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. In order to remove any junk from a well, junk removal tools will be used depending on a particular condition of junk and wellbore.
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.
The junk removal tools are as follows;
There are multiple forms of junk baskets available which are core cutting junk baskets (Figure 1), and a combination junk/jet basket (reverse circulation junk basket, Figure 2). These can be used to recover sidewall core bullets, bit cones, parts of cementing equipment, or other small pieces of debris.
This tool may work in multiple ways:
- They 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, they can use 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, then can provide a directed high-velocity jetting action, which will force materials into the basket.
This footage below demonstrates how the combination junk/jet basket (reverse circulation junk basket) works.
Fishing 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.
Junk Mills are made of tungsten carbide which is used to mill small junks. In some cases, it is easier to break an object into smaller pieces than to retrieve it whole. They can then be recovered with a junk basket. Hard-formation rock bits can sometimes be used to break small objects up, but in other cases a junk mill might be needed, as shown in Figure 4.
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