A drilling jar is a specialized downhole tool used in drilling operations to apply sudden impact forces to the drill string. Its primary function is to help release stuck pipe or bottom hole assembly (BHA) components during drilling or tripping operations. It achieves this by delivering a sharp axial shock, either upwards or downwards, depending on the design and operational requirements.
Types of Drilling Jars
- Mechanical Jars
These jars operate by manually building tension or compression in the drill string. Once a threshold is reached, the jar releases, sending a mechanical shock through the string. They are simple in design and rely on the driller’s control. - Hydraulic Jars
Hydraulic jars use fluid resistance to delay the release of stored energy. As tension is applied at surface, internal hydraulic components gradually allow the tool to release, delivering a more controlled and powerful impact. These are more predictable and effective at deeper depths. - Hydraulic-Mechanical Jars
These combine features of both hydraulic and mechanical jars. They offer a balance of reliability, force, and timing. They can jar both up and down and are often preferred in complex drilling environments.
Working Principle
The jar is placed as part of the drill string, typically within or just above the BHA. When the drill string becomes stuck or meets resistance:
-The driller applies tension (for upward jarring) or compression (for downward jarring).
-The jar “cocks” as the energy is built up.
-When the internal mechanism reaches its release point, the jar suddenly frees, releasing the stored energy as an impact force.
-This shock is transmitted downhole to help free the stuck section.
Even in cases where the pipe is not stuck, jars can be included in the BHA as a precaution against potential sticking or difficult hole conditions.
Placement in the BHA
Drilling jars are usually placed above heavy drill collars to ensure maximum force transmission. Placement is strategic to maximize impact and prevent damage to more sensitive tools.