Material Selection
In general, oil and gas wells are hostile environments. Consequently, careful
consideration must be given to the materials from which completion componentsare manufactur. A wide variety of materials, with a range of physical properties,
have been developed specifically for use in downhole completion components.
In severe cases, it may be necessary, or cost effective, to incorporate a system
which resists the harmful effects of agents present in the wellbore or reservoir
fluid.
Proper selection of completion materials is a key factor in ensuring completion
longevity. However, it is important that the design life of the completion is
compatible with the production profile of the well or field (Fig. 8-1).
Well Life Design
Material Selection Criteria
Completion components must be chosen to resist the damaging effects of
pressure, temperature and corrosion. In addition, recent exploration andcompletion operations have resulted in wells being drilled deeper with higher
pressures and temperatures being encountered.
Material selection criteria for oilfield equipment are typically determined by the
following categories:
- Mechanical properties (function)
- Operating environment
- Cost
- Availability
- Stock size and shape
These categories relate to the selection of metals, elastomers and plastics used
in construction of downhole tools and equipment. They are not listed in orderof priority since they may change for different applications.
Ferrous Alloy
Compositions
and Characteristics
Compositions
and Characteristics
Non-Ferrous Alloy
Compositions
and Characteristics
Compositions
and Characteristics
Packer Component
Materials
Materials
A sample material list for a standard service medium pressure hydraulic set
retrievable packer
Material Applications
Weight or stress bearing components, such as the outer bodies of safety
valves, packers, etc., are know as stagnant flow components. They may be
made of a different material than the internal components (mandrels, flow tube,
flapper, etc.). Internal components that are exposed directly to corrosive well
fluids are known as flow wetted components.
Examples of material applications are shown in Fig 8-4.
Non-Metallic Components
Elastomers and plastics are blended and synthesized organic polymers.
Polymers are repeating units of organic compounds. The flexibility of the
linking bonds between these units is what gives elastomers and plastics the
ability to stretch and then return to their original shape. The primary purpose
of elastomers and plastics in downhole tools is to provide seal materials to
isolate pressure, liquids, gases, or heat.
Many elastomers and plastics are available, each with different inherit qualities.
Elastomers function quite well in most wellbore environments but problems
can arise under the following conditions:
- Certain corrosive environments
- Wide temperature fluctuations
- Extreme pressures
The completion technologist should be aware of downhole conditions,
especially temperatures, to enable selection of the correct elastomers (seal
and o-ring) for a specific application.
especially temperatures, to enable selection of the correct elastomers (seal
and o-ring) for a specific application.
General Seal
Materials Guide
Materials Guide
Elastomers
There are two major types of polymer materials, elastomers and plastics. They
are differentiated on the basis of their elastic properties although there is no
sharp distinction between them. Polymers may be blended with other materials
to create substances with specific properties. An elastomer is a material which
can be stretched at least twice its length and upon release of the stress will
quickly return to approximately its original length.
Materials are added to elastomers and plastics to increase the strength, stiffness,
oil resistance, low temperature resilience, high temperature resistance and to
lower the friction coefficient. Unfortunately, whenever materials are added to
enhance one quality, another quality often suffers.
Polymers are blended for construction of O-rings, seals and packing elements.
The most common type of elastomer is Nitrile. This substance is also know as
Buna-N or Hycar (brand name). Other common materials are Viton and Aflas
which are fairly strong and resistant to degradation from exposure to wellbore
fluids.
Polymer materials are used as high performance packing for moderate
temperatures, pressures and corrosion. When completion tools must be
installed in wells where the temperature is very high, or in an H2S environment,
elastomers of fluorocarbons are used. Fluorocarbon elastomers can be
compounded with many substances including glass and asbestos, thereby
improving resistance to extrusion.
Plastics are also used in the manufacture of completion tools and equipment.
There are two major types of plastics:
- Thermoplastic - Formed by melting a resin, pouring the resin into a
- Thermosetting plastic. - Plastic in a liquid form is poured into a
chemical changes that cause the plastic to harden into the shape
of the mold. Once a thermosetting plastic has been formed it will
not melt, at least not at normal temperature. Examples of
thermosetting plastics are Teflon, Loctite and Eastman 910 (brand
names).
Teflon has probably the greatest oilfield application of the thermosetting
plastics. It has a high resistance to both high and low temperature, very low
friction and is inert to most fluids.
Teflon is used to form seal rings. However, to be efficient they must be
mechanically energized to make and maintain a seal. For this reason, Teflon
seals are usually used as back-up or secondary seals in high pressure
applications.
A number of new exotic plastics have been developed which show a high
degree of resistance to H2S, high pressure, and temperature conditions.
Fig 8-8 illustrates many of the common polymers used in oilfield tools.