The Mechanical Bit Release (MBR) is designed to remotely release the Rotary Core Barrel (RCB) core bit from the Bottom-Hole Assembly (BHA) to allow logging tools to pass through the BHA. The RCB drill bit must be released before logging starts because the internal diameter of the RCB drill bit throat (2.312 in.) is not large enough to pass most logging tools.
The MBR is placed in the RCB BHA above the RCB bit. After the hole is cored to logging depth and conditioned for wireline logging, the RCB bit is released with the MBR. To release the MBR, the Rotary Shifting Tool is made up on the bottom of a standard Inner Core Barrel. The Shifting Tool is lowered on the wireline until it engages the latch sleeve on the inside of the MBR. The wireline is slowly pulled up until an overpull of 4000-5000 lb shifts the Latch Sleeve and allows the latch dogs to fall out, thereby releasing the bit. The four metal latch dogs, lower MBR, flapper valve, lower support bearing, and the bit are left in the hole. This metal "junks" the hole and effectively prevents further coring. A second wireline trip is made with the Shifting Tool in the reversed position to close the Latch Sleeve, which covers the empty dog pockets to reduce the possibility of scraping hole wall material into the BHA.
The MBR is compatible with the RCB BHA.
Benefit: The MBR allows wireline logs to be run without the need to "trip the pipe" to remove the RCB bit on the rig floor.
2) Latch Sleeve/Dog Mechanism
Shifting the Latch Sleeve with the Rotary Shifting Tool releases the latch dog segments. This releases the bit disconnect, dogs, support bearing, float valve, and the core bit into the hole.
Benefit: The core bit can be released in a minimum amount of time, using the Shifting Tool to operate the Latch Sleeve via the wireline.
3) Emergency Bit Release
In unstable formations, the MBR can be run for an emergency bit release.
Benefit: If the BHA is stuck at the bit, the bit can be released to save the BHA from being severed.
Maximum Outer Diameter: 8 in.
Minimum Inner Diameter: 4 in.
Torque Limit: 12,000 ft-lb
Compression Weight: 45,000 lb
Typical Operating Range
Formation: All formation types
Depth Range: All depths
Dropping the core bit, float valve, four latch dogs, and lower MBR leaves metal junk in the hole, which effectively prevents further drilling or coring.
After removal of the bit, the drill string is open-ended, which makes it susceptible to plugging by "swallowing" some of the hole wall as the pipe moves and/or logging tools drag debris into the pipe. This may plug the drill string, which increases the chance of jamming the logging tool when it is entering and exiting the open end of the pipe.
The MBR must be run with a standard Baker float valve (i.e., the LFV cannot be run). The LFV cannot be deployed with an RCB BHA because there is only enough room for the Baker float valve and RCB swivel assembly.
Excessive overpull on the MBR (i.e., 150,000 lb to 170,000 lb) can result in the accidental release of the core bit, which would leave junk in the hole.
The Inner Core Barrel can become "packed off" in the OCB (from cuttings flowback) and unintentionally shift the MBR, which would prematurely release the bit.
After the bit is released with the wireline Rotary Shifting Tool, a second wireline run is required with the Shifting Tool to shift the sleeve down and close the dog windows to prevent the open window in the MBR from scooping up cuttings and plugging the drill pipe between loggings runs.