Hydraulic drilling jars consist of two pistons separated by a valve. When tension or compression is applied to the armed hydraulic jar, fluid in one piston is compressed and flows to the other piston against high flow resistance. The flow rate controls the time required to arm the tool - higher pulling/pushing forces result in shorter arming times. The moving distance is called the stroke. When the stroke reaches a certain position, the compressed fluid suddenly fully releases through a bypass valve, shooting the fluid toward the second piston and instantly equalizing pressure between the pistons. The greater the force applied, the higher the compression of the fluid, the shorter the arming time, and the stronger the jarring force generated.
Advantages and disadvantages of Using Hydraulic drilling jars
Hydraulic jars do not require presetting a trigger threshold force. The timing of firing and the magnitude of the jarring force depends entirely on the amplitude of pulling/pushing. In upward jarring, the jarring force is directly proportional to the pull force - higher pull forces result in harder jar impacts.
Therefore, its advantages are the continuous variable jarring force within operational limits, and larger IDs compared to mechanical jars for the same OD tools.
After re-arming when sufficient stroke time elapses, hydraulic jars will fire again. This gives hydraulic jars unique advantages for high-angle inclined and horizontal well sections where high friction may prevent sufficient pull/push force from being applied to mechanical jars. Hydraulic jars will eventually fire even with minimal applied tension/compression. However, a disadvantage is possible premature firing and dropped assemblies, particularly in vertical wells.
Frequent firing leads to fluid overheating, reduced viscosity, shorter stroke times, and premature firing below-desired pulls. The result is lower jarring intensities.
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