Mechanical Jar

Key Takeaways

• The mechanical jar up-jar cycle begins when the driller picks up weight on the traveling block, stretching the drill string in tension above the jar while the string below the jar (including the stuck BHA) remains immobile; as the driller pulls through the jar's free-stroke range (the distance the mandrel must travel before the latch trips, typically 6-18 inches depending on jar design), the jar mandrel extends while storing the elastic energy of the stretched drill string above the jar as strain energy; when the mandrel reaches the trip point and the latch releases, the elastic energy stored in the stretched drill string above the jar is suddenly released as the collar mass above the jar accelerates downward against the mandrel, delivering an impact force to the stuck BHA that can exceed 100,000 pounds-force in heavy-weight drill collar configurations; the impact force depends on the mass of drill collars above the jar (more mass equals more impact energy), the drill string stretch (more overpull equals more stored energy per inch of stretch), and the jar's mechanical efficiency in converting stored strain energy to impact force at the stuck point.
• Drill collar selection for jarring operations is critical because the impact energy delivered by the jar depends on the mass of heavy collars directly above the jar that can accelerate and deliver kinetic energy to the jar impact mechanism: the standard guideline is to run a minimum of 10-15 drill collars (each 30 feet long and 150-300 pounds per foot) directly above the jar, providing 45,000-135,000 pounds of collar mass that participates in the jar blow; when wells become highly deviated (deviation angles above 45-60 degrees), drill collars tend to lie on the low side of the borehole and much of their weight is transferred to the borehole wall rather than to the axial load on the jar, reducing the effective jarring force delivered to the stuck point; in highly deviated and horizontal wells, fishing and jarring operations often use intensifiers (also called jar intensifiers or accelerators) — spring-loaded devices made up between the collars and the jar that pre-load the jar with additional spring force that supplements the collar impact, improving jar effectiveness in orientations where gravity assistance is reduced.
• Differential sticking is the most common cause of stuck pipe that requires jarring, occurring when the drill string lies against a permeable formation zone where the pressure differential between the hydrostatic mud column and the lower formation pore pressure pushes the pipe against the borehole wall and creates a large contact area covered by filter cake that cannot be overcome by simple pull or rotation; the contact area and the pressure differential determine the differential sticking force (force = pressure differential x contact area), which in a highly permeable sandstone with 1,000 psi overbalance and 10 square feet of contact area equals 1,440,000 pounds-force of sticking force — far exceeding the tensile strength of many drill strings; jarring against differential sticking is effective because the impact force is applied impulsively (in milliseconds), which can break the static friction and hydraulic seal at the pipe-formation contact even when the sustained pull required to overcome the contact area would exceed the string's tensile rating; while jarring, the driller simultaneously attempts to rotate the string (if rotation is possible) and to spot pipe-freeing chemicals (diesel, surfactant, or spotting fluid) at the stuck point to reduce the coefficient of friction and the effective contact area.
• Jar placement optimization in the BHA design is governed by the principle that the jar must be positioned in the drill string at the depth where jarring will be most effective: for a stuck BHA, the jar should be placed as close to the stuck point as possible (because the drill string below the jar is essentially rigid and the energy must travel from the jar directly to the stuck point without attenuation through a long, compliant column), but far enough above the stuck point to allow the jar to trip freely without itself becoming stuck; the standard recommendation is to place the jar 1-3 drill collar lengths above the estimated stuck point, with heavy-weight drill pipe or drill collars filling the interval between the jar and the stuck point; in practice the stuck point depth is estimated by measuring the free point of the string — the depth above which the string stretches elastically under applied overpull, determined by the stretch-per-foot of the string in free pipe — using either the drill pipe manufacturer's stretch tables or a free-point indicator tool run on wireline.
• Jar firing sequence and overpull management during a jarring operation require coordination between the driller (who applies the overpull and controls the jar trip timing) and the company man or well supervisor (who monitors the resulting hook loads, impact vibrations, and any indications of movement in the stuck assembly): the standard protocol is to apply progressively increasing overpull in increments (typically 20,000-30,000 pound increments above the calculated free-rotate load) to build jar impact energy while staying below the drill string tensile rating; the jar is fired repeatedly (5-10 jars per overpull increment) before increasing the overpull to the next level; between jarring sequences, the driller attempts rotation of the string above the stuck point to apply torque at the stuck zone in case the pipe can be twisted free rather than pulled free; if jarring at the maximum allowable overpull (typically 80-90% of the minimum tensile rating of the weakest element in the string) fails to free the pipe, the operation escalates to chemical spotting (soaking with pipe-freeing fluid for 8-24 hours), back-off of the drill string above the stuck point (applying left-hand rotation to unscrew the joint above the stuck collar), or drilling around the stuck assembly with a sidetrack.