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aper No.8 5 CORROSION97

ELASTOMERS IN MUD MOTORS FOR OIL FIELD APPLICATIONS

Hendrik JohnBaker Hughes INTEQ GmbH

Christensenstrasse 1Celle, 29221, Germany

ABSTRACTMud motore the most frequently used downhole drilling motors in modern drilling systems are described in

their application and function.The elastomeric liner in a mud motor acta as a huge continuous seal. Important properties of elastomers

such as chemical resistance, fatigue resistance, mechanical strength, abrasion resistance, bonding to steel andprocessability are discussed. Advantages and disadvantages of NBR, HNBR, FKM, TFEP, and EPDM elastomersfor mud motor application are briefly described. The importance of drilling fluids and their physical and chemicalimpact on motor elastomers are described. Drilling fluids are categorized in: oil based-, synthetic-, and waterbased. Results of compatibility tests in the different drilling muds of the presented categories demonstrate thecomplexity of elastomer development. Elastomers with an equally good performance in all drilling muds are notavailable. Future developments and improvements are directed towards higher chemical resistance at higherservice temperature. This will be possible only with improved elastomer -to-metal bondhg, increased mechanicaland better dynamic properties,Keywords: mud motor, oilfield, elastomer, chemical resistance, mechanical properties, NBR, HNBR, FKM,TFEP, EPDM, drilling fluids, oil based mud, synthetic based mud, water based mud, elastomer-to-metal bonding

INTRODUCTIONMud motors are the most important direct drive units for directional drilling, but are also usad for

performance drilling in the oil field. They are connected directly to the drill bit.

Copyright01997by NACE International. Requests for permission to pubhsh this manuscr ipt in any form, in part or in whole must be made in writing to NACEInternational, Conferences Division, P.O. Box 218340, Houston, Texas 77218-8340. The material presented and the views expressed in thispaper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.al anasundaram - Invoice INV-416942-ZR23KF downloaded on 2/23/2011 11:58:17 AM - Sin le-user license onl co in and networkin rohibited

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As a functional component of a mud motor the elastomer lining is the crucial component which decideeover the succese of a motor run. In Figure 1 the poeition of a mud motor in a drill string can be seen.

The reasons for elastomers being a key factor for the success of mud motors are manifold:. power of motors has doubled in the last 5 yeare. new synthetic organic carrier fluids for drilling muds appeared in the last 5 to 10 years)q reliability and life expectancy has increaeed in the last years. number of applications are increasing rapidly for mud motorsq new applications are steadily emerging:

- deeper wells (higher temperatures)- geothermal applications (high temperatures)- extended reach

Thus the requirements on elastomers have increased very rapidly from different directions: mechanically,chemically, and thermally. A continuous effort in elaetomer development is therefore necessary to keep up with allnew demands on mud motors.

In the following sections a brief discussion of important aspects for the development of elastomers fordownhole mud motors is made with a special focus on the interaction of the various drilling fluids with theelastomers,Function Of Elastomers In Mud Motors

The mud motor can be regarded as a power unit which converts the hydraulic power of the circulateddrilling fluid into mechanical power or rotation of the rotor. The operating principle is the reverse of the MOINEAUpump principle. In Figure 2 the four basic sub assemblies of a mud motor are shown.

. Bypasa valve assembly. motor section with atator and rotor. universal joint assembly

. bearing aesembly with drive shaft

The motor consists of two main components: atator and rotor. In the simplest geometry, the rotor is a steelshaft having a helical geometry with a round cross-section turning eccentrically in a double helically shaped stator.Stator and rotor are forming a series of cavities from the top, the inflow side, to the bottom, the outflow side.Figure 3 shows a section through a mud motor visualizing the open and closed -sealed- cavitiea formed by thestator and rotor and the flow of the drilling mud. The drilling fluid passes the bypass valve with high preaaure andenters the open cavities of the motor. The energy transferred from the fluid to the rotor forces the rotor to turn at acertain speed and causes a pressure drop of e.g. 80 bar (1161) psi

The elastomers in mud motors can be regarded as a continuos seal, sealing off a series of helically woundchambera along the motor axis.

The functioning of this gigantic sealing area makes the elastomeric lining the inevitable functional com-ponent of a mud motor.

During the revolution of the rotor, the rubber is deformed cyclically with a deformation frequency of about5 to 35 Hz (depending on the motor type). The etrain of the elastomer during service can be seen in a crosssection of a motor in Figure 4.

Due to the hysteresis of the elastomer, the strain causes a heat build-up in the elastomer. In addition tothe ambient temperature, given by the circulation temperature of the mud, T., the temperature in the elaetomer,

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T,,,, increases due to the heat build-up, ATHBU, and requirea thus a clearly higher temperature resistance of theelastomer.

T,l = T + ATH,U (1)The stator elastomer is thus subjected to a complex stress regime: dynamic mechanical loads, tempera-

ture, and chemical impact from the drilling mud are superimposed, This makes the use of elastomers in mudmotors quite unique.Properties Of Elastomers For Mud Motors

Due to this combination of forces in varying fluids and temperatures, the elastomers for mud motors haveto possess several functional properties which can be named in groups.

q chemical resistance. fatigue strength. mechanical strength (tensile and tear)q abrasion resistance. bonding to Steel. processabilityThese properties have to be leveled out in order to ettein a compromise for a satisfactory working

elastomer, Figure 5. The target for the formulation of a satisfacto~ working elastomer lies in the overlapping area.Elastomers are being developed for general use or special applications, depending on the mud systems

used, the temperatures anticipated, and the special drilling requirements. In general the focus is on the chemicalresistance and the dynamic properties.

In Table 1a comparison of different elastomers used in the oilfield is shown.From the perspective of usage elastomers on the basis of Nitrile (NBR) are by far the most widely used.

Another elastomer also used to a notable extent is hydrogenated Nitrile (HNBR), Fluorocarbon elastomers (FKM)or tetrafluoroethey lene propylena are used in highly aggressive environments. Ethylene propylene dieneelastomer (EPDM) are limited to special applications because they are not oil resistant.

The ~ of elastomers depends on the chemical nature of the fluid and the temperature atexposure, In addition there is a time effect with a consequence that in an aggressive fluid, at maximum servicetemperature, the life time is reduced substantially. Due to the very low oxygen levels in drilling fluids, oxygen playsno role in the degradation of elastomers in mud motors. Therefore the temperature limits for certain elastomers asstated in the literature 2} cannot be transferred to this application. The lhermal resis- correlates as anapproximation with the chemical resistance. Fluoropolymers have in general an excellent stability in nonpolar fluidseven at high temperatures; the resistance to polar fluids is less good at high temperatures. The stability oftetrafluoroethylene propylene is better in polar fluids than in nonpolar fluids compared to FKM.

NBR and HNBR have a good or satisfactory resistance to non polar fluids and also polar fluids. EPDM hasa poor resistance to non polar fluids but an excellent resistance to polar fluids. In general it can be said that noneof the elastomers performs well in all types of fluids.

As a favorable factor for the chemical resistance of elastomers in mud motors the relatively short exposuretimes can be seen. When compered to exposure timee in the automotive industry there is a factor of 10 inbetween.

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As a first result of these processes a volume change of the elastomer occurs. For the mud motor thismeans the rotor/stator fit -geometry- is altered. In most cases the fluid causes a volume increase - swelling- of theelastomer (when the absorption is greater than the extraction) with the consequence of a tighter rotor/stator fit. Insome fluids a volume decrease - shrinkage- takes place (when the extraction is greater than the absorption). Thisleads to a looser rotor/stator fit. If gaaes are absorbed, e.g. Nitrogen in underbalanced drilling, due to gas uptakeat high pressures, blistera and/or cracks can be the result.

A second result is a change in the mechanical properties. This is because any volume change is linked toa change in mechanical properties as elasticity, hardness, tensile strength etc. In reality this means a degradationof the elastomeric properties will finally lead to cracks or fractures in the elastomer.

A further physical effect which is always inherent to the impact of a fluid, and is separated here only for thesake of clarity, is caused by the temperature. The pure temperature effect generates also a tighter rotor/stator fitand alters the characteristic mechanical properties of the elastomer significantly. The change of the strength andthe elongation is shown for some elastomers in Figures 7 and 8,

Chemical Fffect$.. The chemical effects are related to chemical reactions of the fluid with the polymerresulting in a change of the polymer network. These reactions are essentially

a) scission of bonds, andb) crosslinking

The elastomer responds to the chemical interaction with the fluid by a deterioration of the physical properties, e. g.reduction of strength, elongation and hardness, In most cases a reduction of hardness takes place with theconsequence of an increased wear rate. Temperature acts as an accelerator. As for chemical reactions in general,the rate approximately doubles for a 10 C temperature rise.

Mud C~ ,..~11 data are based on a exposure of rubber samples with a diameter of 1.44 and 0.079

thickness in autoclaves for a period of 3 days. The general test- and evaluation procedures were in accordancewith ASTM D471 and DIN 53519. Volume and hardness chsnges were determined, the hardness as InternationalRubber Hardness Degree (IRHD).

E@ulis, In thefollowingsectionresultswillbe presentedfor thecompatibilityof differentelastomerswithfielddrillingfluidsbelongingto thedifferentcategories:oilbased-,syntheticbased-,andwaterbasedmud.Inorderto limittheamountofdatapresentedforcharacterizationof the interactionof theelastomerwiththedrillingmud,only volume- and hardness change are taken as indicators.

Oil Based Mud, Figure 8 shows the results for the volume change of NBR, HNBR, and FKM elastomers ina typical Mineral oil based drilling mud (oil/water ratio of 80:20 ),

In this figure the intense reaction of the NBWI and HNBRfl elastomers can be seen. In contrast the FKM/1exhibits the anticipated low reaction. In Figure 9 the hsrdness change (IRHD) reflects the relstive strong reactionof HNBR/1, NBR/1 shows at comparable temperatures a slightly lower reaction, and at 200 C shows FKM/1 also asignificant hardness change although the volume change is still relatively low.

Figure 10 shows the volume change of NBR and HNBR elastomers in field mud systems in comparison tothe standardized high swelling test fluid IRM 903 at 125 C (257 F)

Drilling fluids containing diesel are among the most aggressive, only crude oil muds are surpassing dieselin some cases. The very high hardness change caused by a diesel mud is shown in Frgure 11.Figures 12 and 13 show the reaction of NBR, HNBR and FKM in a synthetic -ester-

based mud (oil/water ratio 7525) as volume and hardness change over the temperature. Very high volume and


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The special importance of the drilling fluids and their effect on elastomer properties in mud motors wasshown. It could be shown, that some of the new synthetic muds have a better compatibility with mud motorelastomers than oil baae muds. Because of the variety of drilling fluid chemistry, it is not poaaible to developelastomers which perform equally well in all drilling fluids, However, it is possible to develop elastomers whichserve satisfactory under most conditions.

Future developments of elastomers for mud motors are directed to higher chemical resistance, a shift ofthe operation temperature limits towards higher temperatures, and better dynamic properlies in general. For therealization of higher operation temperatures improvements of elastomer-to-metal bonding are mandatoiy. A furthersignificant increase in the dynamic and strength properties will ba of special interest due to further growth in motorpower. Also special developments for underbalanced- and air drilling applications will be of future importance.

ACKNOWLEDGMENTSI would like to thank Baker Hughes INTEQ for the opportunity to publish this work. Special thanks I owe to

Ms. Gabriela Wiese for the performance of the compatibility tests and the preparation of the graphs with thecompatibility data for this publication.

REFERENCES1. R. K. Clark, Journal of Petroleum Technology, 46,9(1994): p. 8042. K. Nagdi, Rubber as an Engineering Material (Munich, Hansa Publishers, 1993): p. 47


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TABLE 1COMPARISON OF ELASTOMER PROPERTIES OF ELASTOMERS FOR MUD MOTORS

++ excellent+ goodo satisfactorypoor

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OTOR

Figure l- Drill String with Mud Motor

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2- Mud Motor with Subassemblies

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Directionof

Rotation

Stetor(Elastomer)

Rotor

Fluid FIOW

UniversalJoint

Figure 3- Section ofa Mud Motor with Fluid Flow

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Figure 4- Elastic Strain of Elastomere ina Multilobe MudMotor

MECHANICAL STRENGTH

Figure 5- FunctionalPropertiesof ElastomersforMudMotors


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22T20

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0 25 50 75 100 125 150 175 200TemperatureC

Figure 6- Tensilestrength versus temperature for different elastomers

600 ..

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50 75 100 125 150 175 200 225 250Temperature C

Figure 8- Volume change versus temperature for different elasto-mers in an oil based mud

Figure 9

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Hardness change versus temperature for different elasto-mers in an oil based mud

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tillOBM/3 OBM/11 Cm& 011 Eliesel MM 903FluidFigure 10- Volume chamze for NBR and HNBR elastomers indifferent oil b~ed fluids at 125C .2

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Figure 11- Hardness change for NBR and HNBR elastomers indifferent oil based fluids at 125C

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Temperature C

Figure 12- Volume change versus temperature for different elasto-mers in a synthetic ester based mud

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] 5a- .10i r ~a2: -15:~: -20L ! ]+NBRil-E- NBRJI1+ FKNW1-25 -HNBM+ HNBRAI-30 50 60 70 80 90 100 110 120 130 140 150Temperature C

Figure 13-Hardness change versus temperature fordifferent elasto-mers in a synthetic ester based mud

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IFluid

Figure 14- Volume change for NBR and HNBR elastomers indifferent synthetic based muds at 125C

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Figure 15- Hardness change for NBR and HNBR elastomers indifferent synthetic based muds at 125C


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+ FKMU-o- HNBPJI+ HNBRflI

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q

50 75 100 125 150 175 200 225 250Temperature C

Figure 16- Volume change versus temperature for different elasto-mers in water based mud

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Figure 17- Hardness change versus temperature for different elasto-mers in water based mud

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10 , I91

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Figure 18- Volume change for NBR and HNBR elastomers in waterbased mud at 125C

Figure 19- Hardness change for NBRandHNBR elastomers in waterbased mud at 125C

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