A drilling jar is a downhole tool that delivers an axial impact load to the stuck point of a drill string. Jars are often the first line of defense against stuck pipe and can save operators from costly fishing and remedial operations by quickly "jarring" the string free.
Modern drilling jar designs are double-acting hydraulic, i.e., they are capable of delivering an upward or downward impact load to the stuck point by raising or lowering the drill string from the surface. Placement of drilling jars in the bottom hole assembly (BHA) is a critical step in drill string design and will ensure maximum effectiveness of jars in the event of a stuck pipe scenario.
Jars are often placed in the upper portion of the BHA, usually in the drill collars or heavy weight drill pipe (HWDP). Depending on the wellbore, drilling jars can run in either tension or compression. In vertical or low angle wells, jars will typically run in tension since there is sufficient weight below the jar to keep it fully extended. In highly deviated or extended reach lateral wells, jars will run in compression where there is insufficient weight below the jar to keep it in tension.
Any downhole stroking tool, such as a drilling jar, is more susceptible to bending stress failures than a continuous joint, such as a drill collar or HWDP. Because of this, it is important that a jar is safely embedded within the drill collars or HWDP, and never at the crossover point between the two components.
To minimize bending stress, place jars at least two joints above or below this crossover point. The bending stress ratio (BSR) of drilling jars is more compatible with HWDP than drill collars, so it is recommended to place jars within the HWDP if possible.
Vigor could provide you with three different models of drilling jars (Mechanical, Hydraulic, Hybrid mechanical/hydraulic) according to your needs. If you are interested in Vigor's drilling jars, please do not hesitate to get in touch with us.
