Stuck Casing - Remedial Action

In the event the casing string becomes stuck and all attempts to get it free are unsuccessful then , remedial action options shall differ according to the type of wellhead that is being employed and the depth of stuck point in relation with the different formation. The option to set the casing high can only be considered when using Surface Wellheads.

If the casing hanger has not landed and the casing string cannot be freed, it will be necessary to cut the casing, by either:

• retrieving as much casing as possible above the free point prior to sidetracking the hole

or...

• cutting the casing immediately below the wellhead and retrieve the casing hanger. Set a casing patch after rerun the casing hanger.

Setting the Casing High

If circulation is possible and the shoe is near the planned setting depth, the casing can be cemented in place to serve its original purpose.

Prior to taking this action, consideration should be given to:

• the amended casing depths and cementing proposal which must be approved by the Operations Engineer.
• the possibility of having to run an extra string of casing to serve the intended purpose of the stuck casing
• for casing strings comprised of differing weights and grades, the casing design factors must be checked for the new setting depth and approved by the Head of drilling operations.

Allowable Pull on Casing

When pulling on casing, the maximum total surface load on the casing shall not exceed the lesser of either:

the pipe body yield strength or thread yield strength (of top pipe) ÷ 1.6 (SF)

or

(the weakest pipe or thread + Weight in air of casing above it) ÷ 1.6 (SF)

Maximum Allowable Pull on Casing

When pulling on casing, the maximum total surface load (NOT overpull) on the casing shall not exceed the lesser of the following values:

Either

the TOP PIPE body yield strength or thread yield strength
                                             1.6

Or

the weakest pipe or thread + Weight in air of casing above it

                                             1.6

Example

13.3/8in, N-80 68 lb/ft, Buttress thread

Yield strength of the pipe        =          1 661 000 lbs

Yield strength of the thread     =          1 693 000 lbs

Maximum total load = 1 661 000 =          1 038 125 lbs

                                              1.6

9.5/8in casing, P-110 47 lb/ft, BTC from 0-3 000 ft
9.5/8in casing, N-80, 47 lb/ft, BTC from 3 000 ft to TD

Pipe is stuck below 3 000 ft

Yield strength of P-110 pipe    =          1 493 000 lbs (1 500 000 lbs for thread)

Yield strength of N-80 pipe     =          1 086 000 lbs (1 161 000 lbs for thread)

Maximum total load is the lower of the following:

1 493 000 lbs = 933 125 lbs
         1.6

OR

1 086 000 lbs + (3 000 ft x 47 lbs/ft) = 766 937 lbs

                            1.6

In this case the lower value calculated in the second equation would be used.

Notes

• Regardless of the calculated allowable loads, the safety factor for the drill line shall never be less than three. This may turn out to be the limiting factor instead of the casing strength.
• When there are angle changes in the hole and/or internal pressure inside the casing, the allowable surface load shall be restricted to the values given by API Bulletin 502.
• If the casing is stuck in such a way that no circulation is possible, the string can be pressured up and then bled off before applying the extra pull. This will provide an upward force below the floats in addition to the pull applied at the surface. Great care shall be taken when using this method to ensure that formation fracturing does not occur.

Surface Wellheads

Although the casing cannot be landed on its hanger pre-installed in the string, emergency hangers can be installed around the pipe and dropped into the wellhead.

This procedure is not always straightforward and the following cases may have to be considered:

In both cases, the pipe shall be mechanically cut at the proper height to ensure that there is sufficient length of casing above the hanger to allow normal installation of the seal assembly.

The BOP shall not be lifted until either a slip and seal assembly has been run and tested or the cement is hard.