Conditions for a successful back-off:

a) The string shot strength should be sufficient to back-off the pipe without causing damage to the pipe or the connection. The size of the shot should be advised by the Logging Contractor, and depends upon the depth of the back-off attempt, and whether the back-off is in the drill pipe or drill collars.

Successful back-off between drill collars is statistically low. The best string joints for back-off therefore is at the X-over from drill collars to HWDP (if unstuck). This enables fishing assemblies to   be jarred directly on the drill collar string.

b) The weight of the string at the point of back off should be correctly calculated. Normally the buoyancy effect of the mud is ignored, and the weight of the drill string in air is used. The ideal          weight condition at the point of back-off is a neutral condition. Determination of this condition requires careful calculation, however, where there is any uncertainty, the calculation must be made          so as to leave the break in tension.

The following information is necessary for an accurate calculation:

• The weight of the string when it became stuck. (Weight up and down should be recorded).
• Does the string weight include the kelly, and/or the travelling block assembly?
• What was the pump pressure prior to sticking?

The applied pull is equal to:

The indicated string weight before sticking, minus the buoyant weight of the fish to be left in the hole.

c) Sufficient reverse (back or left hand) torque.

Note: The amount of reverse torque to be applied depends on the pipe used, hole depth and                   deviation and so no hard and fast rule can be laid down. As a general guide the table below gives round figure recommendations.

BACK-OFF TORQUES

Drill Pipe Depth              Turns per 310 m               Turns

Under 1200 m                1/4 to 1/2                           1-2

1200 - 2700 m               1/2 to 3/4                            2-6

Over 2700 m                  3/4 to 1                              6-

Note:    Take into account the age and condition of the pipe.

For aluminium drill pipe twice as many turns as for steel drill pipe are required.

d) Tong and slip dies must be sharp, clean and the correct size to bite and hold the pipe, kelly or whatever other part of the string protrudes above the rotary table.

e) When applying torque with the pipe in the hand slips, prior to installing tongs, the slips handles must be tied with soft line just in case they slip and jump during this operation.

f)  When applying torque the elevators shall be latched around the pipe and free below the tool joint so that the pipe can rotate freely through the elevators. The hook shall be unlocked when the pipe is being rotated in the slips.

g) If the pipe is to be lifted out of the slips without the tongs engaged and biting, ensure that there is no residual torque present to rotate the pipe and so create a safety hazard.

h) The bullplug at the top of the swivel must be well maintained so that if a string shot has to be run through the kelly it is not necessary to remove the gooseneck.

i)  If operational conditions do not allow application of reverse torque it may be necessary to "jump" a  box using either Schlumberger, Dresser, JRC or Hotforge.

When jumping a box the charge shall be accurately placed in the box and maximum overpull applied before detonation. To jump a box run 3 m of 80 grain primer cord for every inch of pipe diameter. If a heavier charge is required, the maximum O.D. of the bundle on the shot bar is determined by the minimum I.D. of the pipe.

1. Procedures For Working Torque Down Hole

It is often necessary to work the reverse torque down the string to the desired back-off depth, especially in crooked or deviated holes. To accomplish this:

a) Prior to applying reverse torque, torque the string to the right. Check the torque amount on the Ampere meter. When later applying reverse torque, the reading on the Ampere meter should not  exceed the maximum observed while torquing to the right.

b) Set the string at the calculated back-off weight.

c) Mark the pipe at the top of the slips and refer to this datum mark at all times. (Thereafter, do not use   the weight indicator because wall friction may cause an inaccurate reading).

d) Apply half of the reverse torque required for the back-off and lock the rotary.

e) Use a jerk line and the rotary tongs to pull off the rotary lock and hold.

f)  Pick up the string off the slips and work the pipe vertically a few times (do not go lower than the "weight mark" as the pipe may break at random).

g) Set the pipe back on the slips at the "weight mark" and pull on the tongs to relock the rotary.

h) If it is believed that the pipe will accept the remaining torque then apply it. If not, then apply half the remaining torque and keep repeating steps (c) to (f) until the full amount of required reverse torque is in the pipe.

Note: Using the rotary on the pipe to apply the full reverse torque can damage the pipe and cause the string to break prematurely. Therefore, use the rotary to apply the first half of reverse torque and complete the operation using the rotary tongs.

2. Procedures For Completing The Back-Off

After detonation proceed to:

-   Check for back off. Usually the pipe has spun free with the box and pin completely unscrewed. In some cases, however, the pipe may only partially back-off and when picking up no indication of back-off will be observed.

-   Complete the back-off:

• a) Apply approximately half of the calculated reverse torque to the string and work the pipe up and down ensuring that the neutral point passes beyond/below the point where the back-off   was shot.
• b) If any torque is lost repeat (a) until the back-off is completed.
• c) If no torque is lost then increase the torque a further increment and prepare for the next back- off attempt. (A re-assessment of string weight to the back-off point is probably called for at this  juncture).

Usual experience is that, providing the above procedures are followed, if the pipe fails to back-off when the charge is fired, no amount of subsequent working will unscrew at the selected joint. Furthermore, subsequent attempts at the same pipe joint will almost certainly fail. The next attempt should be at least one, preferably two joints higher. If the hole is filling rapidly, the string should be backed out of the hole and washover commenced as quickly as possible, so that the hole can be brought under control and washover time minimised.

After a successful back-off POH checking all connections for correct make-up torque.

When POH do not rotate pipe and do not circulate unless necessary.

When pulling the electric wireline tool out of the hole, observe for breaks on the CCL (or equivalent). Also observe the tension meter in case the tool sticks.

For efficient performance during fishing, backing-off, and practically all downhole operations a heave and tide indicator is essential.