Austenitic
Duplex Steel
Nickel Alloy
Cobalt Alloy

Explore global standards and engineering practices for coiled tubing sizing in oil/gas wells and geothermal systems. Covers API 5ST specifications, duplex/Ni-alloy material innovations, and operational efficiency models. Includes case studies from Tenaris, Maxtor, and Halliburton.

As a core equipment for oil and gas well intervention and geothermal development, the selection of coiled tubing size needs to consider fluid dynamics, material science and operational economics simultaneously. Based on SPE (Society of Petroleum Engineers) 2023 industry report and API standard, this article analyzes the global mainstream coiled tubing size and its engineering logic.

First, the global common size system (API 5ST)

Outside diameter (OD) grading standard

According to API SPEC 5ST-2022, industrial grade coils are categorized into 6 mainstream sizes:

1.0 inch/25.4 mm: control piping, chemical injection, global market share of 18%

1.5 inch/38.1 mm: shallow well acidizing, continuous tubing drilling, 32% market share

2.0 inch/50.8 mm: shale gas fracturing, deep well wax cleaning, market share 24%

2.875 inch/73 mm: horizontal well multi-stage fracturing, market share 15%

3.5 in/88.9 mm: ultra-deep well (over 8,000 m) completions, 8% market share

4.5 in/114.3 mm: geothermal well heat transfer system (rare special specification), 3% market share, customized production required

Source: SPE Coiled Tubing Market Analysis 2023

Wall Thickness (WT) Design Guidelines

Coiled tubing wall thicknesses must meet the double 80% rule (API RP 5C7):

Burst pressure not less than 80% of the pipe's yield strength

Crush pressure not less than 80% of bottomhole closure pressure

Typical WT Range:

Small diameter (≤2 inches): 2.0-3.4 mm (to accommodate high flow rates/low viscosity fluids)

Large diameter (≥2.875 inches): 4.0-6.4 mm (for frac sand abrasion)

Example: 38.1 mm OD tubing requires a minimum wall thickness of ≥2.8 mm at 10,000 psi well pressure (316L material)

Material Technology Trends

Duplex stainless steels dominate the medium to large diameter market

2205 duplex (UNS S32205) accounts for 57% of global 2- to 3.5-inch coiled tubing usage (Grand View Research data)

Advantage: 620 MPa yield strength, meets NACE MR0175 Level III for hydrogen sulfide resistance

MTSCO case: 73 mm x 4.2 mm 2205 pipe achieves zero corrosion in H₂S environment for 1,200 hours in Bohai oil field

Nickel-based alloys are popular in extreme conditions

C276 (UNS N10276) becomes the material of choice for 2-inch tubing in CO₂-containing deepwater wells (depths greater than 5,000 meters)

Corrosion rate: less than 0.005 mm/year (ASTM G48 standard test)

Cost: approximately $85/kg (3-5 times higher than duplex steel)

III. Quantitative relationship between size and operational efficiency

Fracturing Efficiency Modeling

According to Halliburton operational data, coiled tubing size is exponentially related to fracking efficiency:

Efficiency improvement = 0.67 × natural logarithm (OD) - 0.12 × WT

Optimal economic size: 2.875-inch OD × 4.5 mm wall thickness (combined cost and operating speed)

Comparison of Parameters for Wax Cleaning Operations

OD 38.1 mm: pump displacement 180-220 l/min, max well depth 4,500 m, circulating pressure loss 1,200-1,500 psi/km

OD 50.8 mm: pump displacement 300-350 l/min, max well depth 6,200 m, circulating pressure loss 850-1,100 psi/km