tubulars, Oil & Gas Glossary

Tubulars is the generic oilfield term for any tubular steel goods run into or installed in a well, spanning the entire string inventory of a drilling and completion program: drill pipe, drill collars, heavy-weight drill pipe, pup joints, surface and intermediate and production casing, liners, production tubing, coiled tubing, and surface flowlines and pipeline. The category is often split into oil country tubular goods, abbreviated OCTG, which covers casing, tubing, and drill pipe manufactured to API or proprietary premium specifications. Each tubular is defined by its outside diameter, wall thickness, weight per foot or per metre, steel grade, and connection type, and these parameters together determine the pipe's collapse rating, burst rating, axial tension capacity, and corrosion resistance. Drilling tubulars carry the rotary torque and weight-on-bit needed to make hole, with drill collars supplying compression at the bit and drill pipe transmitting rotation and circulating drilling fluid down to the bit and back up the annulus. Casing is cemented in place to isolate formations, support the borehole wall, protect groundwater, and provide a pressure-tight conduit, while production tubing is the removable inner string through which hydrocarbons flow to surface, kept replaceable so it can be pulled for workover without disturbing the cemented casing. Grades follow API 5CT for casing and tubing and API 5DP for drill pipe, with familiar designations such as J55, L80, N80, P110, and Q125 denoting minimum yield strength in thousands of psi, and sour-service grades like L80 Type 1 or C90 and T95 selected where hydrogen sulphide demands compliance with NACE MR0175 and ISO 15156 to resist sulphide stress cracking. Connections range from API round thread and buttress to premium gas-tight metal-to-metal seals from vendors used across the Western Canadian Sedimentary Basin. In WCSB operations, tubular selection is dictated by well design under AER Directive 010 for casing and cementing minimum requirements, by anticipated pore and fracture pressures, by the corrosive chemistry of sour Nisku, Leduc, and deep Montney intervals, and by the high tensions and dogleg fatigue imposed by long horizontal laterals. A single Montney or Duvernay horizontal can consume well over 4,000 m (about 13,100 ft) of production casing plus a comparable length of tubing or a frac-string, making tubulars one of the largest single line items on an authorization for expenditure after the rig and the completion. Tubular failures, whether a parted drill string, a collapsed casing, or a corroded tubing leak, rank among the most expensive and dangerous downhole events, so material grade, connection integrity, running procedures, and inspection are governed by tight engineering and regulatory standards.

Key Takeaways

OCTG Is The Core Category: Oil country tubular goods, casing, tubing, and drill pipe, form the heart of the tubulars inventory and are manufactured to API 5CT and 5DP. Each joint is rated for collapse, burst, and tension, and the controlling load case, often collapse during a kick or burst during a frac, dictates the grade and wall thickness chosen for each hole section.
Grade Encodes Yield Strength: Designations like J55, L80, N80, P110, and Q125 give minimum yield in ksi: P110 is 110,000 psi (758 MPa) minimum yield. Higher grades allow thinner walls or deeper setting depths but cost more and can be more susceptible to environmental cracking, a trade-off central to every WCSB casing-design decision.
Sour Service Demands NACE Compliance: In H2S-bearing Nisku, Leduc, and deep Montney wells, tubulars must meet NACE MR0175 and ISO 15156 to resist sulphide stress cracking. Controlled-hardness grades such as L80 Type 1, C90, and T95 are mandatory; ordinary high-strength P110 can crack catastrophically in sour environments.
Casing Is Permanent, Tubing Is Replaceable: Casing is cemented and stays for the life of the well, providing zonal isolation and groundwater protection under AER Directive 010. Production tubing is the removable inner conduit, pulled and rerun during workovers to change artificial lift, repair leaks, or re-complete, without disturbing the cemented outer strings.
Connections Are The Weak Link: A tubular string is only as strong as its threads. API round and buttress threads suit many wells, but high-pressure sour gas and long horizontals often require premium metal-to-metal gas-tight connections. Make-up torque, thread compound, and running speed are controlled because cross-threading or galling is a leading cause of tubular leaks.

Collapse And Burst In WCSB Casing Design

Casing design centres on two opposing loads. Collapse pressure peaks when the casing is empty or partially evacuated while external mud or formation pressure pushes inward, a risk during a lost-circulation event or a deep evacuation. Burst pressure peaks when internal pressure from a kick or a hydraulic fracturing treatment exceeds external support. A deep sour Nisku well might set P110 or T95 intermediate casing rated above 70 MPa (about 10,150 psi) collapse to survive the deepest open-hole section, while a Montney frac string must hold treating pressures near 80 to 100 MPa (11,600 to 14,500 psi). The governing case sets the wall thickness, and engineers add a safety factor per AER Directive 010 before finalizing the string.

Drill String Fatigue In Long Horizontals

Drill pipe in a 3,000 m (9,840 ft) Montney lateral rotates through doglegs that cyclically bend each joint, accumulating fatigue damage at the slip and tool-joint areas. Heavy-weight drill pipe and a carefully placed bottomhole assembly manage the transition from compression near the bit to tension up the string. Tubular inspection, including electromagnetic and ultrasonic checks of used drill pipe, catches fatigue cracks before they part the string downhole, an event that can cost CAD 500,000 or more in fishing, sidetracking, and lost rig time on a modern WCSB pad-drilling program.

Fast Facts

The API casing and tubing grade letters do not stand for anything intuitive: the system was deliberately built so that the numeric suffix gives minimum yield strength in ksi while the letter merely sequences the grades. The very first standardized OCTG grades date to the 1920s, when wildly inconsistent pipe quality was wrecking wells across early American fields. A single modern deep WCSB well can contain four or five concentric casing strings telescoped inside one another, conductor through production casing, each cemented in turn, so that a wellhead in cross-section looks like a set of nested steel rings.

Tubulars connect directly to Casing, the cemented strings that isolate formations and protect groundwater, and to Production Tubing, the removable conduit that carries hydrocarbons to surface. Drill Pipe is the rotating tubular that transmits torque and circulates fluid while making hole, and the integrity of every string depends on the Casing Shoe and the cement sheath behind it. Connection metallurgy, grade selection, and load ratings tie all four together into a single well-design discipline.

Real-World WCSB Scenario: A Sour Deep Basin Casing Program

A CNRL deep sour gas well in the Alberta Foothills targets a Nisku interval with measurable H2S. The casing program runs L80 Type 1 surface casing, P110 intermediate above the sour zone, and T95 sour-service production casing across the pay, all with premium gas-tight connections. Total tubular spend, casing, tubing, and connections, reaches roughly CAD 2.6 million on an AFE of about CAD 14 million, and every joint of sour-service pipe carries mill certificates verifying NACE MR0175 hardness limits.

During completion the production casing holds a 92 MPa (13,340 psi) frac treatment without incident, and the T95 string later passes annual sour-service inspection. The disciplined grade selection, though it added cost over plain high-strength pipe, prevented the sulphide stress cracking that has destroyed under-specified sour wells elsewhere in the basin.