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API Oil Tubing & Casing Model List

Casing and Tubing

Standards: API 5CT / ISO 11960
Product Scope: Casing pipes, tubing pipes, couplings, unions and accessories
Casing Size Range: 4 1/2″ to 20″
Tubing Size Range: 1.900″ to 4 1/2″
Grades: H40, J55, K55, N80, L80, C90, T95, P110, Q125, 13Cr, dual-wall and CRA options
Length Range: R1 / R2 / R3
Connection Types: STC, LTC, BTC, NU, EUE and premium connections

What Are Casing and Tubing?

Casing and tubing are the two primary types of downhole tubular goods deployed in oil and gas wells, yet they serve distinct engineering functions. Casing refers to structural pipe strings run inside wellbores, which are generally secured in place via cementing operations. It reinforces the borehole, isolates separate geological formations, protects freshwater aquifers, maintains wellbore stability, and acts as the primary pressure containment barrier throughout drilling, cementing, well completion and production phases.

Once the derrick installation is finished, tubing is run concentrically inside the casing. Tubing creates a controlled flow conduit for oil, gas, produced water or injection fluids to travel between the hydrocarbon reservoir and surface facilities. Unlike casing, tubing is directly exposed to produced fluids, internal pressure, well workover operations, scale buildup, corrosion, and stringent sealing requirements for threaded connections. In most oil wells, tubing can be retrieved or replaced, while casing typically remains in service permanently as an integral structural component of the well.

Casing and tubing are not manufactured from a single fixed steel grade. The majority of casing and tubing compliant with API 5CT are fabricated from carbon steel or low-alloy steel, with primary elemental constituents including iron, carbon and manganese. Products of higher strength grades undergo tighter control over chemical composition and heat treatment. For wells with severe corrosive conditions, specialty materials such as 13Cr, Super 13Cr, Duplex stainless steel or CRA (Corrosion Resistant Alloy) may be selected to boost resistance against carbon dioxide, chlorides or sour service environments.

For buyers, when placing orders for API 5CT casing and tubing, dimensions and steel grades should not be the sole focus. A complete and reliable order requires confirmation of outer diameter, unit weight, wall thickness, length range, connection type, drift requirements, collapse, burst and tensile performance, sour service or corrosive well conditions, inspection scope, and document traceability. All the above parameters shall be fully confirmed prior to product delivery. Casing selection is primarily determined by well depth, formation pressure, mud weight, cementing design and zonal isolation requirements. In contrast, tubing selection is closely tied to flow rate, internal operating pressure, packer compatibility, connection sealing integrity and workover load conditions.

Specification for API 5CT Casing and Tubing

Casing and tubing are generally ordered in accordance with API 5CT / ISO 11960 standards. Threads, couplings and dimensional measurement requirements are normally inspected per API 5B or approved drawings for premium connections when special threaded connections are required. From the buyer’s perspective, product specifications cannot be limited to simple descriptions such as "J55 casing" or "P110 tubing". Prior to production release, a complete purchase order shall clearly define the product type, steel grade, outer diameter, linear weight, connection type, length range, drift requirements, inspection scope and full documentation package.

For project orders, buyers shall regard API 5CT casing and tubing as a complete technical package. The steel grade governs mechanical strength and service suitability; the connection type dictates joint integrity and sealing performance; drift specifications determine internal tool passage capability; and the full documentation package verifies that delivered tubulars comply with approved purchase orders, manufacturing process specifications and acceptance inspection criteria.

Buyer’s decision-making logic: Casing selection is primarily driven by wellbore integrity and zonal isolation requirements. By contrast, tubing selection is determined by factors including flow control, completion compatibility and long-term operational reliability.

How Casing and Tubing Function Within an Oil Well

Oil and gas wells are typically lined with multiple strings of casing rather than a single pipe. Each casing string is run during distinct drilling phases and fulfills unique functional roles. Surface casing safeguards shallow formations and freshwater aquifers. Intermediate casing isolates unstable or pressure-sensitive geological zones before drilling proceeds to deeper well sections. Production casing or liners form the permanent wellbore barrier for long-term production or injection operations.

Once the casing string is installed and cemented in place, tubing is run concentrically inside the production casing. Tubing serves as the primary flow conduit for produced oil, natural gas, water, or injected fluids. To put it simply, casing protects and structurally supports the well, while tubing handles the conveyance of production fluids.

Casing failure compromises well integrity, cement isolation performance and pressure control. Mismatched tubular dimensions may result in leakage, restricted flow rates, failed directional surveys or insufficient tool clearance. This is why casing and tubular goods should be selected as a complete technical wellbore assembly, rather than merely based on size and steel grade.

Grades of Casing and Tubing

Grade selection shall be based on actual well conditions instead of price or strength rating alone. For casing or tubing of the same size, their performance can vary significantly due to different steel grades, heat treatment processes, pressure loads, corrosive environments, connection types and inspection scopes.

Different API 5CT casing and tubing grades are selected to suit distinct well operating conditions:

H40 / J55 / K55: Suitable for shallow wells, surface casing, and conventional production operations with moderate pressure and loading.
N80-1 / N80Q: Applied to medium and deep wells requiring enhanced mechanical strength; N80Q is manufactured via quenching and tempering heat treatment.
L80 / C90 / T95: Selected for service where strict control over tensile strength, hardness and sour resistance evaluation is mandatory.
P110: Used for deep wells, high-pressure intervals, and casing/tubing strings subjected to heavy mechanical loads.
Q125: Specially engineered for ultra-deep wells, high-temperature high-pressure (HTHP) zones and extreme high-load applications.
13Cr / Super 13Cr / Duplex / CRA: Deployed in wells containing carbon dioxide, chlorides or other highly corrosive media.

Key Selection Parameters for Casing & Tubing Grades：

Yield Strength
Defines the fundamental load-bearing capacity of individual pipes and full tubular strings.
Collapse & Burst Ratings
Critical for tubular strings exposed to external formation pressure, internal operating pressure and cementing loads.
Hardness Control
Particularly vital for L80, C90, T95 and all equipment deployed in sour hydrogen sulfide service environments.
Heat Treatment
Determines strength stability, material toughness and overall tubular reliability, especially for N80Q, P110 and Q125 grade materials.
Corrosive Well Service
Elevated levels of carbon dioxide, hydrogen sulfide, chlorides and high downhole temperatures often necessitate 13Cr, duplex stainless steel or CRA corrosion-resistant alloy materials.
Connection Type
STC, LTC, BTC, NU, EUE or premium threaded connections must be matched to the well’s mechanical load and gas-tight sealing requirements.
Inspection Documentation
Records including Mechanical Torque Coefficient (MTC), full heat lot traceability, drift testing, thread inspection, non-destructive testing (NDT) and hydrostatic pressure test reports shall be fully reviewed prior to shipment.

Buyer’s Reminder：A higher steel grade does not always equate to the safest option. The proper grade shall be matched to well depth, pressure profile, corrosive conditions, connection design and inspection requirements.

Casing and Tubing Connections

Connections are of critical importance, as they determine whether casing or tubing strings can be properly assembled, run smoothly downhole, maintain effective sealing under pressure, and withstand axial and torsional loads on-site. Even if the pipe body features the correct grade, size and wall thickness, an inappropriate connection type may result in thread mismatch, coupling rejection, leakage hazards, assembly torque failure or rig operation delays.

Casing connections must be compatible with well depth, cementing loads, axial tension, pressure ratings and running conditions. For tubing connections, additional factors shall be taken into consideration, including annular clearance, packer compatibility, sealing performance, workover loads and internal flow restrictions.Where standard API connections fail to meet requirements such as gas-tight sealing, high torque resistance, high temperature high pressure (HTHP) service, CO₂ injection or critical well completion operations, premium connections may be required.

For tubular purchase orders, straightness tolerances and connection types shall be confirmed simultaneously. The outer diameter may meet specifications; however, if the internal bore clearance fails to match packers, safety valves, downhole tools or logging tools, the tubular string may fail inspection or lead to installation constraints. For casing orders, buyers shall also inspect thread protectors, connection assemblies, thread inspection records and joint compatibility prior to shipment.

Manufacturing and Heat Treatment

Casing and tubing can be supplied as seamless or welded steel tubular products, yet heat treatment stands as one of the critical procedures determining the final service performance of pipes. Its objective is to regulate yield strength, tensile strength, hardness and toughness, so that the pipe bodies can satisfy service requirements for collapse resistance, burst resistance, tensile loading and sour operating environments.For grades including N80Q, L80, P110 and Q125, heat treatment is not a secondary background process. Instead, it serves as a vital control point that shall be consistent with grade manufacturing routes, Mechanical Torque Coefficient (MTC) values and inspection release records.

Typical Production Process of Seamless OCTG：

Heat treatment must not be regarded as a trivial auxiliary process. For grades such as N80Q, L80, P110, Q125 and grades intended for sour service environments, the stability of yield strength, hardness and toughness is directly dependent on the well’s pressure and load design parameters.

Applications of Casing and Tubing

Surface Casing During Cementing

Surface casing is run after drilling the upper well sections. It functions to stabilize shallow formations, protect freshwater aquifers, and form a robust cementing barrier. Buyers shall confirm the outer diameter, unit weight, steel grade, connection type and cementing scheme prior to placing orders. Poor-quality casing may result in inadequate cement support, casing shoe track defects, or hazards to pressure control.

Intermediate Casing for Drilling Through Unstable or Pressure-Variable Zones

Intermediate casing is deployed when drilling plans require isolating problematic zones before advancing to deeper formations. Such trouble zones may feature variable mud weights, circulation losses, unstable shales, or elevated formation pressures. Key inspection items include collapse rating, burst rating, wall thickness, joint strength and cementing integrity.

Production Casing and Liners for High-Pressure Wells

Production casing or liners serve as permanent components of the wellbore wall. Throughout the well’s service life, they must withstand internal pressure, external formation pressure, axial loads, corrosive media and joint sealing demands. Depending on well depth, pressure profiles and completion designs, P110, Q125, high collapse resistance casing or premium connections may be required.

Production Tubing for Oil, Gas and Water Conveyance

Tubing transports produced fluids from downhole to surface facilities. Primary operational risks include leakage, corrosion, internal scale buildup, thermal expansion constraints and joint failure. Buyers shall comprehensively evaluate tubing outer diameter, wall thickness, thermal expansion performance, connection type, internal coating and packer compatibility.

Wells Containing Hydrogen Sulfide and Carbon Dioxide

Material selection becomes far more critical in wells with hydrogen sulfide (H₂S) or carbon dioxide (CO₂). Material options such as L80, C90, T95, 13Cr, duplex stainless steel or CRA (Corrosion Resistant Alloy) can be selected based on sour service specifications, CO₂ partial pressure, chloride content, downhole temperature and project corrosion modeling. Rigorous review of hardness control, heat treatment records and full MTC traceability is mandatory.

High-Temperature High-Pressure (HTHP) Wells and Extended Reach Multilateral Wells

HTHP wells and extended-reach multilateral completions subject casing and tubing to combined loads including pressure, high temperature, tension, bending and joint stress. Premium connections, specialized drift guides, stringent non-destructive testing (NDT) and full-lifecycle traceability are commonly adopted to reduce rework rates, eliminate potential leakage paths and mitigate well intervention risks.

Quality Control and Inspection Records

For tubular goods and fittings, quality control is not merely an in-plant production procedure; it provides verified evidence that every pipe joint complies with the ordered steel grade, dimensions, connection type, drift specifications, pressure requirements and service conditions. The purpose of inspection records is to enable full traceability of each joint, linking pipe marking to heat lot numbers, Manufacturing Tracking Code (MTC), dimensional inspections, non-destructive testing, hydrostatic pressure tests, drift tests and packing slips. Without this complete documentation chain, buyers may encounter receiving disputes, unavailable MTC reviews, failed drift inspections or shipment delays.

The complete documentation chain shall cover the following links:Pipe marking → Heat lot number → MTC → Mechanical testing → NDT / Hydrostatic test / Drift test → Packing list → Shipping documents

This enables buyers to verify that the delivered casing and tubing comply with approved purchase orders, installation drawings and site acceptance requirements.

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