Elastomer With Mud

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Drilling Fluids - Elastomer Issues

Tom Carlson, Technology Leader Solutions Team for the Americas, Halliburton

Energy Services, Halliburton

This paper was prepared for presentation at the Fall Technical Meeting of the ERG held in Galveston, Texas,

September 20, 2001.

Abstract

Elastomers are used in virtually all aspects of day-to-day drilling operations. Elastomers

are used in the manufacture of components for surface equipment such as blowout

preventors, pump parts and floor mats as well as in downhole equipment such as mud

motors and logging while drilling (LWD) tools. In the oilfield of the 21st Century, drilling

fluids have become more sophisticated and can be formulated using unique base oils or

speciality chemicals that might be incompatible with many of the elastomers currently in

use today.

Elastomer exposure to certain types of drilling fluids can reduce the life of elastomeric

components leading to premature failure and unplanned expense and lost rig time.

Consideration as to the type of elastomers used and the constituents of a drilling fluid

must be considered when putting elastomeric materials in service on drilling rigs today.

This paper will discuss various drilling fluid chemistries and some of the testing done in

the past to evaluate the performance of elastomers in drilling fluids.

Introduction

Drilling rigs and downhole tools used in the drilling process will have various components

installed that contain natural rubber or elastomeric materials. The exposure of natural

rubber and/or specialty elastomers to various types of drilling fluids can in some cases

result in a reduction in service life.

Surface equipment and downhole motors and tools can operate with most types of drilling

or completion fluids. These fluids include:

l Mist/Foams/Air

l Fresh water

l Seawater or brine-based fluids

l Oil-based or "synthetic" fluids

l Water in oil emulsions

Page 1 of 9Drilling Fluids - Elastomer Issues (Paper / Article)

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Water-Based Muds

Drilling fluids are generally based on either water or oil as the continuous phase. The

most common drilling fluids are based on water as the continuous phase. With water

muds, swelling clays (such as bentonite) or non-swelling clays (such as attapulgite, "salt

gel") are added to provide viscosity and suspension properties. Inert solids such as barite

or limestone (calcium carbonate) are added to enhance mud density. Chemical additives

are added to the mud to control various properties such as viscosity, yield point, gel

strength, fluid loss, and to control corrosion. Water based muds include the following:

Native Muds — are produced by using freshwater where it reacts with formation clays.

These muds typically have high solids content and thick filter cake.

Fresh Water-Based Muds — used when drilling non-reactive or compacted formations

and where more inhibitive fluids are not required.

Calcium Muds — Lime or Gypsum treated muds are used to inhibit swelling in dispersive

and reactive clays and shales or where acid gases are expected to be encountered.

KCl/Polymer muds — are used where swelling shales are present and to mitigate the

chance of permeability damage to production zones.

Seawater Muds — are widely used in offshore waters where seawater can be used as

the make up liquid of the fluid. The use of seawater based muds is widespread; they are

usually an adaptation of freshwater based muds with the inclusion of thinners, hardness

removers, and polymeric filtration and viscosity control materials.

Saturated Salt Muds — are suitable for drilling salt domes and massive salt sections,

these are sodium chloride based polymer systems.

Lignosulfonate Muds — are the workhorse of water-based mud; they are used in the

less complex applications.

Silicate Muds - Silicate fluids are used to provide a fluid with extremely inhibitive

properties. In ranking capacity for inhibition, silicates approach the performance of oil

muds for inhibition of reactive shales and clays. Silicate muds can replace oil-based invert

emulsions in some applications. Silicate systems are low-solids polymer systems

formulated in seawater or monovalent brines with the addition of a soluble silicate

complex for inhibition. Silicate muds rapidly seal or, at least, partially seal the pore spaces

in a shale or clay platelet to achieve inhibition. A silicate skin or barrier forms that can be

similar to an invert emulsion mud.

Glycol Enhanced Muds - Glycol-enhanced water-based mud systems (GEM) are

generally salt muds that contain a glycol additive. These mud systems offer the following

advantages:





l Enhanced shale inhibition and reduced dilution rates

l Improved filter cake quality

l Improved lubricity

l Enhanced temperature stability

Operational properties such as torque and drag with the newer systems can be similar to

those experienced with conventional oil-base muds (OBMs).

Non -Aquaeous Fluids (NAF)

In almost all NAF or oil-based muds, oil is the continuous phase; and water is the internal

or discontinuous phase. These muds are commonly termed "emulsions". The oil phase

may be crude oil, diesel oil, or highly refined mineral oils. The water phase contains salt,

usually calcium chloride, to provide the osmotic effect desirable in OBM systems. Most oil

muds use a calcium or sodium, fatty acid soap as the primary emulsifier. Water and

treated bentonite clay provide gel strength and suspension properties while fatty acids

provide emulsion stability. Asphaltic materials may be used for High Pressure High

Temperature (HPHT) filtration control. Oil-based muds are more thermally stable than

water-based muds making them less susceptible to rheological fluctuations at high

temperatures. OBM's may also thin appreciably when heated. Oil -based muds provide a

higher degree of lubricity than water based muds and provide for higher protection

against borehole instability. With OBM, elastomer degradation can occur due to chemical

interaction between elastomer compounds and chemical constituents of the fluid

(primarily the base oil).

Ester, Ether and other Synthetic Base Fluids (SBM)

The OBM's in use today are composed of man-made materials such as vegetable esters,

paraffins and olefins. Esters are biodegradable, environmentally acceptable alternatives to

conventional mineral-oil-based invert emulsion systems (MOBM). The "synthetic" systems

are designed in exactly the same manner as conventional OBM with the main difference

being the replacement of the base diesel or mineral oil with ester, olefin, or other

alternative "synthetic" fluid. Ester systems are temperature stable to 300°F, while olefin

systems can be used for still higher temperatures. These alternative based fluids contain

no hydrocarbon aromatic compounds and, under EPA requirements, must contain less

that 0.001% (10 ppm) Phenanthrene (PAH). EPA Method 1654A is used to test for PAH

concentration. Testing has shown that many of today’s elastomers are not compatible

with these new fluids. Because of this incompatibility, it's recommended that all

elastomers be tested in the new base fluids prior to use.





The first synthetic fluid developed was Baroid's PETROFREE ester-based invert emulsion

system. Because PETROFREE is formulated with a vegetable-based ester as the

continuous or external phase, it's safe to handle and readily biodegradable.

Aniline Point

The aniline point of a drilling fluids has generally been used as an indicator of the

tendency of a drilling fluid to degrade elastomer components. Aniline point is the

temperature at which a specific volume of aniline completely dissolves in a similar volume

of drilling fluid sample. This comparison gives a general indication of the temperature at

which there may be a tendency for some of the drilling fluid chemicals to dissolve into the

elastomer where it can change the elastomers' physical properties (e.g. softening and

swelling). Generally it's recommended that oil-based drilling fluids have the highest

possible aniline point in order to minimize elastomer component degradation tendencies1.

Values of 150°F (66°C) and above are recommended. Ideally, drilling fluid aniline points

should be higher than downhole operating temperatures. Operating components at

temperatures above the aniline point of a specific fluid does not necessarily result in

elastomer-related problems. Aniline point guidelines may not apply to many of the newer

drilling fluid systems being used due to the lack of aromatics in the base fluid.

Development of Synthetic Fluids

Type Fluid Year Base MaterialCarbonUnits

PETROFREE 1989Palm Kernal or Coconut fattyacid

C10-C18

Ethers 1991 Petroleum alcohol Various

Polyalphaolefin 1991 Natural Gas C20-C30

Food Grade Paraffin 1992 Petroleum Various

Detergent Alklates 1992 Petroleum Various

Linear AlphaOlefins

1992 Natural Gas C16-C18

Normal Alkane 1993 Petroleum VariousInternal Olefins 1994 Natural Gas C15-C18

PETROFREE LE

Unique LAO blend1996 Natural Gas C14-C18-C18

PETROFREE SF

Internal Olefin2000 Natural Gas C

16-C

18





Downhole Elastomer/Fluid Compatibility

The effects of drilling fluid chemicals, downhole temperatures, and dynamic mechanical

loading on elastomeric components can be very complex.

Drilling fluid/elastomer compatibility tests are continually undertaken by suppliers to

provide data prior to specific drilling applications and to aid in elastomer component

developments. The testing, which should simulate downhole operating conditions,

conforms to recognized elastomer industry standards and to the specific functional

requirements of elastomer components as utilized in drilling components.

Fluid samples consist of laboratory-prepared whole mud and base fluid samples. Once

aged, the elastomer test samples are inspected and mechanically tested. Changes in

various elastomer properties are assessed by relating the results to un-aged reference

sample values and to prior test data. Performance of the elastomer is related directly to

operating conditions in a specific application. The values generally of most concern with

oilfield elastomers are hardness change, swell, mass change, and volume change.

Downhole Motors Elastomers

Downhole mud motors are designed to

operate reliably under a wide range of

downhole conditions. Nonmetallic

components are native to the motor

design; these components provide efficient

and reliable sealing and/or load bearing

capacity. The elastomeric motor

components include the power unit stator,

transmission unit joint covers, radial

bearing linings, and various proprietary

and special ring seals. All elastomer

materials and bonding agents utilized in

downhole motors are selected to resist

abrasion, erosion, commonly-encountered

downhole temperatures, and circulating

PETROFREE LV

Low viscosity ester

2000Palm Kernal or Coconut fattyacid

Various





fluid chemicals. The successful use of an elastomeric motor component depends on its

ability to maintain efficient and effective sealing/loads on mating components (static or

dynamic) for long periods of time.2

Motor stator elastomers are generally classed as "Nitrile" elastomers. Nitriles offer a

reasonable level of chemical and abrasion resistance while maintaining acceptable

mechanical properties at downhole drilling temperatures. The ingredients of the

elastomers and the processing methods are varied to produce different "molded" stator

elastomer characteristics.

The different characteristics of the elastomers allow for reliable operation of motors in

widely varying drilling conditions. Motor elastomers are generally of three types:

l nitrile butadiene rubber (NBR)

l hydrogenated nitrile butadiene rubber (HNBR)

l highly saturated nitrile butadiene rubber (HSN).

Each elastomer type has its own downhole operating envelope. This envelope is related to

the effect of drilling fluids, downhole temperature, internally generated heat, and

mechanical loading from the rotors. Stator elastomers are selected to provide overlap in

their operating envelopes. Elastomers so selected are referred to as "service" elastomer;

and they are utilized in the majority of motor applications. For some specific applications,

"service" elastomers that have especially enhanced chemical and mechanical

characteristics are utilized.

The response of the elastomers to the downhole environment governs their effectiveness

in sealing and bearing loads as well as their resistance to wear or damage. This response

depends on the elastomer microstructure. No single elastomer is suited for all motor

applications. The limitation is due to the variable combined actions of many changeable

downhole parameters, e.g., laboratory prepared system samples and base fluid samples.

Once aged, the elastomer test samples are inspected and mechanically tested. Changes in

various elastomer properties are assessed by relating to values for un-aged reference

samples, to prior test data, and to prior, similar motor data.2

Modern day mud motors can operate up to and exceed 400 hours of service life. Any

reduction in service life can be costly. The process of reducing service life of downhole

motors can be seen in the graphic below. As the motor elastomer components swell or

enlarge due to chemical incompatibilities with the drilling fluid, the rotors can "chunk" or

break off into small bits or pieces. These bits of material can allow for fluid leakage to





occur that will reduce the performance of the motor and eventually stall the motor with

large chunks plugging the motor. When stall-out occurs, the motor must be pulled out of

the well and replaced with a new motor. In terms of economics, a "trip" to pull and

replace a motor can take up to 12 hours of rig time. On deepwater semi-submersible

drilling rigs, such delays can cost the customer up $200,000 per occurrence. In severe

cases, mud motor failures have been experienced in less than 12 hours of normal

operation due to poor elastomer/fluid compatibility. Premature failure of downhole motor

components should and can be avoided by the proper selection of elastomeric

components prior to the start of the job.

Surface Equipment Elastomers

Elastomers are found in almost every area of a drilling rig. From seals on doors to valves

in tanks to blowout preventor elements, the issue of elastomer compatibility and drilling

fluids is a big issue.

In the early 1990's when the use of Synthetic Based Muds (SBM) began, suppliers of

blowout preventor equipment were being notified that their BOP element service life was

diminished when the elements were put in service where SBM was being used. The

various suppliers of BOP elements began testing to optimize the formulation of the

elastomers to minimize the impact of the various new base oils and esters.

What is deemed to be "compatible" is up to the manufacturer. In some cases changes of

up to 10 - 15% in hardness or swell might be acceptable. In other cases, 5% change may

be considered unacceptable.

In addition to elastomers, other materials were tested for compatibility with synthetic

base oils and whole muds. Materials such as paint, tank coatings, electrical wiring and

cable coatings were tested. In many cases, incompatibilities were seen; and steps were

taken to optimize the various materials.

Surface Elastomer Testing

PART COMPOUNDTEST TEMP &

DURATIONRESULTS

RAM NBR 121C - 504 HRS. COMPATIBLE*

RAM SHAFTPACKING

NBRPOLYMITE

121C - 504 HRS. COMPATIBLE

ELEMENT 112-82 NBR 38C - 504 HRS. COMPATIBLE*

HOSE 240-5B NBR 38C - 504 HRS. COMPATIBLE





Conclusions

When it comes to insuring the performance of downhole or surface equipment that

contains elastomeric components, the only answer is to test for compatibility. Guess work

or making assumptions will only cost more in the long run. Testing can be expensive but

in order to satisfy the demands of the market, it’s an expense that must be factored in to

the cost of doing business. Drilling fluids are changing constantly and will continue to

change as the demands for more environmentally friendly fluid systems increase. What’s

good for the environment might not be the best choice when it comes to being compatible

with rig equipment.

References

1. E. Kubena Jr., K.C. Ross, T. Pugh, J. Huycke (Conoco Inc.): "Performance

Characteristics of Drilling Equipment Elastomers Evaluated in Various Fluids," paper

SPE/IADC 21960 presented at the 1991 SPE/IADC Conference, Amsterdam 11-14 March

1991

2. Sperry Drill Technical Information Handbook. www.myHalliburton.com

Acknowledgement

The author would like to thank the management of Halliburton for permission to publish

this paper.

ELEMENT NBR 121C - 504 HRS. COMPATIBLE*

"O" RING VITON 70 102C - 64 HRS. COMPATIBLE

PRESSUREDIAPHRAGM

- 66C - 168 HRS. COMPATIBLE

STANDARD NBR 85C - 168 HRS. COMPATIBLE

"O" RING NEOPRENE 102C - 64 HRS. COMPATIBLEELEMENT NR 85C - 164 HRS. INCOMPATIBLE

ELEMENT NBR 85C - 164 HRS. COMPATIBLE

ELEMENT HNBR 85C - 164 HRS. COMPATIBLE

ELEMENT XHNBR 85C - 164 HRS. COMPATIBLE

ELEMENT NR M1-37/43 93C - 160 HRS. COMPATIBLE

DONUT NR M1-37/43 93C - 160 HRS. COMPATIBLE





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