Sealbore: Polished Bore Receptacles, Seal Assemblies, and Tubing Movement in Production Packers

A sealbore is a precisely machined and polished internal cylindrical surface, built into a downhole tool such as a permanent production packer, a liner top, or a dedicated polished bore receptacle, that is designed to accept a matching seal assembly and form a pressure-tight, sliding metal-and-elastomer seal between the production tubing and the packer bore. The seal assembly, a mandrel carrying stacked seal elements, is run on the bottom of the tubing string and stabbed into the sealbore, so that produced fluid is contained entirely within the tubing and the packer without leaking into the annulus, while still allowing the tubing to move axially relative to the fixed packer. That combination of sealing and permitted movement is the whole point of the design. When a well is produced or shut in, the tubing heats and cools, and it lengthens or shortens by an amount that can reach many centimetres over a deep string; if the tubing were rigidly fixed at both the wellhead and the packer, that thermal and pressure-induced length change would generate large buckling or tensile loads. By landing the seals in a long, honed sealbore, often extended with a seal bore extension below the packer, the seal stack is free to stroke up and down within the polished surface while never leaving the sealed length, so the connection behaves as an expansion joint and the tubing movement is absorbed harmlessly. The sealbore surface itself is honed to a fine finish and close tolerance so the elastomer or metal seals ride against it without cutting, and the assembly is typically held during run-in by shear screws that release at a set weight, letting the seals float. The same geometry doubles as a separation point: because the seals only stab into the bore, the tubing can later be pulled and re-stung, which simplifies workovers and future recompletion. In Western Canadian Sedimentary Basin completions, sealbore packers are standard in production packer installations across Cardium, Viking, and Montney wells and in deeper sour Nisku and Leduc completions, where NACE MR0175/ISO 15156 materials are required for the seals and bore in H2S service. Dimensions are specified in inches alongside mm, pressure ratings in psi alongside kPa, and the completions are permitted and reported under AER Directive 065 for well construction and Directive 020 for well operations.

Locator, Anchor, and Floating Seal Assemblies

How the seal assembly engages the sealbore defines the completion's mechanical behavior. A locator seal assembly stabs in and lands on a no-go shoulder, fixing tubing position while still allowing limited movement above; an anchor seal assembly latches into the bore to resist both up and down loads when the string must be held; and a floating or dynamic seal assembly is free to stroke the full length of the bore to absorb maximum tubing movement. The choice depends on expected thermal cycling, packer type, and whether the tubing must be anchored for artificial lift or left free to expand. Getting this selection right prevents seals from stroking out of the bore under extreme contraction.

Sealbore Length and the Tubing Movement Calculation

Completion engineers size the sealbore and any seal bore extension by calculating total tubing movement from thermal expansion, ballooning under internal pressure, buckling, and piston effects across the well's operating envelope. The polished length must exceed the sum of maximum elongation and contraction so the seals never exit the sealed surface. In a deep WCSB well cycling between injection cool-down and hot production, that calculation can call for a seal bore extension of a metre or more. Under-sizing the bore lets seals stroke out and lose containment; over-sizing wastes cost and rathole, so the movement calculation is a fundamental design step.

Related Terms

A sealbore is a feature of a production packer, the tool that isolates the annulus and anchors the completion, and it receives the seal assembly that carries the actual sealing elements on the tubing. The sealed connection defines the top of the tubing string that conveys produced fluid to surface, and the polished-bore-receptacle version of the sealbore is central to well completion design where tubing movement and future retrievability must both be managed.

Real-World WCSB Scenario: A Deep Sour Nisku Completion Near Drayton Valley

An operator completing a deep sour Nisku gas well near Drayton Valley set a permanent production packer at roughly 3,400 m and needed to manage large tubing movement between shut-in and hot sour production while meeting NACE MR0175/ISO 15156 for the H2S environment. The completion design added a 1.5 m seal bore extension below the packer and specified a floating seal assembly with sour-rated elements, so the seals could stroke through the full range of thermal elongation without leaving the polished bore. The sealbore hardware and sour-service seals added roughly CAD 180,000 to the completion.

Over the first two years of cyclic production the sealbore connection held annulus pressure at zero, confirming a competent seal, and when a downhole gauge later required replacement the tubing was simply unstung from the bore and re-run on the same packer. Avoiding a packer milling and re-set on the deep sour well saved an estimated CAD 900,000 and several days of rig time, a direct return on the sealbore-based retrievable design.