilizers • Excessive bend in mud motors • Tortuous hole geometries, high dogleg angles • High WOB with low RPM • Insufficient mass of drill collars • Insufficient stiffness in DP and BHA • Poor drilling practices • Lack of lubrication in drilling mud • Lithology – interbedded formations, formation interfaces, hard and/or abrasive stringers, formations with high friction coefficients. • Reaming/back-reaming, hole opening, drilling out casing, control drilling Symptoms • Large & erratic surface RPM & torque fluctuations, especially noticeable on a top drive • BHA/Rotary table stalling • Whirring sound from top drive • Cutter/insert damage; bit/stabilizers wearing undergauge • Shoulder damage to bits with heat checking, dull characteristics. Nose ringout on larger diameter bits • Poor hole cleaning; under-gauge or washed out hole • Shocks/vibrations measurements from MWD and BlackBox® • Connection fatigue cracks; fractures of BHA components; connection back-off • Fractured or cracked motor drive line components such as bearing mandrels, driveshafts or driveshaft adapters • Thrust bearing cage damage (cage bar deformation or fractures) • Excessive drive assembly wear at articulating engagement points 29
  36. 36. Motor Handbook, 7th Edition Solutions • Reduce WOB and increase RPM • Increase weight and stiffness of BHA (Inclination <60°) • Select NOV PDC bit that includes Torque Control Components (TCC). The TCCs control the depth of cut and help prevent torsional vibration • Run a VibraSCOPE™ of the BHA and identify stable RPMs • Measure torsional vibration with the BlackBox® recorder and calculate optimum RPM ranges for each formation • Increase the diameter of the drill pipe to allow for more efficient torque transfer to the bit and BHA • Improve the tortuosity of the borehole by minimizing the motor bend setting and sliding over longer intervals • Use stabilization or roller reamers to improve borehole quality • Improve the lubrication qualities of the drilling fluid Lateral Vibration (Bit/BHA Whirl) Lateral vibration is caused by the high bending stresses in the BHA and drillstring resulting in the assembly impacting on the borehole wall. Lateral vibration can be extremely damaging to all drillstring components and result in energy being removed from the system that would otherwise be used to drill ahead. High lateral vibration will result in a reduction in penetration rate and premature bit and BHA failure including motor housing fractures. Causes • Harmonic resonance of drill string • Excessive RPM; Stick slip • High tortuosity of the wellbore • Formations with high coefficients of friction and restitution • Lack of BHA stabilization • Incorrect parameter selection • Excessively heavy BHA in a high angle hole • Low WOB with high RPM • Lack of lubrication in mud • Reaming/back-reaming, hole opening, drilling out of casing • Incorrect bit selection Symptoms • Poor penetration rates and higher MSE than expected for that formation • Damage to the bit is probably in the shoulder or randomly scattered across the body. Dull characteristics will be chipped and broken or missing cutters. • Sensitive electronic tools may be damaged or destroyed • Connection fatigue cracks or fractures on BHA components • Real-time lateral vibration data from various MWD tools • Rotary torque fluctuations • Rapid eccentric (during forward whirl) or concentric (during backward whirl) BHA component OD wear 30
  37. 37. Operation Solutions • Pick up off bottom and hold string stationary until all energy is released (typically a couple minutes). Go back on bottom with reduced RPM and higher WOB. • Increase WOB and reduce RPM and try to confirm with downhole measurements. • Run a VibraSCOPE™ of the BHA and identify stable RPM • Measure lateral vibration with the BlackBox® tool and calculate optimum RPM/WOB ranges for each formation. • Increase the diameter of the drill pipe to allow for more efficient torque and weight transfer to the bit and BHA • Improve the tortuosity of the borehole by minimizing the motor bend setting and sliding over longer intervals • Improve the lubrication qualities of the drilling fluid • Use stabilization or roller reamers to improve borehole quality • Rotate the drill string at a Lower RPM and higher WOB • Bit selection to include lateral mitigation features include design with high Lateral Stability Index (LSI) and SteeringWheel® Gauge technology Axial Vibration (Bit Bounce) Axial vibration is caused by a cyclical loading and unloading of the bit and the BHA in the axial direction. It is also often referred to as “bit bounce”; however be very clear that it rarely results in the bit leaving the bottom of the hole. It normally results in a rapid cyclical movement of the neutral point in the BHA that causes the WOB to rapidly increase and decrease. Extreme torsional vibration can also contribute to axial vibration resulting in the cyclical shortening and lengthening of the drill string. Axial vibration is normally associated with (but not limited) drilling with large diameter roller cone bits in hard formations. Causes • Axial harmonic resonance of drill string. • Excessive RPM • Hard formations with high compressive strengths. (Hard sandstones, hard limestones, quartzites, conglomerates & igneous rocks, etc.) • Incorrect parameter selection. Parameter selected gives the BHA a propensity to go into axial harmonic resonance • Drilling with large diameter roller cone bits • Excessive WOB with high RPM Symptoms • Large WOB fluctuations (shaking hoisting equipment) • Poor penetration rates and higher MSE than expected for that formation • Damage to the bit demonstrates impact damage characteristics, broken cutters on all cones particularly the outer rows. Internal inspection of bearings may find Brinell marks. • Run NOV Downhole BlackBox® HD tool and measure axial accelerations in the drill bit. • Real-time axial vibration data from various MWD tools • Damaged BHA components • Sensitive electronic tools may be damaged or destroyed 31
  38. 38. Motor Handbook, 7th Edition Solutions • Change RPM and WOB combinations to try and get a stable drilling situation and the MWD axial sensor informs you that the vibration has gone away. • Run a VibraSCOPE™ of the BHA and identify stable RPM zones with a low risk of axial vibration. • Measure axial vibration with the BlackBox® HD tool and calculate optimum RPM/WOB ranges for each formation drilled. • Change drill bit from Roller Cone to PDC. • Replace rotary BHA with a motor BHA that generates high ROPs with less WOB and higher RPM. Rotary RPM While rotating the drill string can provide benefits such as improved hole cleaning and reduced drag, rotating a bent motor creates high bending stresses in the motor housings, and can eventually lead to fatigue fracture. This risk grows as the hole curvature increases and higher bend settings are used. Each motor configuration shown in SECTION IV: MOTOR PERFORMANCE DATA displays a chart of “Maximum Adjustable Bend Settings for Rotary Drilling”. This chart shows the maximum bend setting, in degrees, for continuous rotation of the drill string at various hole curvatures. Note that the practice of alternating between rotating and sliding through a curve will create micro-doglegs that are higher than what may be shown on a survey report. These limits apply to ideal hole conditions with smooth drilling operations. Challenging conditions such as poor hole quality, hard formations, high WOB, vibration, stick-slip, and stalling, may necessitate further reduction in the bend setting used, to prevent motor housing fracture. Rotating the drillstring while subjected to bending loads produces fatigue loading on the motor. These bending loads can be produced even when using adjustable bend settings that are within the recommended values. Note: NOV recommends a maximum bend setting of 1.83° for rotary drilling. The maximum recommended drillstring rotational speed for NOV motors is 50 RPM. This 50 RPM limit helps extend the life of the tool by limiting fatigue cycles and helping to avoid excessive vibration and whirl. Rotary speed should be decreased as necessary based on local conditions. It may be feasible to exceed the 50 RPM limit if using an MWD system to monitor vibrations and collar RPM in real-time at the rig, so downhole vibrations can be mitigated. NOV can provide evaluation of critical RPM speeds for BHA configurations using our custom VibraSCOPE™ analysis program. Motor Pressure Drop Exceeding the maximum recommended differential pressure across the motor will reduce the stator life. Circulation rates exceeding the recommended values also reduce the rotor and stator life. To understand the conditions affecting the pressure drop across a power section, the following should be recognized: 32
  39. 39. Operation • The pressure drops across the top/dump sub, adjustable assembly, driveshaft assembly and bearing mandrel of the bearing assembly are dependent on the circulation rate only (i.e. torque has no effect). This pressure drop increases as the circulation rate increases. This “no load” pressure drop across the motor at a specific flow rate is shown for each motor configuration in SECTION IV: MOTOR PERFORMANCE DATA • The pressure drop across the power section increases linearly as the torque increases, assuming a constant circulation rate. • The effect of mud weight alters the pressure drop across the motor at no load. However, it has a negligible effect on the pressure increase due to the torque “on bottom”. Bit Pressure Drop Continuous excessive pressure drops across the bit can cause early seal failure in sealed bearing motors. The bit pressure drop should be limited to 1,500 psi (10,000 kPa) for continuous drilling. The ‘ML3’ mud-lubricated motors, and ‘ST1’ and ‘ST2’ sealed bearing motors incorporate the use of flow restrictors, which bypass some amount of flow away from the bit. The percentage of flow that bypasses through the flow restrictors will vary between applications, as it is dependent on several factors: • Mud weight and viscosity • Bit pressure drop • Clearance between flow restrictors Note that other tools run below the motor which create additional pressure drop will effectively increase the “bit pressure drop” experienced by the motor, so will cause more flow to bypass through the flow restrictors. These pressures must be added together to obtain a total “bit pressure drop” to consider when calculating restrictor bypass flow. The ‘ML3’ mud-lubricated motors require a certain amount of flow through the bearings to provide lubrication. The amount of flow that passes through the bearings will depend on the bit pressure drop – bits run with a Total Flow Area (TFA) too large will not create sufficient back pressure to force the required flow through the bearings. The result can be seized bearings, from overheating due to lack of lubrication. Contact NOV Downhole for guidelines for maximum TFA to provide sufficient lubrication. Power Section Fits The power section elastomer may swell or shrink over the course of the job, depending on the mud type and downhole environment. This affects the fit between the rotor and stator in the power section, either making the fit tighter or looser. An ideal fit is essential to maximize elastomer life and to achieve optimum drilling performance from the power section. If the fit is too tight, excessive heat will build up in the elastomer due to friction which decreases elastomer strength and in extreme cases, lead to chunking of the elastomer. If the fit is too loose, excessive slippage (leaking across cavities in the power section) will result which leads to decreased RPM and torque at increased pressures. This is referred to as a weak motor. Fit is calculated using the following formula: Fit = (Rotor Major Diameter – Lobe Height) – Stator Minor Diameter 33
  40. 40. Motor Handbook, 7th Edition LOBE HEIGHT R TO Ø RO JOR A M R MA OTO JO R R Ø ROTOR MAJOR Ø ROTOR MAJOR Ø - LOBE HEIGHT - LOBE HEIGHT STATOR MINOR DIAMETER Figure 6: Power Section Fit Typically, there is one rotor profile size for a given power section model and fit is adjusted with different stator elastomer profiles. There are 4 general categories of fits: • Undersize (US)… provides the tightest fit (most interference) • Standard (STD) • Oversize (OS) • Double Oversize (2xOS)… provides the loosest fit (most clearance) Factors that can affect fit for a power section are: • Type of Drilling Fluid – Oil Based Mud, Water Based mud, Air/gas (See Table 4: Basic Chemical Compatibility Chart on page 36.) • Elastomer Type – NBR, HNBR, PowerPLUS™ • Bottom Hole Temperature (See Downhole Temperature on page 39.) • Abrasive Mud – High solids or sharp particles in mud wear the elastomer causing a looser fit. HEMIDRIL even wall power sections are typically only required to have a Standard fit option. Because of the even thickness of the elastomer, significant deformation of the elastomer is less prominent making fits other than standard unnecessary. Drilling Fluids Proper use of drilling fluids (a.k.a. drilling muds) is an essential part of the overall drilling process. The main functions of a drilling fluid include: • Removing drilled cuttings from the borehole • Transmitting hydraulic pressure • Suspending cuttings and LCM when not drilling • Controlling formation pressures • Promoting wellbore stability • Cooling and lubricating bit and BHA • Preventing/limiting corrosion A large variety of additives are used to achieve these different functions plus have many other specific applications. While drilling fluids can vary greatly in purpose and composition, they typically fall into one of the following types: • Air/Gas (See Air or 2-Phase Drilling on page 21) • Water based mud (WBM) • Oil based mud (OBM) • Synthetic based mud (SBM) 34
  41. 41. Operation Drilling fluids with a pH below 4 or above 10 can cause damage to the stator elastomer and seriously attack the plated components. Circulation through the rotor and stator can minimize this damage and should therefore be maintained when operating in drilling fluids close to the limits of this pH range. Allowing the drilling fluid to stagnate will aggravate the problem. The motor should be flushed and serviced as soon as possible – refer to Rig Site Maintenance on page 42. Drilling mud with a density of more than 16.7 PPG (2.00 kg/L) will cause abnormal erosion of motor internals due to suspended materials within the mud. Silicate mud is also known to be extremely abrasive. Well-mixed medium to fine lost circulation material (LCM) can be used without plugging or motor damage. If coarse lost circulation material is to be used, a circulating sub should be installed above the motor assembly to bypass the motor. A good rule of thumb is no more than 2 1/2 lbs (1.13 kg) of LCM per barrel. Abrasives in the drilling mud will reduce the life of the motor in several ways: • Erosion of the stator liner leading to more leakage and higher likelihood of stalling, • Erosion of the rotor coating leading to damage of the stator liner • Erosion of driveline components leading to component fracture • Seal damage in the bearing assembly leading to mud invasion and bearing seizure Generally, high gravity solids are not highly abrasive and are required for pressure control in the wellbore, and therefore NOV does not specify an upper limit for HGS. It is recommended that the content of potentially abrasive low gravity solids (LGS) be minimized (below 5% if possible) to extend the life of the mud motor. LGS contents of 7% or below for lower mud weights and to 6% or below for higher mud weights and higher temperatures are standard recommendations by NOV Fluids Services to extend the life of the mud pump liners. Rock types that are potentially abrasive are: Sandstone, Siltstone, Conglomerate, Chert, Granite, Tuff, and Gneiss. It is recommended that sand content be maintained at 1% or below. Run times may need to be shortened to avoid downhole motor failure if the above recommendations are not followed. Never try to cement through a motor as the motor and bit-jets shear the cement and the cement will immediately flash set. Oil Based Drilling Fluids The aniline point of an oil-based mud is an indication of its tendency to cause swelling of elastomeric parts (e.g. stator, seals), and is a measure of the oil’s aromatic content. The swelling tendency increases with a lower aniline point. The aniline point gives a measure of the solvent power of a petroleum product for aniline, which is related to its solvent power for many materials. This solubility increases with increasing temperature. Note that the aniline point may vary significantly between suppliers of the same product, for example, diesel. The lower the aniline point, the more readily the oil will permeate and swell the stator elastomer liner and reduce its hardness and strength. The swelling increases the interference between the rotor and stator and results in heat build-up that leads to rapid destruction of the stator elastomer liner. 35
  42. 42. Motor Handbook, 7th Edition NOV has seen higher frequencies of stator chunking incidents when run in drilling mud with aniline point below 140°F (60°C). Elastomeric compounds seem to perform better in mineral based mud systems than in diesel oil based systems. Low toxicity oil base systems are easier on elastomers because they contain fewer aromatics. Also Nitrile elastomers with high acrylonitrile content (ACN≥39%) have a lower swelling in low aniline point fluids and therefore are the elastomers of choice in such applications (NOV’s Standard HNBR and PowerPLUS NBR elastomers meet this criteria). Power sections with larger clearances are available to minimize the effects of swelling. Table 4: Basic Chemical Compatibility Chart Elastomer Description (Monoflo® Elastomer Code) Standard NBR (RR) Continuous BHT Standard HNBR (OC) PowerPLUS NBR (PRR) HEMIDRIL (HD) Water Based Mud Below 220°F (Below 104°C) Preferred Std Fit 220°-280°F (104°-138°C) 280°-320°F (138°-160°C) Preferred OS Fit Preferred 2xOS Fit Continuous BHT Not Preferred Not Preferred Not Preferred Preferred Std Fit Preferred OS Fit Preferred 2xOS Fit Preferred Preferred Preferred Oil Based Mud Below 220°F (Below 104°C) 220°-280°F (104°-138°C) 280°-320°F (138°-160°C) Preferred Std Fit Preferred OS Fit Caution 2xOS Fit Acceptable Std Fit Acceptable OS Fit Preferred 2xOS Fit Preferred Std Fit Preferred OS Fit Caution 2xOS Fit Above 320°F (Above 160°C) Not Preferred Caution 2xOS Fit Not Preferred Preferred Preferred Preferred Caution OBM Run Practices Due to the varied molecular weights of hydrocarbons in many oil-based drilling fluid systems, it is not recommended that components that have elastomeric components be used for more than one downhole trip. Lower molecular weight hydrocarbons have a higher propensity to penetrate into the elastomer matrix causing problems not limited to loss in elastomer modulus, accelerated heat aging, changes in elastomer volume, leaching of crucial elastomer ingredients, and rubber-to-metal bond degradation. While some elastomers are more resistant to the differences in drilling fluids, the uncontrollable variance of oils, diesel, and other organic fluid additives make the predictable run life of the elastomeric elements impossible to determine. Therefore, whenever possible it is not recommended to subject components that have elastomeric elements for more than one downhole trip, independent of duration, temperature, loading, or any other factors that would typically affect an elastomer. H2S (Sour) Service NOV motors are manufactured from steels of hardness in excess of that allowed by the National Association of Corrosion Engineers (NACE) Specification MR0175. This hardness is required to achieve the strength necessary for use in the drillstring, but it renders it susceptible to sulfide stress cracking (SSC). As a consequence the drilling environment should be controlled if these tools are to be used in sour environments. 36
  43. 43. Operation The drilling environment may be controlled using one or more of the following (Reference NACE MR0175): 1. Maintenance of the drilling fluid hydrostatic head to minimize formation fluid in-flow. 2. Use of chemical sulfide scavengers. 3. Use of a drilling fluid in which oil is the continuous phase. NOTE: Low aniline point oil based drilling fluids can damage stator liner and cause premature failure. The following specifications provide recommendations for drilling sour wells and for the control of the drilling environment: 1. American Petroleum Institute (API) Recommended Practice The RP7G Section 9. 2. The National Association of Corrosion Engineers (NACE) Specification MR0175, “Sulfide Stress Cracking Resistant Metallic Materials for Oilfield Equipment”. 3. Alberta Recommended Practices for Drilling Critical Sour Wells. The time that a motor is exposed to H2S in a typical drilling application is insufficient for H2S to cause damage to the elastomers (e.g. stator liner)within the motor. The Effects of Chlorides on Power Sections Drilling fluids containing chlorides can reduce rotor and stator life due to corrosion, especially at elevated temperatures. Special attention should be paid to the internal coatings when the chloride concentration is in excess of 30,000 PPM. The motor should be flushed and serviced as soon as possible if it has been exposed to chlorides – refer to Rig Site Maintenance on page 42. Drilling fluids are blends of several chemicals in water or oil based media. Some of these ingredients can negatively impact the performance of an elastomer compound. KCl, a common ingredient in water based mud (WBM), is used as a source of potassium as a Base Exchange ion to stabilize drilled shales. Published test data supports the belief that KCl does not have any detrimental effects on the elastomers used in power section stators; however, this is not always true. When changes in mechanical properties of an elastomer occur as a result of exposure to a fluid containing chlorides, the most likely catalyst is not the chloride, but one or more of the ingredients within the fluid. Although the impact of chlorides to the mechanical properties of elastomers is minimal, high chloride water based fluids can permeate through the elastomer, especially high acrylonitrile base polymers. Fluid that permeates through the elastomer to the base metal is able to interact with the adhesion bond between the elastomer and metal tube. This can result in leaching of materials from the bond system and corrosion of the substrate material, resulting in a progressive loss of adhesion strength. This bond hydrolysis phenomenon is accelerated when high temperatures (250°F [121°C] and above) are involved. A weakened bond system can allow the elastomer to separate from the tube under normal loading conditions, leading to premature stator failure. When possible, use an alternate K donor such as potassium acetate, potassium carbonate, potassium lignite, or potassium hydroxide for drilling fluids that can negatively affect the stator elastomer. If an alteration to the drilling fluid is not possible, it is recommended to use PowerPLUS™ with the appropriate fit selection based upon temperature and fluid interaction for applications using a WBM with high chloride content. 37

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