LWT Field Technique
The LWT collar is inserted into the drill string on the last bit trip or on the reamer run and does not require any change in drilling plans or the extra rig time required to change to a specialized BHA. LWT tools are pumped at TD stage and log un-tethered in memory during the normal pipe trip out, there is little additional rig time used compared to pipe conveyed or shuttle logging and no chance of bridge off as with conventional wireline.
Throughout LWT operations, the fully retrievable logging tools and nuclear sources are safely housed inside the LWT steel and/or composite drill collars. As tools are never exposed to well conditions, the risk of damage or loss is practically eliminated. Full well control including pipe rotation, reciprocation and mud circulation are maintained at all times during logging. Unlike LWD, the LWT tools are not a permanent part of the bottom hole assembly, but instead are deployed and retrieved from the drill string only when log data is required.
The detailed procedure for acquiring log data using Logging While Tripping (LWT) is as follows:
- Set up surface depth system on rig (drum encoder/crown sensor, hook load sensor or EDR interface).
- Insert specialized drill collar in drill string near BHA (added at any point during or after drilling).
- Program logging tools for appropriate “sleep” time before data begins recording to memory and synchronize tool clock to surface depth system clock.
- Once TD or zone of interest is reached, LWT tools are deployed into BHA collar by pumping, slickline, or surface deployment depending on application.
- Once set in collar, logging tools “wake up” and begin recording as the pipe is being tripped out of the hole.
- Any required circulation, reaming, or well control can take place as with normal pipe tripping operations.
- At surface, depth system records depth versus elapsed time during the pipe trip.
- Tool reaches surface and logging data is downloaded from memory.
- Logging data is referenced to surface acquired depth and logs are generated.
- Rig operations resume with minimal time impact due to LWT logging operation.