Casing Head vs Tubing Head: Key Differences, Functions & Selection Guide

A casing head is the lowermost wellhead component, installed directly on the surface casing string. Its primary functions are to support the weight of subsequent casing strings, provide a seal between the surface casing and the next casing string, and provide side outlets for fluid returns during drilling and cementing operations.

Casing heads are available in three main connection types:
- Slip-on casing heads: Welded to the surface casing, providing a permanent, leak-tight connection. Common for onshore wells.
- Threaded casing heads: Screwed onto threaded surface casing. Used for low to medium pressure applications.
- Flanged casing heads: Bolted to a casing flange welded on the surface casing. Preferred for high-pressure and offshore applications.

The casing head serves as the foundation for the entire wellhead stack — everything above it depends on its structural and pressure integrity.

A tubing head is installed above the uppermost casing head or casing spool, at the top of the casing program. Its primary functions are to support the full weight of the production tubing string, seal the annulus between the tubing and the production casing, and provide access to the tubing-casing annulus for monitoring, gas lift, or chemical injection.

Key design features of tubing heads:
- Tubing hanger landing profile: The internal profile that receives and supports the tubing hanger, which in turn supports the tubing string.
- Annulus outlets: Typically two or more side outlets for accessing the A-annulus (between tubing and production casing). These may be threaded, flanged, or studded connections.
- Top connection: The top flange or studded connection that mates with the tubing head adapter or directly with the Christmas tree.

Tubing heads are designed to accommodate the tubing hanger running and landing procedure, which may involve hydraulic setting tools or mechanical lockdown mechanisms.

Understanding the wellhead stack sequence helps clarify the roles of each component:

Typical wellhead stack (bottom to top): 1. Conductor Pipe — Installed first, supports the wellhead structure 2. Surface Casing — Cemented in place 3. Casing Head — Installed on surface casing, supports next casing string 4. Intermediate Casing — Run through casing head, cemented 5. Casing Spool — Installed between casing heads (if multiple intermediate strings) 6. Production Casing — Run and cemented 7. Tubing Head — Installed on top of the last casing head/spool 8. Tubing String — Run and landed in the tubing hanger within the tubing head 9. Tubing Head Adapter — Connects tubing head to Christmas tree 10. Christmas Tree — Surface flow control assembly

The casing head appears early in the drilling program (step 3), while the tubing head appears at the completion stage (step 7), often months later.

Both casing heads and tubing heads must meet or exceed the maximum anticipated surface pressure for the well. API 6A standard pressure ratings apply equally to both components.

Material Considerations:
- Casing heads encounter drilling fluids during well construction — ensure compatibility with mud chemicals and lost circulation materials.
- Tubing heads are exposed to production fluids for the life of the well — material selection should account for long-term exposure to produced hydrocarbons, CO2, H2S, and completion fluids.
- For sour service wells, both components require NACE MR-01-75 compliant materials (API 6A material class CC minimum).

Sizing: Casing heads are sized to accommodate the surface casing OD and the next casing string. Tubing heads are sized to accommodate the tubing OD and provide adequate annular clearance.

Based on years of industry experience, here are the most common mistakes when ordering casing and tubing heads:

1. Wrong size: Ordering a tubing head sized for the wrong tubing OD. Always cross-reference your tubing program. 2. Missing annulus outlets: Not specifying the number, size, and pressure rating of annulus outlets on the tubing head. Verify your completion design requirements. 3. Incompatible connections: Casing head bottom connection must match your surface casing; tubing head top connection must match your tubing head adapter or Christmas tree. 4. Overlooking material class: For sour wells, failing to specify CC or higher material class. This is the most expensive mistake to fix — it requires replacing the entire component. 5. Assuming interchangeability: Casing heads and tubing heads from different manufacturers may not be compatible even if they share the same nominal size. Always verify dimensional compatibility.