Wireline cables are most susceptible to damage during their first few runs into and out of a wellbore, when the cable is essentially “new.” Although this seems contrary to what you would expect, the exposure to the wellbore environment helps “season” a new cable and prepares it for life into and out of a wellbore. These first several runs are critical for ensuring that you get the maximum trouble-free operation from your cable.
A new cable as delivered by the manufacturer undergoes important changes when it is first put into service. These changes are tension, temperature and rotation. Cable tensions and wellbore temperature during field operations are much higher than during manufacture. These repeated higher tensions at elevated temperatures produce an embedment of the inner armor into the conductor insulation and a reduction of the diameter of a new cable in the range of 0.005″. It is advantageous if your manufacturer’s process uses a series of pressure rollers to partially embed the inner armor into the plastic core before the outer armor is applied. This process reduces the diameter changes typical of a new cable, which starts the seasoning process in advance of running into a well. See Figure 1. The insulation depicted in red shows the indentation made during the pressure rollers process.
All cables used in oilfield service have an armor design that develops a torque proportional to the load on the cable. The torque of the outer armor wires is always greater than the opposing torque of the inner armor wires. This is because there are generally more armor wires on the outer, and the distance from the center is greater (increasing leverage). Under load the outer armor dominating will attempt to rotate and unwind the cable until there is a torque balance between the armor layers.
During manufacturing, a cable is subjected to only a few hundred pounds of tension, so there is essentially no torque in a new cable as delivered. When installed on a truck the spooling tensions are significantly higher than during manufacturing and the cable is not free to rotate so the cable will develop significant torque. During the first field operations, this new cable will try to rotate to equalize the built up torque, as well as to support the weight of the tool string. To illustrate the magnitude of this problem consider a new 5/16″ cable deployed in a straight 20,000 ft. well. The total rotations that a new cable end would need to rotate in order to equalize the torque could be over 400.
One way the problems associated with cable rotation can be reduced is if the manufacturer uses a special coarse blocking material that increases the friction between the inner and outer armor, thus reducing rotation. After repeated operations, this initial blocking material is slowly lost. With standard cables using galvanized steel armor wires, the spaces between armor wires now become filled with the corrosion by-products of zinc and iron. These by-products combined with the roughening of the wire from corrosion serve to increase the friction between the inner and outer armor, thereby reducing cable rotation.
Cables built for operations in H2S and extreme corrosive well conditions use special alloy armor wires. Alloy wires do not pit or generate corrosion by-products. Without this friction from corrosion, alloy cables rotate freely throughout their life, after the initial blocking material is lost.
Operations using alloy cables (Figure 2) are frequently in wells containing toxic well gases where hydraulic pack-offs are generally kept much tighter than in regular operations. These conditions result in additional unwinding and loosening of the outer armor. Alloy cables throughout their life must have the armor tightened and set with post forming. There are no standard rules for servicing alloy cables, but it is a good practice to bring a new alloy cable to a service center after the first job and thereafter from every 10 to 20 runs.
All cables that become loose, but in particular new cables, are more susceptible to damage including: 1) drum crush; 2) outer armor wires being “milked” into a “bird cage” as shown in Figure 3; and 3) a reduced breaking strength. Importantly, the breaking strength of a normalized cable comes from all of the inner and outer armor wires combined. When a cable becomes loose, the load is shifted from all the wires to the inner wires only. This dramatically reduces the breaking strength as shown in Figure 4.
Tips to prevent damage to new cables and increase cable life: (These suggestions may not be practical in all situations, however it is important to understand that if employed, these practices will increase cable life.)
• To allow a new cable to rotate and become “normalized,” it is important to choose the first operations carefully. Select operations with minimum mechanical drag downhole, and where little or no pack-off pressure is required, and where the hole is fairly straight. In practice bore holes are never straight and the end of the cable is never completely free to rotate, so it takes a new cable several trips into the hole to spin out and become totally “normalized.” Follow the same rules any time a new part of the cable comes off the drum for the first several times.
• Coming out of the hole, cable tension is increased due to friction and the weight of the tool string. This higher tension will cause the cable to rotate and the outer armor to unwind. Line speed is a critical parameter; the faster you run in the hole, the less tension the cable experiences. Coming out of the hole at high speeds creates increased tension and captures the cable in a loose condition. When going back into the hole, tension is reduced and the cable will rotate to tighten the outer armor. To allow a cable to wind and unwind normally, a good operating rule is:
– While going into the hole do not allow the tension at any depth to fall below two-thirds of the static tension at that depth.
– Come out of the hole at a speed not greater than the speed that increases the tension by more than 11/3 of static tension at that depth.
• When special operating conditions do not allow for normal cable speeds or when tight pack-offs are needed, the cable will unwind and develop loose outer armor. To correct this condition it will require that the cable be taken to a cable service shop to have the armor tightened and post-formed (Figure 5), otherwise the cables ultimate breaking strength could be jeopardized.
• For new cables, when running into the hole, stop every 1,000 to 2,000 feet (or whenever the customer permits) to allow the cable to regain tension and properly spin out. At this point pull back 50 to 100 feet prior to running further into the well.
• Avoid deviated wells if possible until the cable is seasoned.
• Avoid any pressure with a hydraulic pack-off and ensure the flow tubes are minimum 0.004″ to 0.006″ clearance.
Article by: By: Terry Moffatt & Terry Madden/Quality Wireline and Cable, Inc.
ABOUT THE AUTHORS: Terry Moffatt, P.Eng., ME, PET, is president of Quality Wireline and Cable Inc. His experience includes co-founder and president of Wireline Works Inc.; and president and founder of PROMORE Engineering Inc. He has 20 years experience in the design and installation of permanent monitoring systems using wireline, and in the manufacture of electro-mechanical wireline.
Terry Madden is the U.S. sales manager of Quality Wireline and Cable Inc. His experience includes 28 years at Maddens Cable Service Inc. where he managed a service center and installed and troubleshot wireline. He also spent 2 years in technical sales at Wireline Works Inc. His expertise is troubleshooting wireline cable field problems.