Definition And Differences Between Tubing & Casing

In oil and gas extraction and well engineering, Tubing and Casing are two critical pipeline components used to support wellbores, transport fluids, and ensure downhole safety. Below is a detailed analysis of their definitions, applications, structures, and industry standards:

一、Core Definitions and Key Functions

1. Casing

• Definition:
  Casing is a large-diameter steel pipe run into the wellbore in sections during drilling, fixed between the well wall and borehole with cement slurry to form a permanent support structure.

• Key Functions:

• Wellbore Reinforcement: Prevents formation collapse and infiltration of groundwater or shallow fluids.

• Stratigraphic Isolation: Separates different pressure zones (e.g., high-pressure hydrocarbon layers from freshwater zones) to avoid cross-flow.

• Support for Subsequent Operations: Provides a foundation for running tubing and installing completion tools.

• Application Stages (run in sequence during drilling):

• Surface Casing: Protects loose surface formations and establishes the initial wellhead.

• Intermediate Casing: Isolates complex formations (e.g., salt layers, high-pressure water zones).

• Production Casing: Reaches the target hydrocarbon zone to protect tubing.

2. Tubing

• Definition:
  Tubing is a small-diameter steel pipe run into the production casing after well completion, used to transport fluids (oil, gas, water) from the reservoir to the surface and serve as a carrier for downhole tools (e.g., packers, pumps).

• Key Functions:

• Fluid Conveyance: Forms a channel from the formation to the surface, withstanding high-pressure fluid flow.

• Downhole Control: Works with packers and safety valves to enable layered production, pressure control, or workover operations.

• Corrosion Protection: Reduces formation fluid corrosion through material selection (e.g., corrosion-resistant alloys) or internal coatings.

 二、Key Differences Comparison

三、Industry Standards and Materials

1. Common Standards

• Grades (e.g., H40, J55, K55, N80, L80, C90, T95, P110), with higher grades indicating greater strength/corrosion resistance.

• Thread types (e.g., API round threads, buttress threads, gas-tight threads like VAM/TPC).

• Dimensional tolerances and mechanical tests (tensile, flattening, collapse resistance).

2. Material Differences

• Casing: Prioritizes external pressure resistance (preventing collapse from formation stress), using low-carbon steels (e.g., J55, K55) for most wells and high-strength steels (e.g., P110) for deep/high-pressure wells.

• Tubing: Prioritizes internal pressure resistance and corrosion resistance (e.g., in H₂S/CO₂-rich acidic environments), using alloy steels (e.g., L80-9Cr, C90-13Cr) or stainless steels (e.g., 316L).

 四、Practical Application Examples

1. Casing Scenario:

• A shale gas well runs three casing layers: surface casing (339.7mm, J55) for surface stabilization, intermediate casing (244.5mm, N80) to isolate shale formations, and production casing (139.7mm, P110) to reach the reservoir, all cemented in place.

2. Tubing Scenario:

• A offshore horizontal well uses 73.02mm (2.992") L80-13Cr tubing with internal epoxy coating for corrosion protection. Combined with packers, it enables layered production, with an electric submersible pump at the tubing bottom to lift crude oil to the surface.

五、Conclusion

• Casing is the "skeleton of the well", focusing on support and isolation to ensure wellbore stability.

• Tubing is the "vascular system of the well", focusing on fluid transport and control for efficient production.
  Working together, they form the "infrastructure" of oil and gas wells. Their selection must consider formation pressure, fluid properties, and well structure, key to the safety and economics of hydrocarbon extraction.