During drillstem tests (DSTs), if a packer or tailpipe becomes stuck, the hydraulic jar is used to provide an upward-moving shock that will help pull the string loose.
The jar consists of a housing and a mandrel that move relative to each other. An oil chamber, separated by a flow restrictor in series with a check valve, is located between the housing and mandrel.
Applying weight to the packer causes the jar to close. If the tools below the jar become stuck, an overpull is applied to the string, which causes the jar to begin metering hydraulic oil. As the oil flows slowly through the restrictor from the upper chamber into the lower chamber, elastic energy is stored in the string. The mandrel housing moves until a seal ring is uncovered, at which time the housing rapidly accelerates to produce an upward impact on the stuck tools.
After the jar has tripped, the string weight is set back down on the packer to reset the tool. Oil flows rapidly through the check valve and back into the upper chamber, allowing jarring to be repeated as necessary.
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