Sour Service Wireline Cable

A sour service wireline cable replaces the galvanised steel armour of a conventional logging cable with a corrosion-resistant alloy, and the bare copper conductor with nickel-plated copper. Everything else — the contra-helical double armour, the fluoropolymer insulation, the geometry — stays the same.

The reason is specific and unforgiving. Hydrogen sulphide attacks the iron in steel: first it eats the zinc coating, then it embrittles the wire underneath until strands snap on the way out of the hole. A cable that came out of a sour well looking fine can fail on the next trip, and the failure mode is a toolstring on the bottom.

The threshold is not a matter of opinion. NACE MR0175 / ISO 15156 defines sour service as an H₂S partial pressure of 0.05 psia (0.3 kPa) or more. Multiply H₂S concentration in ppm by bottom-hole pressure in psi, divide by one million, and if the answer reaches 0.05 you are in sour service and galvanised armour is no longer the right material. The Standards tab works that through with numbers, including the version of the rule that circulates with the divisor missing.

We build the alloy armour in three grades — S75 nickel-chromium, S77 nickel-iron and MP35N cobalt-nickel-chromium-molybdenum — across four diameters from 7/32″ to 5/16″. Swapping to alloy costs you breaking strength and, less obviously, it multiplies the armour’s DC resistance by four to six. That second one catches people out, because the armour is the return conductor. Both effects are tabulated on the Specifications tab.

Send a current fluid analysis, the bottom-hole pressure and the temperature — the same three numbers that drive material selection everywhere else in our oil and gas cable range. Without a fluid analysis nobody can specify this cable honestly, including us.

Construction

The three armour alloys

Iron content is the whole story. H₂S attacks iron; the less iron the armour contains, the less there is to attack. MP35N is close to iron-free, which is why it survives conditions that finish the others.

Nickel-plated conductor

Every sour build uses nickel-plated copper, not bare copper. This is not belt-and-braces. Insulation on a wireline cable takes crush damage from the armour over its life, and a single breach in a sour well exposes bare copper to H₂S along the whole flooded length — not at one point. Nickel plating buys the time to notice.

Insulation

Dual-layer FEP/ETFE at 260 °C on all sour builds: the FEP does the dielectric work, the ETFE skin faces the armour and takes the crush. For wells hotter than 260 °C see the high temperature wireline cable page.

What does not change — and one thing that does

Wire counts, wire diameters and finished OD are identical to the galvanised build of the same size — 12 inner over 18 outer, same lay. A sour cable drops onto the same drum and runs over the same sheave as the conventional cable it replaces.

The conductor is the exception. On two of the four sizes the alloy build uses a different conductor from the galvanised one: the 7/32″ goes from 16 AWG 19-strand to 18 AWG 7-strand, and the 9/32″ from 15 AWG to 16 AWG. Conductor resistance rises accordingly — 13.1 to 22.0 Ω/km on the 7/32″. It is on the specification table, but it is easy to miss when you are comparing armour grades and assume the rest of the cable stayed put.

Applications

Sour gas and sour oil wells, and the CO₂-rich wells that corrode by a different mechanism but end the same way.

What alloy armour does not fix

It does not raise the temperature rating — that is the insulation's job. It does not make the cable stronger; it makes it weaker. And it does not remove the need to wash and inspect the cable after a sour run.

Specifications

Alloy armour builds by size

All builds: FEP/ETFE insulation, 260 °C, 457 MΩ·km minimum insulation resistance, same OD and same minimum sheave diameter as the galvanised cable of that size.

What the swap costs, part one: strength

Recalculate the toolstring before the first sour run. An 18 % strength loss on a 9/32″ is not absorbed by the safety factor — it is the safety factor.

What the swap costs, part two: the armour is your return conductor

Alloy armour has four to six times the DC resistance of galvanised steel, and on a monoconductor cable the armour carries the entire return current. This is the number that surprises people, because nobody thinks of the armour as a circuit element until the telemetry starts failing.

Worked through on a 5,000 m run of 1/4″, loop resistance is conductor plus armour:

Three times the loop resistance means three times the voltage drop at the same tool current. Check the surface power supply headroom and the telemetry budget before ordering, not after — a tool that ran fine on galvanised cable can be underpowered on the alloy version of the same cable.

Standards and selection

The threshold, from the standard itself

NACE MR0175 / ISO 15156 defines sour service as an H₂S partial pressure of 0.05 psia (0.3 kPa) or greater in the gas phase. Partial pressure is concentration times total absolute pressure:

Worked example

A well at 4,000 psi with 50 ppm H₂S: (50 ÷ 1,000,000) × 4,000 = 0.20 psia. Four times the threshold — alloy armour. The same 50 ppm in a 500 psi well gives 0.025 psia, below the threshold, and galvanised armour with proper greasing is defensible. Concentration alone tells you nothing; it is concentration times pressure that matters.

A warning about the version of this rule you may have seen

A rule of thumb circulates in the field as “ppm × psi ≥ 0.05, avoid galvanised”. Taken literally that is wrong by a factor of a million — 1 ppm in a 1 psi well would trip it. The ÷ 1,000,000 is the missing step, and once you put it back the rule of thumb and the standard agree exactly. If a supplier quotes you the short version, that is worth noticing.

CO₂

The companion field rule is CO₂ percentage × bottom-hole pressure in psi reaching about 130 psia of CO₂ partial pressure. CO₂ corrodes by general metal loss rather than by cracking, so it is less catastrophic and more predictable — but it still ends galvanised armour's useful life early.

Standards

Frequently asked

Which grade do I need — S75, S77 or MP35N?

S75 for moderate sour and CO₂. S77 where the fluid analysis is severe. MP35N where it is severe and the toolstring is heavy, because it keeps the most strength of the three. If you are between two grades, the deciding factor is usually how many runs the cable has to survive before it is replaced.

Can I keep my galvanised cable and just grease it more?

Below the NACE threshold, protective grease on a well-seasoned line is legitimate practice. Above it, grease buys time and nothing more. The failure mode is hydrogen embrittlement inside the wire, not surface rust, and grease does not reach inside the wire.

Why did my alloy cable's signal get worse?

It probably did not get worse — it started worse. Alloy armour has four to six times the resistance of galvanised, and on a monoconductor cable the armour is the return path. See the loop resistance table on the Specifications tab.

Does the cable need different handling after a sour run?

Wash it. Alloy armour resists attack, it is not immune to it, and residual well fluid sitting in the armour interstices between runs is the worst case. Wash, dry and inspect — the same discipline that a conventional monoconductor wireline cable needs, done more seriously.

What do you need from me to quote?

A current fluid analysis with H₂S and CO₂ in ppm or percent, bottom-hole pressure, bottom-hole temperature, depth and toolstring weight. Anyone who quotes sour cable without asking for a fluid analysis is guessing.