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Sidetracking out of Casing with Whipstock

The ‘Window Master’ whipstock is a one trip system to mill afull size hole in casing in the required direction. The whipstock is mechanically set and requires a bridge plug to be set at a suitable depth so that weight can be put down on it to initiate the setting process.

Dogleg Severity

The dogleg caused by the whipstock is initially +/- 6o/100 ft. However by the time the pocket is milled, to allow the full gauge watermelon mill to exit the casing, the effective dogleg will be +/- 2o/100 ft, due to the tendency of the pilot mill to slide down the formation. This should be taken into account when planning the sidetrack.

Torque and Directional Control

The torque applied in milling the window will have some effect on the actual direction of the window. Once the casing has been exited, directional control surveying is difficult, due to the effect of the casing, until sufficient departure has been achieved.

Pre-Job Planning

Introduction

When planning a sidetrack,ensure that casing collars and/or centralisers will not be milled. It should be borne in mind that if the hole angle is close to vertical a Gyro (UBHO included in BHA) will be required to orientate the Whipstock face. An MWD can be used if the hole angle is higher than 6o using high side readings.

Gyro’s

If planning to use a Gyro, ensure the bottom static temperatures do not exceed the working limit of the gyro, if they do circulate and spot cold mud immediately prior to running the gyro.

Sidetracking

The Sidetracking programme should include the longitude as well as the latitude, or if using the UTM system, the Central Meridian should be included. In addition it is important to:

  • make drawings of everything that will be RIH, especially the stroke of the whipstock and distance from the starter mill until the full size watermelon mill will be out of the casing.
  • set the bridge plug, as close as possible to the depth, where the window has to be milled to activate the setting anchor where a kelly is used (a top drive allows more flexibility).

Note: Once the anchor is activated it will no longer be possible to lower the string.

  • space out the string such that a minimum of 10 ft, but preferably 20 ft +, of the kelly is above the table once the whipstock is set, otherwise the window mill cannot be retracted from the whipstock assembly due to the presence of the kick-off pad (the 8in water melon mill, positioned on top of the window mill, has to mill this pad first to give a 8in clearance).

Note:Once the whipstock is set, the kelly can’t be raised, unless the Whipstock is pulled up as well (if still possible) which could have serious well control implications. For this reason a CBL is required and milling should only take place if hydrocarbon bearing zones are isolated

  • plan for a milling fluid with a good carrying capacity for metal eg., high yield point, and sweep hole at regular intervals
  • plan the job with the contractor and consider the mill requirements for the type and weight of casing and the formation behind the casing
  • ensure fishing equipment for fishing whipstock is available
  • locate ditch magnets in flowline.

Whipstock and Milling Assembly

The whipstock and drilling assembly comprises:

  • whipstock anchor
  • whipstock
  • window mill, undergauged, c/w shear sub
  • watermelon mill, undergauged
  • flex joint
  • watermelon mill, full gauge
  • 1 joint HWDP
  • UBHO sub
  • 9 joints drill collars
  • 12 joints HHWDP
  • Jar
  • 3 joints HHWDP.
  • drillpipe to surface.

Notes

  • UBHO should be aligned to the whipstock face.
  • Mills shall be gauged.
  • Shear bolt shall be secured with a lock ring and installed in downshear mode.
  • Ensure ‘bit’ breaker is available for window mill.
  • Assemble and torque up milling assembly prior to picking up Whipstock assembly and Anchor.

Running and Orientating Whipstock

When running and orientation the whipstock the following procedure should be followed:

1. RIH the Whipstock assembly to one joint above the bridge plug.

2. Space-out the string.

3. Run Gyro while working torque out of the string.

4. Orientate whipstock face to the required Azimuth +/- 30 deg.

5. POOH Gyro.

Notes:

  • RIH at controlled speed and set slips gently to avoid weakening shear bolt.
  • Use drillpipe wiper while RIH.
  • Check UBHO depth, in relation to pipe tally, while running the gyro wire line.

Setting Whipstock

To set the whipstock:

1. Tag bridge plug with the kelly installed with amaximum of 2 tonnes and CHECK spaceout, (minimum 10 ft, preferably 20 ft plus, of kelly above the table).

2. Pull back 3 ft and circulate to milling/drilling mud if required.

3. Take up and down readings with and without pumps on.

4. Lower the string and activate the anchor, by applying weight on top bridge plug, the shear pin should shear with +/- 10 000 lbs.

5. Set the anchor with 15 000 - 18 000 lbs down weight.

6. Check anchor set by pulling 5 000 lbs above pick-up weight.

7. Shear the window mill from the whipstock in stages of 10 000 lbs until the bolt shears +/- 35 000 lbs.

Milling Window

The following are only guidelines and should be adjusted to suit the actual conditions after discussion with the contractor supplying the equipment:

1. Break circulation and PUMP at +/- 3 bbl/min.

2. Pick-up to neutral and attempt to ROTATE string at +/- 20 rpm, record the required torque.

3. Slowly increase pump rate to +/- 5 bbl/min and increase rotary speed to +/- 45 rpm while maintaining neutral weight.

4. Make initial cut with 1 000 - 3 000 lbs WOB, maintaining these parameters for at least 3 ft.

5. Increase parameters gradually to +/- 80 to 100 rpm, 4 000 -
10 000 lbs WOB and pump rate according to mud parameters and returns seen on surface.

Note :  As a rule of thumb for milling casing in general, when milling well cemented casing, the penetration rate can be increased by using higher weight and rotary speed. Conversely, when milling badly cemented casing, low weight and rotary speed should be used.

6. Mill and drill pocket sufficient enough to ensure that the full size watermelon mill has exited the window.

7. Backream gently to starting position and ream window once more.

8. Check window clear by lowering the mills through it without rotating. If more than 3 000 lbs excess drag is observed REAM the window once again (excessive reaming will lead to excessive wear on the Whipstock slide).

9. Sweep hole and POOH.

10. Re-measure the diameter of the upper watermelon mill when the milling assembly is on surface. If the mill is more than 1/10in undergauge a reaming trip with a full gauge mill should be made.