In order to fish for wireline tools out of the hole, we must use different equipment than would be used for pipe recovery. Common wireline tool issues center around the cable being tangled or wadded in the hole, as well as the fact that attempts at fishing can pull the wireline out of the rope socket or part, further complicating tool retrieval.
Which Part is Stuck? Cable or Tools
As soon as a wireline assembly becomes stuck, the operator will need to determine whether the problem is in the cable or the tool. Usually, one would apply normal logging tension on the cable and allow it to sit for a few minutes. During this time, four things should be recorded:
- Current depth of the tool
- Type and size of the cable
- Surface tension of the cable just prior to becoming stuck
- Cable-head’s weakpoint rating.
The cable will be marked at the rotary table, and a T-bar clamp will also be securely fitted to the cable just above the table. Should the cable break, then the clamp holds on to the cable end at the surface, so that the whole cable does not fall down the hole and cause additional blockage. The operator will then need to apply 1000 lbf of tension on the cable, and make a note of the distance that the cable mark moves at the rotary table. This figure shows the stretch produced in the elastic cable. It is then possible to estimate the length of free cable, using a stretch chart or from prior knowledge of the cable’s stretch coefficient. Should the length of free cable be the same as the current logging depth, then the problem does not lie with the cable; rather, the tool is stuck, and not the cable. If the length of free cable is less than current logging depth, then the cable is stuck at some higher point in the hole.
If it is the tool which is stuck, and not the cable, then pulling on the cable will cause one of three results. The tool may come free, the weakpoint can break and the tool will remain in the hole but the cable can be removed, or the cable will break at the point of maximum tension.
Causes of Tool Getting Stuck
When the cable cuts through mud cake, differential pressure sticking may occur. This is because one side of the cable is exposed to some degree of formation pressure, whereas the other is exposed to the hydrostatic mud column. Due to this significant difference in pressure, the cable will be pressed harshly into the formation, and friction against the formation stops the cable from moving any longer. Other reasons why sticking may occur include ledges, particularly severe doglegs, borehole caving, or the borehole becoming corkscrewed. As the length of the tool increases, as well as when there has been a long amount of time since the last conditioning trip, the chances of sticking will go up.
When a wireline tool or cable gets stuck, there are several different ways that they can be recovered.
One option is a side-door overshot as shown in Figure 1. This method is similar to a regular overshot, except that it features a removable side door, so that the tool can be put together around the wireline at the well head itself. It is then possible to run the tool on some tubing or on the drillpipe, downhole alongside the wireline in order to make direct contact with the tool. This stops the wireline from being at risk of parting. It is not recommended that side-door overshots are used with deep open hole intervals. This is because it introduces the potential for keyseating, or differential sticking in the mud cake.
Throughout modern drilling, the most successful method to retrieve stuck logging tools is through the cut-and-thread method. This involves cutting the wireline at the surface, and then threading it through a pipe string while the pipe is lowered, until it engages with the logging tool. The line must be secured at the surface, and rope sockets need to be fitted to each end to form a spearhead both emerging from the top of the well, and a spearhead overshot at the logging end. A stand of pipe will then be hung in the derrick, allowing enough of an overshot at the bottom to catch the logging tool, or at least the wireline rope socket. When the upper end of the line is spooled down through the interior of the pipe until the overshot connects with the spearhead at the bottom, then the pipe will be run into the hole. This is repeated with additional stands until the bottom of the string is close enough to the fish. When this is achieved, the spearhead overshot can be disengaged and the overshot can be circulated clean, before it engages with the tool. When the fish has been grasped securely, the wireline will be pulled free from the rope socket, and then spooled out of the hole, and the tool itself recovered with the fishing string. Although the cut-and-thread method takes a lot of time, and comes with a certain amount of risk, it vastly improves the chances of recovering the wireline and tool fully, and is much quicker than trying to engage with the wireline in an open hole.
If it is not possible to use either a side-door overshot or a cut-and-thread, then an alternative is to break the weakpoint, and then recover the cable and use the drill pipe to fish for the logging tool. If tool recovery is not an option, then a last resort is to push it to the very bottom of the hole, and then plug it using cement.
Wirelines that are wadded or tangled can be retrieved with a wireline barb or rope spear. This penetrates the debris, engages with it, and then allows the debris to be pulled away, as shown in Figure 2. This is one of the most basic forms of fishing tool, and gives strong results when used in the right way.
Junk Removal Tool
Junk refers to any objects or debris which have been dropped into or lost in the hole. Junk can include all manner of things, from downhole tools and bottomhole assembly components, to bit cones, or even hand tools which have been accidentally dropped into the hole. In some cases, it may be clear what the junk is, such as when something has been visibly dropped down the hole. On the other hand, though, it may sometimes be unclear just what is causing the problem. While drilling is taking place, junk can be detected by an irregular torque, or by the drill being unable to move ahead when a new bit has been run. There are three main ways that junk can be dealt with; which method is chosen will depend on the size of the junk itself, and how hard the formation is. The junk can be recovered whole, split into smaller pieces so that these pieces can be recovered or that they are too small to cause any additional issues, or finally pushed into the side of a soft formation or the bottom of a formation with a large enough rathole. If none of these are possible and the junk continues to interfere with well operations, then the well made need to be sidetracked or abandoned.
There are multiple forms of junk baskets available, as shown in Figure 3 and Figure 4. These include core cutting junk baskets (Figure 3), and a combination junk/jet basket (Figure 4). These can be used to recover sidewall core bullets, bit cones, parts of cementing equipment, or other small pieces of debris.
Junk baskets may work in multiple ways:
- The tools might penetrate the formation and cut a relatively short core, digging out debris from the bottom of the hole and then trapping it inside of an inner barrel.
- Alternatively, tools can perform reverse circulation, leading to drilling fluid circulating around the exterior of the basket, and thereby sweeping junk into the top part of the tool before moving further up the annulus. It is also possible for these tools to cut small cores.
- Finally, They should provide a high fluid velocity jetting action, which will force materials into the basket.
Magnets can be run either on a wireline or on a drill pipe. Smaller ones can pull around 2 lbf, while larger ones can pull up to 3000 lbf, equivalent to 13345 N. These magnets are designed to only exert their magnetic field downwards, so they do not cause any damage when they are lowered through casing. Permanent magnets will be run on a drill pipe, and include circulating ports to allow for cuttings to be washed away so that the magnet can make contact with metal fish. On the other hand, electromagnets are run on wirelines, and only switched on when they reach the fish. They can be run in and out of holes quickly, but a disadvantage is that they lack any fill-cleaning capabilities, and therefore cannot engage fish that are covered with debris or fill. They are useful for retrieving iron-containing metal objects.
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