What is backoff operation?
The backoff operation is a procedure used in oil and gas well fishing jobs to unscrew a stuck pipe string at a specific threaded joint above the stuck point. This method is particularly popular for drill pipes and drill collars because it leaves a threaded connection at the top of the remaining pipe, making it possible to screw back into the fish with a workstring and fishing tools.
The backoff process involves applying a left-hand torque to the pipe string while firing a shot of prima cord explosive. The resulting explosion produces a concussion that partially unscrews the threads at the targeted joint.
For tubing or coupled pipes, backoff is still a common method, although their finer threads and higher thread interference make the process slightly different. In some cases, it may be more economical to back off the tubing, circulate the well, screw it back together, and continue the process until the fish is removed.
In addition to backing off stuck pipes, string shots can also be used for releasing stuck packers or fishing tools, removing corrosion, opening perforations, jumping collars or tool joints, and removing jet nozzles from drill bits.
The Method of Backoff Operation:
- Tightening and Reciprocating: The pipe string is first tightened with right-hand torque and then moved up and down (reciprocated) while maintaining the torque. This ensures a good connection and distributes the force throughout the string.
- Applying Left-hand Torque: Next, torque is applied in the opposite direction (left-handed) to the string. This torque is also “worked downhole” by reciprocating the pipe. The amount of left-hand torque required varies based on factors such as the depth of the stuck point, hole conditions, and pipe type. A general rule of thumb is to use three-quarters to one round per 1,000 feet of free pipe for tubing and half to three-quarters of a round for drill pipes.
- Firing the String Shot: Ideally, the pipe at the separation point should be neither under tension nor compression. In reality, a slight tension is preferred. A small explosive device called a string shot is then detonated, creating a shockwave that loosens the threads at the desired joint.
- Unscrewing and Removing the Pipe: With the threads partially unscrewed, the pipe can be further unscrewed using surface equipment and then removed from the well.
Important Considerations:
- Pipe Type: Backoff works well with drill pipe and drill collars due to their coarse threads and metal-to-metal seals. Tubing, with its finer threads and tension, can also be parted using backoff, but other factors need to be considered.
- String Shot Selection: The string shot’s strength and fuse type depend on the pipe size, depth, wellbore fluid, and temperature.
- Outside Backoff: This is a variation where the string shot is placed outside the pipe in the wellbore annulus. It’s suitable for hard rock formations and situations where the pipe is plugged and cannot be cleaned for an internal shot.
Additional Uses of String Shots:
There are several purposes which string shots can be used as listed below;
- Breaking free stuck packers or fishing tools
- Removing corrosion from pipes
- Opening clogged perforations in the wellbore casing
- Separating a stuck drill collar from a tool joint
- Removing nozzles from drill bits to improve circulation
- Dislodging drill pipes stuck in formation keyseats
What is Outside Backoff ?
When a drill pipe gets stuck in a wellbore, and traditional methods of retrieval are impossible due to a plugged pipe, an outside backoff technique can be a viable option. This method is most successful in near-vertical wells with hard rock formations, and not recommended for wells with softer formations like shales or unconsolidated sands.
Here’s how it works:
- A special tool called a side-door sub (also known as an outside backoff collar or hillside sub) is lowered down the wellbore on a wireline. This sub has a side opening that allows access to the space outside the stuck pipe.
- Another tool called a string shot, which is essentially a small explosive charge, is then lowered through the side opening of the sub and positioned near the connection point of the stuck pipe.
- With the string shot in place, torque is applied to the drill pipe, and then the string shot is detonated. The explosion creates a shockwave that can loosen the connection and allow the drill pipe above the stuck point to be retrieved.
Important points to note for outside backoff:
- Outside backoff is a technique for near-vertical wells and is not suitable for directional drilling.
- This method is ideal for hard rock formations and should be avoided in softer formations.
- The string shot design ensures it lands near the pipe connection for optimal impact.
References
The Guide to Oilwell Fishing Operations: Tools, Techniques, and Rules of Thumb (Gulf Drilling Guides) by Joe P. DeGeare, David Haughton, Mark McGurk
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