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Wellhead systems for land drilling
When a well is drilled on land, an interface is required between the individual casing strings and the blowout preventer (BOP) stack. This interface is required for four main reasons:
- To contain pressure through the interface with the BOP stack.
- To allow casing strings to be suspended so that no weight is transferred to the drilling rig.
- To allow seals to be made on the outside of each casing string to seal off the individual annulus.
- To provide annulus access to each intermediate casing string and the production casing string.
When drilling a well on land, a spool wellhead system is traditionally used, as shown in Fig. 1. This wellhead is considered a “build as you go” wellhead system that is assembled as the drilling process proceeds. The spool system consists of the following main components:
- Starting casing head.
- Intermediate casing spools.
- Slip casing hanger and seal.
- Tubing spool (if well is to be tested and/or completed).
- Studs, nuts, ring gaskets, and associated accessories required to assemble the wellhead.
Starting casing head
The starting casing head (see Figs. 2 and 3) attaches to the surface casing (conductor) by either welding or threading on to the conductor. The top of the starting casing head has a flange to mate with the bottom of the BOP. The flange must meet both size and pressure requirements. The starting casing head has a profile located in the inside diameter (ID) that will accept a slip-and-seal assembly to land and support the next string of casing. The slip-and-seal assembly transfers all of the casing weight to the conductor while energizing a weight-set elastomeric seal.
Intermediate casing spools
The intermediate casing spool is typically a flanged-by-flanged pressure vessel with outlets for annulus access (see Fig. 4). The intermediate casing spool (or spools) is installed after each additional casing string has been run, cemented, and set. The bottom section of each intermediate casing spool seals on the outside diameter (OD) of the last casing string that was installed. The bottom flange will mate with the starting casing head or the previous intermediate casing spool. The top flange will have a pressure rating higher than the bottom flange to cope with expected higher wellbore pressures as that hole section is drilled deeper.
The intermediate casing spool also incorporates a profile located in the ID, which accepts a slip-and-seal assembly similar to the one installed in the starting casing head. This slip and seal will be sized in accordance with the casing program.
The tubing spool, as shown in Fig. 5, is the last spool installed before the well is completed. The tubing spool differs from the intermediate spool in one way: it has a profile for accepting a solid body-tubing hanger with a lockdown feature located around the top flange. The lockdown feature ensures that the tubing hanger cannot move because of pressure or temperature. The flange sizes vary in accordance with pressure requirements.
Casing weight is transferred to the starting casing head and intermediate spools with two different types of hanger systems:
- A slip-and-seal casing-hanger assembly.
- A mandrel-style casing hanger.
The slip-and-seal casing-hanger assembly (Fig. 6) has an OD profile that mates with the internal profile of the starting casing head and intermediate casing spools. Integral to this casing-hanger assembly is a set of slips with a tapered wedge-type back and serrated teeth that bite into the OD of the casing being suspended.
When the casing has been run and cemented, the BOP is disconnected from the casing spool, and lifted up to gain access to the spool bowl area. After the slip-and-seal casing-hanger assembly is installed, the traveling block will lower the casing, and set a predetermined amount of casing load onto the slip-and-seal casing-hanger assembly. The teeth on the slips will engage the pipe OD, and transfer the suspended weight of the casing to the starting casing head. As the slips travel down, they are forced in against the casing, applying greater and greater support capacity. As the slips continue to engage the pipe, a load is placed on the automatic weight-set elastomeric seal assembly, sealing the annulus between the casing and the casing head. This installation creates a pressure barrier and isolates the annular pressure below the slip-and-seal casing hanger from the wellbore.
Traditionally, mandrel hangers (Fig. 7) are used only to suspend tubing from the tubing head. Occasionally, they can also be used in intermediate casing spools as an alternative to the slip-and-seal casing-hanger assembly. The mandrel hanger is a solid body with a through-bore ID similar to that of the tubing or casing run below, and it also has penetrations for downhole safety valve line(s) and temperature and pressure gauges, if required. Traditionally, in spool wellheads, elastomeric seals are used to seal the annulus between the casing-spool body and the casing or tubing hanger.
The seals used on spool wellhead systems are traditionally elastomeric. This is primarily because the seal must be energized against the casing-bowl ID, and must also seal against the rough finish of the casing OD.
This elastomeric sealing system is used for the slip-and-seal assembly, as well as the bottom of the intermediate casing or tubing spools. The slip-and-seal assembly (Fig. 8) provides a primary annulus seal, while the elastomeric seal in the bottom of each casing and tubing spool also provides a seal. The casing-spool flange connection becomes a secondary seal for both annulus and wellbore pressure. The elastomeric seals are manufactured using different materials to allow for various pressures, produced fluids, and other environmental conditions. The exception is the seal between each flange face, which is a metal-to-metal sealing ring gasket that provides a pressure-tight seal between each of the spool flanges. Ring gaskets are also used between the wellhead and the BOP stack, as well as the valves used for annulus access.
While drilling the well, it is required that the seal bores in each of the intermediate casing spools and tubing spools be protected. A series of wear bushings (Fig. 9) are supplied to protect the seal areas discussed during the drilling operation. The wear bushings are run on a drillpipe tool (Fig. 10) with J-lugs located on the OD that interface with J-slots located in the top ID section of the wear bushing.
It is also required that the flanged connections between each spool and the BOP be tested during the drilling and completion phases. The tools required are available from the equipment supplier. The tool used for testing the BOP is typically a plug type with a heavy-duty elastomer seal.
For onshore wells, during the drilling operation, access to each annulus is required for the following reasons:
- To provide a flow-by area for returns during cementing of casing strings.
- To provide access for possible well kill operations.
- To monitor the annulus for pressure below the slip-and-seal assembly.
Product material specifications
When ordering wellhead equipment, the following should be considered:
- All surface wellhead equipment and gate valves should be manufactured to the latest edition of the American Petroleum Inst. (API) and Intl. Organization for Standardization (ISO) standards. These standards define equipment specifications as follows:
- Material class: based on produced fluids; AA, BB, CC, DD, EE, FF, and HH (please see the example for gate-valve trims, shown in Fig. 11).
- Temperature range: 75 to +350°F.
- Please review the relevant API specifications for your application or consult your equipment supplier for further information.