A string of casing can be run at a later date after a liner has been installed and joined to it as long as the liner hanger assembly top (or weight set packer top, if it has been run) has been provided with a setting sleeve with tie-back extension (standard machined or PBR).
Reasons to run a tie-back string of casing include:
- damaged production casing due wear or corrosion
- upgrading of the production annulus to a higher working pressure, e.g. reservoir pressure under-estimated.
The tie-back string is run with a tie-back stem and after spacing out the string is hung off (the string may have been cemented before being hung off).
The tie-back stem shall be 15 ft long to fit in a 15 ft PBR or standard machined tie-back extension with three sets of pressure-actuated moulded seals with half mule shoe entry guide on bottom.
The following procedure should be used when running a liner tie-back string:
1. RUN IN with the casing with shoe track configured as follows:
- tie-back stem
- one full joint of casing
- orifice float collar (without back pressure device) or baffle collar.
2. TAG the top of the tie-back extension or PBR by installing a circulating head when approaching it and circulating slowly while observing for a pressure build-up. As soon as the pressure build up occurs, SHUT OFF the pumps . The pressure shall only be allowed to build up to a maximum value 250 psi.
3. BOTTOM OUT the string by applying 1 000 lbs weight down. PRESSURE TEST the casing and tie-back seals to 3 000 psi, BLEED OFF pressure.
4. PULL BACK, SPACE OUT the casing, and INSTALL the hanger assembly. MAINTAIN accuracy within 0.3 m while spaced out to the 1 000 lbs weight down position.
Note: In case the string is not to be cemented, the space out will be advised.
5. LOWER casing again while pumping. After a pressure build up occurs (indicating that the first seal has entered the seal bore) CHECK the space-out and CORRECT if necessary. BOTTOM OUT to double check the space out.
6. PULL the string above the tie-back position.
7. SLOWLY LOWER string while a decrease in string weight will indicate that the stem seals are entering the tie-back extension or PBR. BLEED OFF pressure and OPEN casing to atmosphere while lowering the string until the casing is landed.
8. CIRCULATE the casing contents + 25 % and CEMENT as per the Cementing Programme. TEST casing as required in Pressure Testing -Tie-Back Packers of this chapter after bumping the plug.
9. BLEED OFF test pressure. HOLD enough pressure to contain the hydrostatic differential pressure from the annulus.
10. In the event of a stem seal leak, to avoid back flow, CLOSE casing while watching for a pressure build-up in the string. If no build-up of pressure occurs, BACK OFF landing string or INSTALL emergency hanger assembly. If build-up occurs, DELAY rigging down until the cement is hard.