A drilling jar converts the potential energy stored in the elongated drill string into kinetic energy. After the jarring action occurs, this kinetic energy generates an impact wave that transmits a jarring force to the stuck drill string, helping to free it.
The energy of the impact wave relates to the acceleration of the jarring tool, while the duration of the wave relates to the length of the drill string. Their relationship is as follows:
Kinetic Energy = 1/2MV^2
Where M equals the weight of the drill string above the jar, and V is the velocity of the jar during impact.
There are three main types of jarring tools: mechanical jars, hydraulic jars, and hydraulic-mechanical combinationjars.
Mechanical Jars
Mechanical drilling jars utilize a series of springs, detents, and release mechanisms to produce the jarring impact. They jar upwards when the overpull reaches a pre-set limit and jar down when the overpull reaches another pre-set limit. The jar only activates within these set limits, which are normally above the forces experienced during normal drilling. During regular operations, the mechanical jar is either in a neutral position or an overpulled state, but never pre-loaded to jar down as this could damage the drill string below.
The release mechanism can be configured at the surface or downhole, depending on the jar's design. There are two main mechanical jar designs:
1) Torsion spring style - These are pre-loaded at surface for the upwards and downwards jarring loads. Jar firing is activated by applying 10-15% torque variations to the drill string (left-hand torque decreases overpull, right-hand increases).
2) Extended sleeve with grooves and latch springs - The load to fire the downhole jar is reduced by increasing the mud pump rate. The Anadrill EQ Mechanical Jar uses this principle.
Hydraulic Jars
Hydraulic drilling jars consist of two piston areas separated by a valve. When tensional or compressional forces are applied to activate the jar, fluid in one piston area is compressed and forced through a restricted flow path into the second area. The flow rate controls the firing time - higher forces give quicker firing times. This stroke distance is called the "power stroke". When the power stroke reaches its limit, the pressurized fluid vents through a bypass allowing the differential piston areas to quickly equalize pressure and impact.
Hydraulic jars do not require pre-set firing loads - the firing force is proportional to the magnitude of overpull or compression applied. Higher forces yield higher impact loads. A key advantage is that hydraulic jars can produce a variable range of jarring forces within their ratings. They also typically have larger internal diameters than mechanical jars for a given tool size.
If time allows, a hydraulic jar will re-cock and fire again once the power stroke completes. This makes them advantageous in high angle or horizontal wells where limited overpull may be available. However, it can also lead to inadvertent firing downhole, especially in vertical sections.
Frequent firing can lead to hydraulic overheating which reduces fluid viscosity, shortens the power stroke time, and causes premature firing before the desired overpull - reducing impact force.
A main advantage of mechanical jars is that they only fire when pre-set force limits are reached. They are more resistant to inadvertent firing and have longer operational lives compared to hydraulic jars.
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