Casing Head vs. Casing Spool: Key Differences and When to Use Each
At first glance, the terms “casing head” and “casing spool” can seem interchangeable, and in casual conversation, they sometimes are. But in the engineering sense, these two components play distinct roles in the wellhead assembly, and selecting the right one — or understanding how they work together — is critical to well integrity and operational efficiency.
What Is a Casing Head?
The casing head (also called a casing head housing or surface casing head) is the bottommost component of the wellhead assembly. It is permanently welded or threaded directly onto the surface casing string and provides the foundational support and sealing surface for all components installed above it.
The casing head serves several critical functions: it provides the structural base for the entire wellhead stack, it contains the casing hanger that supports the next string of casing or tubing, it provides a sealing surface for the annular space between the surface casing and the next string, and it includes side outlets (typically 2-1/16″ or 3-1/8″ flanged or threaded) for annular access, pressure monitoring, or cementing operations.
Casing heads are manufactured to API 6A specifications with body pressure ratings matching the anticipated wellbore pressures. Once a casing head is landed and cemented, it becomes a permanent part of the well.
What Is a Casing Spool?
A casing spool is a flanged spool body that sits above the casing head (or above another casing spool in multi-string wells). While it performs similar functions to the casing head — housing a casing hanger, sealing the annulus, and providing side outlets — the key difference is that casing spools are bolted flange-to-flange connections, not permanent weld-on components.
This makes casing spools replaceable and interchangeable, which is a significant advantage in well workover scenarios or when field conditions change. A well with multiple casing strings will typically have one casing head (at the bottom) and one or more casing spools stacked above it, each one accommodating a progressively smaller diameter casing string.
When to Use Each
The casing head is always installed on the first (surface) casing string. It is a permanent weld-on installation. From that point, each subsequent casing string requires a casing spool to be flanged onto the wellhead stack above the previous component.
In a simple single-string well, you may have only a casing head followed directly by a tubing head spool. In a complex deep well with three or four casing strings, you will have a casing head plus two or three casing spools stacked in sequence, each one sized to match the specific casing string it supports.
Material and Dimensional Standards
Both casing heads and casing spools must be sized to match the OD of the casing string they support, and their flanges must be compatible with the mating components above and below. Non-standard wellhead configurations often require custom adapters or crossovers to bridge dimensional or pressure-class mismatches.
CRC Wellhead designs and manufactures casing heads, casing spools, and complete wellhead assemblies from our Edmonton facility. We work to API 6A standards and can accommodate non-standard configurations, legacy wellhead dimensions, and custom sour service requirements. Contact our team for more information.