As per API RP11S5, cable insulation isolates the electrical potential between conductors and other conducting materials. Insulation also minimizes leakage current from the conductors.
Insulation selection depends on many factors mainly related to produced fluids properties and well behavior; such as: gas type and concentration, well environment, gas to oil ratio, pressure cycling, exposure to downhole chemicals…
There are two classes of cable insulation materials used in oil and gas industry:
- Thermoplastics (like polypropylene)
- Thermoset compounds (like EPDM)
Thermoplastics are polymeric materials that will soften or flow at elevated temperature but will harden when cooled (reversible process). Examples of thermoplastic materials are polypropylene, polyethylene, polyvinyl fluoride, polyvinylidene fluoride and polyamides.
Polypropylene is relatively inexpensive insulation material useable in low-temperature environments. It is useful in the temperature range -10 C / 14 F to 96 C / 205 F. This temperature range is estimated for ideal conditions where there is neither chemical exposure nor applied mechanical forces.
Cable with propylene insulation should not be handled when temperatures are less than -10 C / 14 F. Risk of insulation cracking if bending is applied at lower temperature.
There are several detrimental well conditions which are known to affect polypropylene:
- Premature cracking is initiated by the presence of carbon dioxide (CO2).
- External forces, such as cable clamps, applied on a cable operating near the upper temperature limit leads to premature deformation.
- Propylene is susceptible to accelerate aging from contact with copper material. To overcome this issue, most manufacturers applied a tiny coating to isolate the copper from polypropylene.
- Polypropylene allows gas to migrate between the conductor and insulation. For application where this would be a problem, a conductor / insulation gas blocking material must be applied.
Thermosets are polymeric materials modified through a chemical reaction and became permanently shaped when cured. It is irreversibly cross-linked and cannot be remolded. Typical thermoset materials includes EPDM (Ethylene Propylene Diene Monomer) and EPM; styrene butadiene rubber (SBR); and cross-linked polyethelene (XLPE).
For ESP cable, EPDM is the most commonly used thermoset material. This due to its better resistance to CO2 environment and its resistance to many type of well treatments. Also, EPDM is able to retain a good flexibility at extremely low temperature (-40 C / – 40 F). Furthermore, EPDM materials are generally preferred in higher temperature oil well applications.
EPDM materials swell in oil; this issue can be reduced by proper formulation. Also, EPDM is highly dependent on outer constraining coverings to retain its integrity.
Composite insolation system:
Some cables use a composite insulation system composed of basic and auxiliary insulation, where films or thinly extruded material are used as the auxiliary insulation.
- Films: are generally thin tapes of materials which are applied directly over the conductor. Films are suitable for high temperature (up to 232 C / 450 F), and their most common applications are motor lead extension (MLE) and motor winding insulation. Their major limitation is their extremely high cost.
- Extruded Auxiliary Insulation: several materials are extruded as auxiliary insulation directly over the conductor or over the insulation to provide added dielectric strength and chemical resistance. The material is less susceptible to damage from penetration of well fluids than the EPDM covering. Therefore, it provides greater protection and may extend the life of the cable. Thermoplastic polyvinylidene fluoride (PVF) and ethylene chloro-tetra-fluoro-ethylene (ECTFE) are used for temperatures up to 149 C / 300 F, and fluorinated ethylene propylene (FEP) is used for temperatures up to 204 C /400 F.
API Recommended Practice 11S5 (RP 11S) – First edition, February 1, 1993.