Jarring 10-16

© 2005 Drilling Systems Ltd DrillSIM Operator’s Manual

10.9.2 Exercise-1 (Snapshot: vjar-1.snp)

A vertical well is drilled with the following BHA:

• 12 ¼” bit

• 19 joints (30 ft) 8 ½”x2.1/4”; 170 lbs/ft drillcollars

• 8 ½” hydraulic jar

• 27 joints (30 ft) 5”x3”; 50 lbs/ft heavy wate drillpipe

• 5”; 22.2, premium S-135 drillpipe

The mudweight is 12.0 ppg.

The rig is a heavy land rig equipped with a Top Drive System (TDS)

The details of the vertical well and the drilling assembly are shown in the spreadsheet below:

The drillstring became stuck during drilling. A stretch test indicated that the string is stuck at the bit or very close to the bit.

Calculations to be made:

As the string became stuck while drilling, it is necessary to start jarring upwards. As the jar is of the hydraulic type, it has to be cocked before it can be fired upwards. There is no accelerator fitted in the drillstring.

The first value to calculate is the hook weight at which the jar is in neutral position.

To calculate the weight of the drillstring above the jar, we need to calculate the drillpipe length:

Bit depth: 11,389 ft Length of BHA below the jar: 570 ft Length of jar: 30 ft Length of BHA above the jar: 30 ft Length of Drillpipe below RKB: 9,949 ft

Jarring 10-17

© 2005 Drilling Systems Ltd DrillSIM Operator’s Manual

This requires: 54.11090949,9

= stands

As full stands are used the number of stands in use is: 111

Total length of drillpipe in use: 111 x 90 = 9,990 ft

Length of drillpipe above the RKB: 9,990-9,949= 41ft (stick up)

Next calculate the weight of the drillstring (in mud) above the jar.

As the mudweight in use is 10.2 ppg the buoyancy factor for steel in this mudweight is

8166.043.650.121 =−

Drillpipe weight: 9,949 x 22.2 x 0.8166= 180,360 lbs Drillpipe stickup weight: 41 x 22.2= 910 lbs BHA above the jar: 30 x 170 x 0.8166= 4,165 lbs HWDP weight: 27 x 30 x 50 x 0.8166= 33,072 Total weight: 218,507 lbs Block weight: 35,000 lbs Top Drive Weight: 50,000 lbs Hook load for neutral point at the jar: 303,507 lbs

As the jar mandrel have to be moved down to cock the jar ready for firing upwards enough downward force has to be applied to overcome the cocking force required and the friction of the drillstring in the hole (above the jar). Total down-force required:

Drillstring / hole friction: 11,389 x 1.05= 11,958 lbs Jar cock force: 9,000 lbs Total weight: 20,958 lbs

Maximum hookload required to cock the jar:

Hook load for neutral point at the jar: 303,507 lbs Down weight: 20,958 lbs Maximum hookload to cock the jar: 282,549 lbs

Note: the hookload is defined as the value indicated on the weight indicator on the driller’s control panel.

On the simulator and in real life, the driller will slack off the hookload slightly below this value to ensure the jar will indeed cock. The largest uncertainty is the hole friction. This can only be estimated from data obtained during drilling.

It is quite normal to assume that the driller will slack off the hookload to say 250,000 lbs.

However, the hookload must not be slacked off too much in order to prevent the jar from firing down. With the drillstring being stuck on bottom this must be prevented at all cost.

For safety reasons calculate the maximum hookload at which the jar will fire downwards.

Hook load for neutral point at the jar: 303,507 lbs Drillstring / Hole friction: 11,958 lbs Jar down force: 35,000 lbs Maximum hookload: 256,549 lbs

Jarring 10-18

© 2005 Drilling Systems Ltd DrillSIM Operator’s Manual

The driller must ensure that the hookload will not be slacked off close to this value to prevent jarring down.

After the jar has been cocked the minimum hookload must be calculated to determine when the jar will fire upwards.

Hook load for neutral point at the jar: 303,507 lbs Drillstring / Hole friction: 11,958 lbs Jar up force: 90,000 lbs Maximum hookload: 405,465 lbs

In the field and on the simulator the driller will pull to at least this value to ensure the jar will fire.

The last calculation is to determine what the maximum hookload is before drillpipe failure will occur. Usually a 10% safety margin is used, but if the condition of the drillpipe is questionable the safety margin could be increased.

90% of breaking strength: 540,000 lbs Add: Weight of the TDS: 50,000 lbs Weight of the blocks: 35,000 lbs Maximum hookload: 625,000 lbs

Exercise for the DrillSIM-5000 simulator.

1. Provide the student with the well, drillstring, jar and Drilling Rig details.

2. The student to perform the calculations as shown above.

3. Load the snapshot (vjar-1) on the DrillSIM-5000 simulator.

4. Cock the jar by slacking off the hookload to 275,000 lbs.

5. Pull the drillstring to a hookload of 420,000 lbs.

6. Wait until the jar fires upwards (remember the time delay).

7. Record the jar fire delay time.

8. After firing, re-cock the jar.

9. Pull the drillstring to a hookload of 450,000 lbs.

10. Wait until the jar fires upwards (remember the time delay).

11. After firing, re-cock the jar.

12. Repeat steps 7; 8 and 9. Increase the hookload in increments of 30,000 lbs.

13. After the maximum hookload has been applied and he jar has fired, print the jarring log and analyse the results.

14. Reload the snapshot (vjar-1).

15. Cock the jar by slacking off the hookload to 275,000 lbs.

16. Pull straight to the maximum allowable hookload.

17. Note that the drillstring will be free with only one blow of the jar.

18. Calculate what the maximum overpull is at the jar when maximum drillstring pull is applied.

19. Make a print-out of the jar log.

20. Discuss the results with the student.