A packer is a piece of steel with a rubber seal, and the rubber is the part that decides whether the zone stays isolated. Packing element selection is often reduced to a temperature number, but the element has to survive the fluid, the pressure differential, the expansion of the casing and the way the packer is set. Getting it wrong does not show up at installation; it shows up as communication between zones months later, when the fix requires a workover.
Where a Packing Element Has to Hold
The element sits between the packer body and the casing, and it has to seal the annulus against the differential that the completion will see. In a multi-zone completion that differential can reverse as different zones are produced or injected, so the element is loaded in both directions. It also has to compensate for casing wear, thermal movement and the relaxation that follows setting. A seal that holds on the day of installation and leaks after the first thermal cycle was the wrong element, not a bad installation. In an injection well the same element sees the differential reverse on every shut-in, which is why injection service is specified separately from production service.
Elastomer Choices by Fluid and Temperature
Four material families cover most applications. Hydrogenated nitrile is the general-purpose choice for oil and moderate temperature; a low-temperature variant keeps flexibility where the well starts cold; FKM handles aggressive chemistry; and AFLAS is the grade for sour gas and high temperature. The selection is a matrix of temperature, H2S content, CO2 and aromatics, and the element supplier should be given the fluid analysis rather than a single bottomhole temperature. The completion fluid matters as much as the produced fluid: a brine that is benign at surface can swell or extract plasticiser from an element at 150 degrees C.
Expansion Elements in Irregular Wellbores
An expansion element is designed to be set, released and re-set. The spring-shouldered design retracts cleanly, which matters on a packer that has to be moved after a test, and it avoids the wire or band shedding that can leave debris in the hole. The rated performance is 150 degrees C and 10,000 psi across 10 set-and-release cycles, in sizes from 4.5 to 7 inches. That cycle rating is the number to compare against the work program, because a packer that is set three times in a workover is outside the assumption behind a single-set element. The retraction behaviour is what protects the casing on the way out, and a torn element on release is a fishing job rather than a re-set.
Compression Elements for HTHP Annulus
Compression elements are squeezed axially by the packer, and the HTHP variants are built for large annular spaces. The air-sealed compression element is tested to V0 grade and seals at 204 degrees C and 10,000 psi in 7-5/8 and 9-5/8 inch casing. The open-hole version extends the range to 260 degrees C at the same pressure and is run with backup rings and anti-extrusion devices, because an element in an ovalised open hole has nowhere to go except into the clearance. The anti-extrusion device is the part that keeps the rubber from being extruded past the backup, and leaving it out to save stack length is a false economy.
Match the element to the fluid analysis, not only to the temperature.
Count set-and-release cycles against the element rating.
Use backup rings whenever the element sees an open hole.
Packing Element Installation and Test
Installation decides as much as material. The element has to be lubricated and seated square, the packer has to be set at the specified load, and the annulus test has to be held long enough to show a stable pressure rather than a slow bleed. On a water-swelling element the volume change is above 1,000 percent in clean water and the expansion takes 2 to 10 days, so the test schedule has to allow for the swell rather than assume it happens on the trip. A test that holds pressure overnight tells the operator more than a ten minute check, because the element has time to relax and the packer has time to move.
A packing element is a wear part with a pressure duty. Give it the right compound, the right number of cycles and a clean setting surface, and the zone stays isolated for the life of the completion.
For more information, please contact China Vigor at info@vigorpetroleum.com or call +0086 29 81161513.
