Accelerator

In drilling operations, an accelerator is a downhole tool placed in the drill string above a jar tool to store additional mechanical energy that is released instantaneously when the jar fires, multiplying the impact force delivered to a stuck drill string. When a jar fires, it generates an impulse force by rapidly releasing the tension stored in the stretched drill string above the jar. The accelerator enhances this impulse by acting as a spring: as the jar compresses and then fires, the accelerator's compressed spring element releases its stored energy into the jar blow almost simultaneously, combining with the jar force to deliver a sharper, more powerful impact than the jar alone could generate. Accelerators are most effective when used with hydraulic jars in heavy-weight or long-reach directional strings where the high string weight above the jar reduces the available free-fall energy. Without an accelerator, a jar in a heavy-weight string may deliver a slow, gradual pull rather than the sharp impact needed to break a stuck pipe free.

Key Takeaways

The physics of jar and accelerator operation: a jar is essentially a mechanical fuse in the drill string. It stores tension (on an overpull) or compression (on a downward blow) until a trigger mechanism releases it, generating an impact. The jar converts the elastic strain energy stored in the stretched or compressed drill string into a kinetic energy impulse. The accelerator adds a spring element immediately above the jar: as the jar fires, the accelerator spring discharges its stored energy into the jar blow in the first milliseconds of impact, before the heavy drill collars above can respond. This makes the impact pulse faster and more energetic than the jar alone, concentrating the force into a shorter time window that is more effective at breaking mechanical bonds holding a stuck pipe.
Accelerators are used with hydraulic jars rather than mechanical jars in most modern applications. A hydraulic jar uses a metered fluid bypass to control the firing time: the jar is activated by overpull (or compression) and fires after a time delay as fluid bleeds through a calibrated orifice. The time delay allows the driller to build the desired string tension before the jar fires. A mechanical jar fires at a preset load without a time delay. Hydraulic jars with accelerators are the standard stuck-pipe fishing tool configuration in Alberta and British Columbia horizontal wells, where the long horizontal section means the drill string has very high drag and the jar cannot take advantage of free-fall weight from above.
The accelerator is rated by its stored energy capacity, typically expressed in joules or foot-pounds of spring energy. Common accelerators have energy ratings of 3,000 to 15,000 joules. The energy rating is matched to the jar's rated impact force and the weight of the drill string above the jar. If the accelerator is too weak relative to the jar, it does not significantly increase the impact force. If it is too strong, it can overstress the jar mechanism or the fish (the stuck pipe being jarred loose).
Placement of the accelerator in the bottom hole assembly (BHA) is critical. The accelerator must be placed between the jar and the heavy drill collars above it, so that the spring energy is discharged into the jar blow before the inertia of the heavy collars can dampen the impact. Placing the accelerator below the jar (closer to the bit than the jar) negates its benefit entirely because the spring energy would then compress the fish rather than driving the jar blow. In a standard fishing BHA, the order from top to bottom is: drill pipe, heavy weight drill pipe, accelerator, jar, safety joint, fishing tool (overshot or spear).
In cement operations, a different type of accelerator (a cement accelerator additive) is used to shorten the thickening time and setting time of cement slurries. Common cement accelerators include calcium chloride (CaCl₂), sodium chloride (NaCl at certain concentrations), sodium silicate, and potassium chloride. These chemical accelerators are added to the mix water and work by increasing the rate of calcium silicate hydrate formation (the reaction that causes cement to set). This cement accelerator is entirely separate from the mechanical accelerator tool and shares only the name.

How a Jar and Accelerator Work Together

Think of a jar and accelerator as a slingshot and a spring working together. The jar is the slingshot: you stretch the elastic (the drill string under tension) until it releases and snaps the projectile forward. The accelerator is a compressed spring loaded alongside the elastic band: when the slingshot fires, the spring pops simultaneously and adds extra energy to the projectile at the exact moment of release.

In the real drill string, the driller applies overpull above the jar by slacking off the traveling block and then picking up more weight than the string should need to support, stretching the pipe above the jar. The hydraulic jar lets the string stretch for a calibrated time while the hydraulic fluid bleeds through its orifice. When the orifice clears, the jar fires: the upward tension in the string drives the jar mandrel upward, delivering an upward blow to the fish below the jar.

At the exact moment the jar fires, the accelerator spring, which has been compressed by the overpull, releases its stored energy downward into the jar mechanism, adding to the upward blow delivered to the fish. The combined impulse lasts only a few milliseconds but can deliver forces of 100,000 to 400,000 pounds, far more than could be achieved by simply picking up the drill string slowly.

Fast Facts

Mechanical jar tools have been used in rotary drilling since the early 20th century, when primitive jar tools were used to free stuck cable tools and early rotary strings in shallow wells in the US Gulf Coast. Hydraulic jars became the standard in the 1950s as wells became deeper and the need for controlled firing time became apparent. The first downhole accelerator tools designed specifically to enhance hydraulic jar performance were commercialized in the 1980s as extended-reach and horizontal drilling became more common in the North Sea and western Canada. Manufacturers of downhole accelerators active in the Canadian market include National Oilwell Varco (NOV), Hunting, and Weatherford. In the Montney and Duvernay plays, where horizontal laterals can be 3,000 metres long and string drag is very high, jar-accelerator combinations are included in the BHA as a standard stuck-pipe precaution on most wells.

When Accelerators Are and Are Not Used

Accelerators add cost to the BHA rental and to the risk of a fishing job if the accelerator itself gets stuck. They are therefore not used on every well but are specified when the risk of differential sticking or mechanical sticking is elevated.

In Alberta and British Columbia, accelerators are routinely specified for: horizontal wells in the Montney, Duvernay, and Cardium where the lateral section has high drag; wells through high-clay shale intervals where clay swelling and differential sticking risk is elevated; wells where the bottomhole temperature is high (above 150°C) and the hydraulic jar's fluid-bypass calibration is temperature-sensitive; and any well where a lost-circulation zone or formation overpressure transition is anticipated, because these conditions are associated with stuck pipe risk.

For short vertical wells in conventional formations where stuck pipe risk is low, the accelerator is typically left out to simplify the BHA and reduce the fishing risk from the accelerator tool itself.

The drilling accelerator is also called a jar intensifier or jar booster. Related terms include jar (a downhole tool that generates an impact force to free stuck pipe by suddenly releasing stored energy from the stretched drill string; the primary use case for accelerators is enhancing jar performance), stuck pipe (a drill string that cannot be rotated or moved vertically due to differential pressure, mechanical keyseating, wellbore collapse, or cement setting; the drilling problem that jars and accelerators are designed to address), fishing (the process of retrieving equipment lost or stuck in the wellbore; a fishing BHA typically includes a jar and accelerator above the fishing tool to provide impact force if the fish does not come free easily), bottom hole assembly (BHA, the lower portion of the drill string including the bit, stabilizers, drill collars, and specialty tools; the jar and accelerator are placed near the top of the BHA above the weight collars), and differential sticking (a form of stuck pipe where the drill string is held against the wellbore wall by the pressure differential between the mud column and a low-pressure formation; the most common cause of stuck pipe in weighted mud systems; freed by reducing mud weight or by jarring with a jar-accelerator combination).

How a Jar-Accelerator Combination Freed a Stuck String at 4,800 Metres in the Foothills

An operator was drilling a Triassic Halfway Formation target in the Foothills of northeast British Columbia at approximately 4,800 metres measured depth. The horizontal section was 1,800 metres long and had been drilled in an oil-based mud system at 1.45 SG. While making a connection at 4,780 metres, the drill string stopped reciprocating: the pipe was differentially stuck against a permeable Montney sandstone interval that was slightly underbalanced relative to the mud column at that depth.

The driller immediately began jarring upward. The jar was a hydraulic jar with a calibrated firing load of 80,000 kilograms. An accelerator with a 6,000-joule spring rating was positioned immediately above the jar. The driller applied 100,000 kilograms of overpull, held it for the jar's firing delay (approximately 90 seconds), and the jar fired with a sharp crack that was felt at surface through the rotary table vibration. The string moved 15 centimetres upward.

After 12 jar cycles over 4 hours (alternating upward and downward blows), the string broke free and full reciprocation was restored. The total overpull applied was 110,000 kilograms (well within the 140,000 kilogram rated capacity of the 5-inch drill pipe at that depth). Post-free analysis estimated that the accelerator contributed approximately 40 percent of the total impact force per blow. Without the accelerator, the jar alone would have delivered weaker blows and might not have freed the string before the maximum overpull limit was reached, potentially requiring a sidetrack at a cost of CAD 2.8 million. The fishing operation cost 4 hours of rig time at CAD 35,000 per hour, or CAD 140,000 total, compared to the avoided sidetrack cost.