The one-sentence difference
Casing is the pipe that becomes part of the well; tubing is the pipe that travels. A casing string is cemented in place for the life of the well. A tubing string is the string the programme pulls and re-runs over the life of the well.
The confusion is understandable. Both are specified under the same standard, API Spec 5CT. Both are described by outside diameter, weight, grade and thread. A buyer new to oilfield supply can be forgiven for treating them as the same product in different diameters — until the first time a tubing coupling is ordered against a casing specification, or a completion is designed around casing that was never meant to be pulled. In the ranges this factory publishes, casing runs from 4-1/2 in to 20 in, and tubing is named in sizes from 1.05 in to 3-1/2 in. The published coupling stock list runs from 1.315 in to 4-1/2 in across eight named sizes.
Size and position in the well
A well is telescopic. The published size list names sizes across the range — 20, 18-5/8, 16 and 13-3/8 in at the large end, 9-5/8 and 7-5/8 in in the middle, 7 in and 5-1/2 in below them — and the design states which size each section of the programme takes: the surface casing, the intermediate strings and the production casing or liner that reaches the reservoir. The published tubing list names sizes such as 2-3/8, 2-7/8 and 3-1/2 in. When someone gives you a size, the first question is always: which string does it belong to?
Duty: loads, pressure and time
The duty for each string is the one its design states. The casing design states the loads for each interval, and the grade each section takes is set by the casing design against those loads; the published list names J55, K55, N80, L80 and P110, with the chromium designations listed separately. The tubing design states its own duty, including the pulling frequency the programme expects. Where a design names corrosion-resistant tubing for the production string, this catalogue lists L80-13Cr in 2-3/8 in to 4-1/2 in.
Threads and connections
Casing threads are the API round and buttress family — STC, LTC and BTC — with premium gas-tight connections where the design names one. Tubing threads are named by end finish in the thread-type list: EUE, the external upset end, or NUE, the non-upset end, whose coupling is the slimmer one. Couplings reflect the split too: casing couplings and tubing couplings are both phosphated, magnetic-particle inspected and marked in accordance with API Spec 5CT, as the factory's coupling pages describe. Casing packing in the published material is by steel strap or steel frame, or as the customer requires; coupling packing is cartons, wooden cases and pallets, or as the customer requires. The published thread-type list names NUE, EUE and the special-clearance form for the tubing side.
Accessories behave differently
Both strings use pup joints — short lengths that adjust total string length — but for different landings: casing pup joints for the casing landing the programme states, tubing pup joints for the spacing the completion design names. Crossovers appear in both worlds wherever size or thread changes. The company's casing literature states that casing and tubing pup joints of various sizes can be machined to order. The published text also names conversion joints and crossover couplings as customisable to customer requirements. The accessory logic is shared; the design document behind the order is not.
How each is bought
The two families are specified differently at enquiry stage. A casing specification is written interval by interval: each section its own size, weight, grade and thread, priced line by line, with the inspection and documentation the tender names. A tubing specification is written string by string, and its couplings and pup joints are stated as separate line items alongside the pipe, each with its own size, grade and thread. The practical rule for buyers: never let one purchase order blur the two families. Keep casing lines and tubing lines separate, each with its own full specification.
Receiving, tallying and the yard test
State for each family what your receiving check has to verify — for casing, grade and thread section by section; for tubing, the items your own completion documentation names — and the quotation confirms the marking and documentation that can be provided with the order to support that check. Because the tubing string will be pulled and re-run, what its receiving record should carry beyond location is settled by the buyer's own records practice, stated in the enquiry — not by a tally rule this page states. Marking discipline is the defence: couplings are phosphated and marked in accordance with API Spec 5CT, and for pup joints, the measured-length marking your tally needs is stated in the enquiry, for the quotation to confirm whether it can be accepted and in what scope.
There is also a documentation question worth settling at enquiry stage. Casing is cemented in place for the life of the well, so state the documents you need section by section in the enquiry: mill and inspection documents for each interval, in the form your well file needs. A tubing enquiry can state the traceability form it needs — per-batch or per-joint — rather than assuming one. State the documentation your records need in the enquiry; the written quotation confirms what can be provided with the order.
Can tubing be used as casing, or casing as tubing?
Not by substitution on a purchase order. The products differ in size range, connection practice and design intent even where diameters overlap. If a design genuinely calls for an unconventional use, it will say so explicitly, with its own specification.
Which one uses premium connections more?
Premium patterns appear on casing and tubing strings alike in the catalogue, and which intervals carry one is stated in the design. The driver differs: casing connections are specified for the string cemented in place, tubing connections for the string the programme pulls and re-runs — state the duty your design assumes in the enquiry.
Request a Quotation
Send your size, grade, thread and quantity. MOQ, packing and lead time are confirmed in the quotation for your exact specification — no generic promise replaces a written quote.