Downhole logging tools

Stuck and lost tools are a normal occupational hazard of logging. In spite of our best efforts to avoid tool loss, Schlumberger strings have been lost on ODP Legs 101, 113, 117, 122, and 175. These strings are expensive and therefore are insured against loss in a hole; however, the shipboard loggers try their best to avoid sticking a tool, to recover a stuck tool, and to fish for a lost tool.

If a tool is lost downhole, a reasonable effort must be made to recover it in order to satisfy obligations to Schlumberger and the insurance provider. The recovery effort should follow accepted practices and include multiple recovery attempts if technically feasible. USIO/LDEO must be notified immediately by the Logging Staff Scientist or the drilling superintendent if a tool is stuck or lost.

If all reasonable efforts have been made to recover a stuck or lost tool without success, then the decision to abandon the tool must be made collectively by the Logging Staff Scientist, Operations Superintendent, Rig Superintendent and the Schlumberger engineer. A report must be filed by the Operations Superintendent and delivered to the Logging Staff Scientist. In the event of loss involving a radioactive source, the hole must be cemented to safely entomb the sources, and then abandoned.

Strategies

There are four main types of tool "sticking" situations:

1. The tool is either stuck in a bridge or stuck by cavings (possibly beneath a bridge).
2. The tool is not stuck but cannot be pulled up past a bridge.
3. The tool is stuck in the base of pipe.
4. The tool is not stuck but cannot get into pipe

Several strategies are available for dealing with stuck tools:

1. Pulling harder on the cable
Pulling harder on the cable is recommended as the first course of action when a tool appears to be stuck. Pulling may not exceed the combined cable weight plus weak point strength, or 50% of cable strength, whichever is less. With this method, there is a higher chance of recovery in situations #2 and #4 than #1 and #3.

2. Adding pipe (if using the CSES)
If the CSES is used, stands of pipe may be added to break through a bridge or cuttings (situations #1 and #2). Cable tension should be maintained when lowering pipe to prevent: (A) cutting through or kinking a slack cable with the pipe; or (B) sudden dropping of the tool when the tool is freed (a l0'-30' free fall of the tool may be enough to snap the weak point). Once the tool is free, you can pull it well into pipe, raise pipe, and go back down to resume logging.

3. Cutting and stripping
Cutting and stripping involves clamping the cable at the drill floor, cutting it, then either adding or removing a stand of pipe. For every 30 m of pipe added or removed, the cable must be threaded in or out of the pipe and re-clamped. In situations #1 or #2, one would add pipe to break through the bridge. In situations #3 and #4, one would remove pipe, eventually pulling the tool on deck with the bottom hole assembly. One disadvantage of cutting and stripping is that all of the cut cable will be discarded (perhaps 1000-3000m) and this may not leave enough cable on the spool for subsequent logging. The Logging Staff Scientist is responsible for making this determination. Cutting and stripping is also not the most favorable alternative because it is time consuming. Cutting and stripping is not needed for situations #1 and #2 if the CSES is in the string, but a modified type of cutting and stripping may be possible with situations #3 and #4 with the CSES.

4. Using the Kinley crimper/cutter
The Kinley crimper and cutter system greatly increases the safety of downhole tool recovery operations. The crimper/cutter procedure is extremely sequence sensitive. The crimper slides down the wireline and stops about 10m above the base of the bottom hole assembly (BHA), then a hammer is sent down to fire the crimper which crimps the logging cable against the BHA. A successful crimp must be observed by the Schlumberger engineer by checking for an electrical short inside the cable. If successful, the cutter is dropped and the cut logging cable is reeled in. The tool is held inside the BHA and recovered by pulling pipe to the rig floor. Crimping and cutting works well for situation #4 but is no guarantee of success. During Leg 175, the Kinley crimper was used to secure the tool in the pipe but it failed to adequately crimp the cable. As the tool and drill pipe were being pulled to the surface, the toolstring dislodged itself and fell to the seafloor where it could not be retrieved.

5. Additional strategies
In 1988, Glen Foss (Operations Superintendent at ODP/TAMU), put together a detailed memo on wireline stripping operations. This is highly recommended reading. The part relevant to recovery of stuck tools is given here.

In addition, USIO/LDEO has compiled a list of very dangerous situations to avoid when logging, along with strategies to avoid and cope with them.

To some degree, each stuck/lost tool situation is unique, and it is impossible for any guidelines we give to always be appropriate. Thus, the recommendations given in the following table should be considered as suggestions only, not requirements: