Selecting Drilling jars is a major step in drill string design. Here we shall cover the Drilling jar working principle, placement, types, operations & calculations.
In general, the purpose of drilling jars placement in the BHA is to generate upward or downward impact loads to free stuck pipe or release a fish. There are many factors that affect the decision of when to use jars and where to position them for maximum benefit.
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Nowadays, every drill string run in hole use a drilling jar whatever its types. But drilling jars are mainly required in the following situations:
- There are sloughing formations in a given area or at a particular depth.
- There are sensitive swelling shales.
- The Circulating System used does not have good suspension properties to suspend cuttings.
- There is costly equipment in the bottom hole assembly, such as monel Drill Collar, downhole motor or Measurement While Drilling tools that need to be recovered.
Positioning & The Drilling Jar Placement In BHA:
To be of most benefit, drilling jar placement in the Bottom Hole Assembly BHA should be a small distance above the point where sticking is likely to occur. If the jars are placed far above the stuck point, some of the jarring action is wasted in stretching the pipe between the jars and stuck point. Jars are rendered useless if they become stuck or are placed below the stuck point.
Prediction of the location of the stuck point ( check also free point calculation for stuck pipe) is not straight forward and depends on the types of formations drilled, wellbore conditions and driller’s experience in the area.
For example, if a BHA with three Stabilizers is used to drill sloughing shale, it is likely that the shale will slough and pack around all three stabilizers. In this case the jars are often placed above the top stabilizer. Usually, few drill collars are placed above the jars to achieve effective jarring force on the drill collars below the jars.
If differential sticking is likely to occur, all drill collars could become a stuck pipe problem. In this case, it would be advantageous to place the jars in the Heavy Weight Drill Pipe HWDP above the drill collars.
The two main considerations for jar placement are preventing jar fatigue failure and maximizing jar impact at the probable sticking location.
Fatigue Failure Prevention
In the olden days, when holes were vertical or near-vertical, the “rule of thumb” was to make drilling jars placement in “tension” (i.e., above the buckling neutral point). More recently, jar companies say that it is acceptable to run jars in “compression” (below the neutral point) but not at the “neutral point” itself. This is under the misconception that the Tension = 0 at the neutral point.
To simplify matters, we should apply the following rule: Don’t run jars buckled at any time. Obviously, this rule prohibits being drilling jars placement below the neutral point in vertical or near-vertical hole sections.
In high angle holes, the compressive load a jar may carry without buckling will depend on many factors, but can be easily estimated using jar dimensions and the Dawson-Pasley relationship for drill pipe buckling. Regarding the prohibition against running jars in the “neutral point”, this will not always be practical in high angle holes.
For example, in the recent North Sea extended reach well, with a hole inclination of 75 degrees, bit weights varied between 5,000 and 25,000lbs. as stringers were drilled. In the soft formations it was necessary to limit the ROP to clean the hole, thus the lower bit weight. The higher bit weight was necessary to drill the harder rock. The upshot was that the “neutral point” was constantly moving over a range of about 1,800 feet in the string (and past the jars).
Although the drilling jars may have cycled from open to closed, the change in position occurred at low energy levels because care was taken not to add bit weight too fast and no problem occurred with the jars. If you expect a situation like the one above, you should discuss the circumstances with your jar company and develop operating limits to prevent cycling the jar while too much energy is stored in the drill string.
Maximizing Jar Impact
The position at which the string will become stuck is unfortunately not known beforehand, so the drilling jar placement is usually positioned so as to give the highest average freeing force along with the BHA. This drilling jar placement calculations can be done with the help of computer simulation and for many drill string and borehole configurations will turn out to be one drill collar below the top of the BHA.
Hammer & Anvil
This is best understood by envisioning a hammer and an anvil. The mass of the hammer is the single drill collar above the jar. The anvil is of course the rest of the BHA. For upward jarring, the energy for jarring is temporarily stored as elastic stretch in the drill pipe, then suddenly released as the jar trips. The opposite applies for downward jarring, as the jar temporarily supports, then suddenly releases weight slacked off from the surface. For upward jarring at a given string stretch, the mass of the drill collars above the jar will effect the jar impact.
So drilling jar placement and its impact is related as too much drill collar mass above the jar and collars will not gain sufficient speed by the end of the jar travel to hit with the desired force. Also too little and the blow may be insufficient because of inadequate mass coming to an abrupt stop at the end of jar travel.
If the risk of getting stuck is very high, a drilling accelerator should be used. A drilling accelerator is a spring element that provides the energy for the impact of the jar after it trips, thus replacing some of the function of the drill pipe stretch. This can be more effective because the accelerator is less affected by borehole friction than drill pipe and it has a more favorable dynamic force/displacement characteristic. The placement of the drilling jar accelerator is between the HWDP and the top drill collar.
Drilling Jars Types
Hydraulic drilling jars Working Principle
Below we shall give an overview on hydraulic drilling jars working principles. For more details, you can visit Hydraulic Drilling Jar Operation & Working Principle
The most common type of hydraulic drilling jars working principle operates on a time delay sequence wherein hydraulic fluid is metered through a small opening for the initial extension of the mandrel. After moving a small distance over several minutes, the fluid opening size increases dramatically and the jar opens unrestrained. Finally, when the jar has reached the end of its stroke a tremendous jolt is achieved by rapidly decelerating the collars and Drill Pipe above the jars that had built speed during the unrestrained portion of the opening cycle.
The magnitude of the jars impact depends on the tension applied to the jars when they are fired.
The time required for a hydraulic jar to fire depends on:
- the type of hydraulic fluid used
- the size of the metering hole
- the temperature of the hole.
Repeated firing of jars generally increases the hydraulic fluid temperature and, therefore, lowers the viscosity of the fluid and the time required for firing to occur to a point where the jar becomes useless.
The drilling jar working principle can be explained with the aid of this schematic diagram. When the drill string is pulled, the piston moves up inside the housing and forces the hydraulic oil to move down through the small annular clearance between the piston and the housing which restrains the movement of the piston. It takes several minutes for the piston to move out of the small diameter housing. Once the piston moves into the larger ID area, the piston velocity increases dramatically, and when the hammer hits the anvil a tremendous jolt is achieved. The magnitude of the impact is directly proportional to the amount of tensile pull applied on the jars before firing.
The hydraulic jar working principle for firing delay is determined by a combination of load applied and time. Do not exceed the manufacturer’s recommended maximum jarring load and do not exceed the tensile strength of the fishing tools or work string.
Mechanical Drilling Jars Working Principle
Below we shall give an overview on Mechanical drilling jars working principle. For more details, you can visit Mechanical Jar article
Another common type of jars are mechanically operated ones. Mechanical Jars Working Principle is to preset jar at surface or in the shop to fire at a given tension. Mechanical jars use a detent device to delay the firing of the jar until the stretch has been applied to the work string. Detent is defined as a device such as a catch, dog, or spring-operated ball used for positioning and holding one mechanical part in relation to another so that the device can be released by force applied to one of the parts.
Drilling Jar Operations
Jars are approximately thirty feet long. Heavy-duty lift subs for tapered shoulder elevators are typically used for handling.
Jar Safety Clamp
When handling the Jar above the rotary table, the safety clamp must be on the Jar. This keeps the jar in the safe, extended position while being handled. The safety clamp is removed when the Jar is lowered into the hole, see figure 7
Picking Up The Jar
Place the lower drill assembly in the slips.
- Place thread protector on the tool joint thread.
- Clamp elevators around the Lift Sub.
- Pick up Jar with elevators.
Note: Leave safety clamp on the Jar.
- Put Jar in mouse hole.
- Tong up lift sub.
- Make up Jar into lower drilling assembly in rotary table.
- Remove the drill collar clamp
- Lower the string until the slips can be set in the slip section of the Jar.
- Remove Lift Sub.
- Make up next full stand of drill collars of HWDP into top of Jar.
- Tong up drill collars.
Note: Leave Safety Clamp on Jar.
- Pick up drill collars or HWDP.
- Remove the slips.
- Slightly lower the string.
- Remove the Jar Safety Clamp.
- Lower the drilling assembly into the hole.
Establishing the Jar Load
With the Jar in the hole, control of the tool is in the hands of the drawworks operator. Use the Weight Indicator Reading and the Working String Weight above Jar, to establish jar load. Compare the load to the Specification , for the minimum and maximum detent load. In the following examples of drilling jar operation it is necessary to calculate the ‘working’ string weight above the jar and drag before calculating jar load. This weight is calculated as follows:
Working String Weight Above the Jar = Drag + String Weight From The Drilling Jar To Surface.
Drag = Weight Indicator Reading Up – Weight Indicator Reading Down.
CAUTION: To prevent damage to the Drilling Jar, do not exceed maximum detent working load (Jar Specification Table) during up-jar cycle or down-jar cycle.
Example 1: Jar Load “Up”
Up Load on Drilling Jar = Final Weight Indicator Reading Up, before impact – Working String Weight Above the Jar.
Final weight indicator reading up, before impact, is ……..250,000 lb
Working string weight above the jar is …………………………..150,000 lb
Which results in a jar up load of ………………………………………100,000 lb
Example 2: Jar Load “Down”
Down Load on Drilling Jar = Final Weight Indicator Reading Down, before impact – Working String Weight Down.
Final weight indicator reading down, before impact, is 120,000 lb
Working string weight above the jar is ……………………….. 150,000 lb
Which results in a jar down load of …………………………….. 30,000 lb
Changing the Jar Load
Impact can be changed by adjusting the working load on the Drilling Jar.
Changing the Jar Cycle
If the delay time between blows is too short, it can be extended by applying more load when setting the tool. Extending the delay time also makes it possible to apply higher working detent loads, increasing the impact force. shows the normal relationship between load (at the jar) and time (before impact), for a given jar size. The chart can be used to establish the delay time for a given pull (or push) load.
Step 1: Establish the jar load ‘up’ within the range shown in Drilling Jar Specifications Table.
Step 2: Apply pull to the drill string per the established final weight indicator reading, then wait for the Drilling Jar to impact. There will be a small loss of indicator weight just before impact, which corresponds to the retraction of drill string length. There should be a clear indication on the weight indicator after the Drilling Jar has impacted.
Step 3: To repeat the operation, slack off 10,000 to 15,000 lb below the working load down and immediately apply the previous up-jar load.
CAUTION: Do not permit spudding down or dropping larger loads than the jarring mechanism is designed to withstand.
Step 1: Select a jar load ‘down’, within the range shown in Drilling Jar Specifications Table, or within the weight range just above the Drilling Jar.
Step 2: Slack down per the established final weight indicator reading, then wait for the Drilling Jar to impact.
Step 3: Pick up on the string until the weight indicator is above the ‘working’ string load by 10,000 to 15,000 lb, then immediately slack off to the previously selected down jar load.
Step 4: Wait for the Drilling Jar to impact down according to delay time vs. overpull manufacturer charts
Step 5: Repeat Step 3 for additional blows.
Down-jar impacts may not be transmitted through shock tools run in the lower drilling assembly. When jarring down with small amounts of drill collars or HWDP on top of the jar, select a load range that will not buckle the drill pipe, run above the jar.
Up And Down Jarring Operations
Step 1: Select jar load for up and down, as described in Examples 1 and 2.
Step 2: Carry out the up-jar sequence, as described in the Up-Jar Operation.
Step 3: Once the Drilling Jar has impacted up, slack off until the selected down weight on the Drilling Jar is achieved, as described in Down Jar Operation.
Step 4: The weight indicator will reflect when the Drilling Jar impacts down.
Step 5: Repeat Steps 2 through 4 for continuing operation.
Setting The Drilling Jar Prematurely
If the Drilling Jar is prematurely set, the string must be suspended in the elevators and allowed to impact. Following the impact, it may be run to depth. If it is set in the hole, leave the elevators on the pipe until the impact, before continuing tripping pipe operations. When coming out of the hole, do not slack off more than six inches before setting the slips in the rotary or the Drilling Jar may set for an up-jar impact.
Coming Out Of The Hole
- Attach the Safety Clamp on the polished shaft of the jar, as the Drilling Jar comes through the rotary table, and before setting the rotary slips.
- Moderately tighten the two bolts of the safety clamp. Do not over-tighten safety clamp bolts.
Step 2: Set rotary slips on slip section of the Jar. Break off and stand back drill collars or HWDP. (Jar may be changed out at this point.)
Step 3: Make up and tong up lift sub into the Drilling Jar, then hoist out the next stand
Step 4: Stand back Jar with stands of drill collars or HWDP. The Drilling Jar should be at the top of the stand.
Drilling Jar Calculations
Jar Open Force In Drilling Jar Calculations
The jar pump open force (POF) (also called jar extension force) is the pressure difference between the external (wellbore) and internal (drill string) pressures. This differential pressure is the result of the different internal/external surface areas exposed. When a differential pressure exists between the inside of the jar (higher pressure) and the outside of the jar (lower pressure) it produces a resultant force that opens the jar. This differential pressure is present under normal circulating conditions due to the pressure drop across the drill string MWD, Motor and Bit plus any annular pressure drops from the bit up to the jar.
A differential pressure will also exist if pressure is trapped inside the drill string and the same pressure is not trapped in the annulus at the jar. This occurs when the string is packed-off below the jars and pressure is not bled off at the standpipe. The effect of pump open force on jarring can be considerable. For example, if 2000 psi is trapped inside the jar when the string is packed off below the jar the pump open force can be as high as 60000lbs depending on the jar size and type being used. The pump open force charts will be provided by its manufacturer.
Drilling Jar Force Calculations Formulas
Drilling Jar Calculations Formula for calculating the surface weight required to fire jar up (once it has been cocked). Apply a measured weight of at least Uj.
Uj = UpWt – BHA Wt below Jars – POF + Jar up trip force
Drilling Jar Calculations Formula for calculating the surface weight required to start cocking the jar to enable it to be fired up. Setdown to at least a measured weight of Uc (more may be required).
Uc = DnWt – BHA Wt below Jars – POF – Jar fric
Drilling Jar Calculations Formula for calculating the surface weight required to fire jar down (once it has been cocked). Setdown to at least a measured weight of Dj.
Dj = DnWt – BHA Wt below Jars – POF – Jar Dn tripforce
Drilling Jar Calculations Formula for calculating the surface weight required to start cocking the jar to enable it to be fired down. Apply a measured weight of at least Dc.
Dc = UpWt – BHA Wt below Jars – POF + Jar fric
NB1: In a non-vertical well,
BHA.Wt.below.jars = BHA.Wt.below.jars x cos (Inclination).
NB2: trip force = the force (tension or compression) at the jar that is being used to fire it.
NB3: Jar friction is usually about 5000lbs. It is ignored for the firing stroke but included in the calculation for the cocking stroke.
These Drilling Jar Calculations provide approximations to the actual surface forces required.