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Introduction

Temperature

Pressure

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Cables Operating Under High Pressure and Temperature

Bill Bowers
3/21/89

Pressure

There really is no practical upper limit to the hydrostatic pressure rating of a cable as the conductors are constructed essentially void free. The microscopic voids that do exist in a conductor are pressure equalized by the -permeable nature of thermo plastic coating over the copper wires. Camesa cables have successfully operated in oil well applications where bottom hole pressures have exceeded 30,000 psi.

The most serious cable problem resulting from operation at very high temperatures and pressure occurs when the cable is removed from the hole. At this time it is not uncommon to experience electrical leakage. The cause of this electrical problem is as follows: 

When the cable is exposed to very high temperature and pressures for a period of time there will be a migration of water (pure H»0) and methane gas (CH4) through the plastic insulation. This is true of any therroo plastic in varying degrees. The plastic acts as a molecular sieve and'.allows only .the smallest of molecules, H.,0 and CH4, to pass. This water and gas collect in the microscopic voids in the conductor stranding. Even though during manufacturing all steps possible are taken to minimize these voids, by filling the spaces between copper strands with blocking materials, there is invariably some unfilled microscopic voids for the gas and water to fill. When the cable is removed from the well and returned to atmospheric pressure and surface temperatures, the gas trapped in the conductors voids will try to expand forcing the water out the end of the conductor. When this occurs it can cause a leakage path at the bottom hole connector on the cable in the cable "Head." 

If the connectors are cut off the cable after an extended exposure to very high temperature and pressure, water can sometimes be observed oozing out of the strands of copper. Sometimes in high gas content wells the water will "spit" out of the conductor under the pressure of the gas. Normally this condition will exist for only several hours, but in extreme cases as long as a week.

It is very important to remember that this gas and water have penetrated through the permeable plastic -jacket -and it is NOT A RESULT OF A PIN HOLES IN THE INSULATION. This statement is easily proven by making a high voltage test of the insulation.

With the cable completely disconnected at both ends (collector and head connectors removed) a high voltage test of the conductor to armor insulation will show there is NO electrical leakage even while water is dripping out of the ends of the conductors.

The explanation is that pure water is an excellent insulator and dielectric and therefore does not degrade the electrical properties of the plastic as it passes through. Once the water reaches the conductor, it is contaminated by the copper, copper stranding lubricants, and water blocking materials; therefore the water oozing out the end of the conductor will be contaminated and conductive.

The microscopic collection of moisture in the copper conductors does not in any way deteriorate the performance of the cable. It will, however, discolor the copper, and cause temporary termination problems. There are several operating "tricks" that can be used to minimize these problems in many applications but these problems have to be considered individually.

Great progress has been made over the past 30 years since this problem was first encountered but in the case of extreme temperature and pressures there is no way of completely eliminating the problem with out sacrificing other desirable operating characteristics of the cable. 

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