A casing centralization system is normally selected to support a planned run to total depth while providing the required standoff for cement placement. However, if the casing is POOH and subsequently re-run, the centralizers and stop collars can be subjected to three full passes through the wellbore: RIH, POOH and RIH again.
The primary requirement during retrieval is that the equipment remains structurally intact and correctly retained on the casing. Re-use may be possible following inspection, but the more important consideration is avoiding component failure and debris being left downhole.
How does POOH change the loading?
During RIH, bow-spring centralizers repeatedly flex as they pass through changes in wellbore geometry. In highly deviated and horizontal sections, casing weight also produces substantial lateral load against the low side of the wellbore, increasing friction and side loading.
During POOH, centralizers may have to pass back through:
- doglegs and high-contact-force sections;
- casing shoes and liner tops;
- local restrictions and tight spots;
- washed-out or irregular hole sections;
- milled or exit windows; and
- completion or subsea equipment with reduced internal diameter.
A stop collar resisting axial centralizer movement during RIH will experience loading in the opposite direction during retrieval.
In addition, POOH frequently follows an unsuccessful attempt to progress the casing. Before the decision to pull is made, the string may have been rotated, reciprocated, circulated or subjected to elevated pickup forces. The centralizers and stop collars can therefore be exposed to significant and abnormal loading prior to and during POOH. This is particularly significant in highly deviated and ERD wells.
The centralizer therefore requires a combination of:
- sufficient flexibility to compress through restrictions;
- resistance to axial compression and buckling;
- spring recovery after deformation; and
- structural integrity under repeated loading.
Why structural integrity during POOH matters
The most serious consequence of centralizer failure during retrieval is not necessarily that the component cannot be re-used. It is that part or all of it may remain in the well.
Detached centralizer or stop-collar components can become downhole debris, potentially requiring cleanout or fishing before further operations can continue. If recovery is unsuccessful, the consequences can extend to another casing attempt or sidetrack.
A field example from a highly deviated offshore well in Indonesia illustrates this risk. The well reached inclinations of up to 70° and included washouts and restrictions. During POOH, zinc-alloy solid-body centralizers and stop collars broke apart.
Following an unsuccessful fishing operation, 19 pieces of 9 5/8 × 12 1/4-in. centralizers and stop collars remained downhole. The original wellbore was abandoned and a sidetrack drilled. Centek S2 single-piece bow spring centralizers and 00SO stop collars were then selected for the sidetrack, which was run successfully with no centralizer failures and achieved the required cementing performance.
Axial-compression testing assesses the ability of a bow spring centralizer to withstand increasing axial load without buckling, collapsing or separating into components that could remain downhole. In comparative testing, Centek's S2 withstood 33% higher axial-compression load than the competitor bow spring products tested.
The role of the stop collar
Because forces completely reverse during POOH, a stop collar must deliver equal holding power in both directions. While API RP 10D-2 provides the industry-standard protocol for measuring stop-collar holding force based on up to 4 inches of axial travel, critical operations may demand immediate, zero-slip resistance to prevent downhole component bunching.
Stop collar selection should therefore consider:
- anticipated centralizer running force;
- well inclination and geometry;
- restrictions and changes in diameter;
- the method of centralizer installation; and
- the consequences of axial movement.
The magnitude of required holding force varies considerably by application. For example, Centek's UHD stop collar is rated up to 90,000 lbf for high-load applications, including flush-joint casing where the connection itself does not provide a physical barrier to axial centralizer movement.
Evidence from repeated casing runs
Field cases where casing has been retrieved provide useful evidence of how centralizers and stop collars behave after repeated mechanical loading.
Dammam ERD well
In Dammam, 9 5/8-in. casing was run through a 12,216 ft horizontal section before being POOH due to operational reasons.
The centralizers and stop collars were recovered in good condition and the casing was subsequently re-run to TD. Across the complete operation, the equipment traveled 54,366 ft, including two trips to TD.
Rajasthan, India
In Rajasthan, hole instability required the casing to be pulled. All 139 S2 centralizers installed in the open hole were recovered intact. By comparison, three of five competitor centralizers installed on the shoe track were lost downhole.
Ecuador
In Ecuador, a liner could not progress beyond 7,803 ft and had to be POOH.
Following retrieval, the centralizers showed no bow deformation and the stop collars had remained in position. After a well-conditioning trip, the equipment was re-run successfully.
Across the operation, the centralizers traveled 25,456 ft in a 9,850 ft well design and passed through an exit window twice.
Engineering for an unplanned retrieval
POOH is not the planned operating condition for a casing centralization program, but it is a foreseeable contingency in difficult wells.
For highly deviated, ERD, tortuous or restricted wellbores, the centralization system should therefore be considered not only in terms of restoring force and standoff, but also in terms of its response to repeated and reversed loading.
If casing has to be retrieved, successful re-use of the equipment may reduce delay and replacement requirements. From an engineering perspective, however, the more fundamental requirement is that the centralizers and stop collars remain intact and return to surface with the casing rather than becoming debris that complicates the well further.
