A casing patch can be required in the following situations:
- the casing has to be set high before the casing hanger has landed, (well control requirements may dictate that an emergency slip cannot be set)
- a leak has been found in a casing string.
The following guidelines shall be used in the preparation of the casing patch programme provided by the service company:
1. Ensure that ditch magnets have been installed prior to cutting the casing.
Supporting the casing above the cut shall improve the effectiveness of the cutting operation.
2. Clean the hole by pumping high viscosity pills periodically during the cutting operation.
3. Calliper the stub of the recovered casing to ensure that the OD of the casing downhole is accurately known.
General Guidelines
- In cases where the casing must be cemented, the top of the cement shall be located such that casing stretch is allowed and the cement gives the necessary support required after the casing is cut in preparation for applying the patch.
- The following data can be assembled while WOC:
- The free casing weight.
- Can the casing be cut below the rotary table and be left free standing ?
- Does the casing have to be supported after cutting ?
- When the casing cutter is run, how will returns be obtained ?
- Will the position of any of the centralisers interfere with the planned cutting and patching operation ?
- The casing joint lengths.
- The length required for the patch running string. The spacing shall be arranged so that the middle of the patch is approximately in the centre of a casing joint.
If the casing extends above the rotary table, either back out the upper joints or cut the lower casing below the rotary with a cutting torch under a hot work permit. If a power swivel or mud motor is available to drive the casing cutter, proceed as follows:
1. After ensuring that magnets are installed after the shaker screens, run the casing cutter and cut the casing at the designated depth. When cutting casing without supporting the casing above the cut, the cutter can torque up and stall out. If this happens, stop the pump and restart the rotary to ensure free rotation. The pump can now be re-started and cutting continued. The hole shall be cleaned with viscous slugs periodically during the cutting operation.
2. After the cutting operation is completed, fish the free casing out of the hole using a casing spear. The casing stub of the recovered casing shall be calipered to ensure that the OD of the casing downhole is accurately known. RUN a mill to remove burrs from the top of the stub and to chamfer the inside and outside of the stub tip. Pull the mill and check the wear pattern to confirm that the stub has been correctly finished. If a casing hanger is to be run, match the patch to the hanger length.
3. RIH with the patch complete with seals and grapples. A circulating head shall be installed on the running string.
4. The patch shall be landed with the pump running and a pressure increase will indicate the entry of the stub into the seals. As soon as a pressure rise is observed, shut off the pump and open the circulating head to atmosphere to prevent pressure from building up which could support the running string weight.
5. If a hanger is to be used, land the hanger.
6. With the patch over the stub, pull +/- 30 000 lbs the grapple has latched then pressure test the patch. The patch and seal type will determine the test pressure which will be advised by Heads of Drilling.
7. If a hanger is used, re-land it and if a slip and seal assembly is to be used, pull to the required value and install the sealing assembly.
8. If hangers are used, the following procedure shall be employed:
A spear shall be run through the landing string and patch.
Latch onto the casing about 5 m below the top of the casing stub.
Pull enough overpull on the casing stub to support the free casing weight in air plus an extra allowance for patch slip movement. EnsureE that the landing string does not slip.
9. Pressure test the patch.