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Temperature Rating

Well Conditions
& Cable Dynamics

Downrating

Evaluating Cables

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The Temperature Rating of Well Logging Cables

Signs of Temperature Damage and Evaluating Cables

Signs of insulation damage from temperature are usually most prominent at the cablehead, and include:

• Monocables- appearance of deep. uniform ridges, or a single prominent or "high" ridge on the insulated conductor

• Multi-Conductor- deep ridges in the tape layer, deep ridges in the outside of individual conductors, and flattening of individual conductors where they lay against the tape layer

• In extreme examples, large amounts of plastic can be found between the armor layers

The types of problems that can occur due to temperature damage are:

• Immediate electrical shorts

• Insulation breakdown under voltage, as the plastic layer becomes thin

• Communication problems, due to the change in electrical characteristic of the cable.

The problem most immediately noticed is, of course, an electrical short. This is usually the case with the monocables used in the cased hole environment.

If a single "high ridge" is found in the plastic of a monocable, "cold flow" may have occurred. Cold flow happens when plastic is moved under pressure at a temperature below its melting point. This can cause the plastic to lose its physical properties, including ductility. The plastic becomes brittle at the ridge. An electrical short may occur here due to the fact that the brittle plastic won't tolerate mechanical stress. The plastic can crack at the ridge due to the normal handling and twisting of the insulated conductor required to build a cablehead. Then, a short can occur when the cablehead goes into fluid in the well. Also, the ridge can crack under the armor somewhere up the cable's length due to the stresses that occur from sheaves, also resulting in an electrical short.

An electrical short can also occur when plastic "cold flow" occurs to the point of only thinning the plastic layer. A thinner plastic layer means a lower voltage rating at this point, so a short can occur when a tool is powered to a voltage greater than the thin layer of plastic will tolerate.

In the case of a damaged, thin insulation layer, especially with the open hole multi-conductor cables, it is possible to experience signal communication problems between logging tools and the surface before an electrical short occurs. Because the plastic insulation is thinner, total cable capacitance increases. Therefore, signal attenuation increases, and communication between tools and the surface may become difficult or be lost.

If plastic damage is suspected, it is appropriate to test the cable using a "megger" to check insulation resistance, and to apply high voltage with a "hi- pot to check for maximum voltage capability. The amount of high voltage applied should be according to the manufacturer's maximum voltage rating. Please note that many "meggers" operate at only 500 volts.

While the condition of the plastic is usually at the worst at the cablehead, it has been observed in short radius directionally drilled wells that isolated damage can occur at large distances from the cablehead. The same situation can exist in the case of a well workover in which a window is cut in casing through which wireline work is performed. In these cases, the condition is comparable to having a small diameter sheave downhole. The combination of softened plastic due to well temperature, and the stresses generated at the "kickoff" point or casing window can cause plastic to move within the cable core, downrating the voltage carrying capability or causing an electrical short to occur.

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