Overview of Wellhead Systems
Introduction
-Wellhead is the surface foundation on which the well is built up during the drilling operations.
-The base for the BOP and after it for Christmas tree for production operations.
Objectives of the wellhead
-Withstand tensile loads of suspended tubular
-Provide a pressure sealing capability
-Isolate the well from the external environment
-Isolate between casings and downhole formations
-Maintain pressure during well control operations, well testing, or shut-in periods.
Components of Wellhead
-Base plate
The load acting on the casing head housing might be high(in the case of a deep well for instance). in this case, a base plate is used to distribute the load and increase the wellhead’s stability.
The base plate is set at the bottom of the cellar.
-Casing head housing
It is the lowest part of the wellhead assembly and is almost always connected to the surface casing. It supports subsequent drilling wellhead and completion string.
Its main functions are:
-Provides for attachment to the surface casing string.
-Supports BOP during drilling operation
-Provides for suspending and packing of the next casing string.
-Provides outlets for annular access
-Provides for testing BOP while drilling
Casing head housing(CHH) types
Threaded: Although safer, threaded CHH requires to have the uppermost casing thread at a precise depth. If the surface casing has to be set off depth(fill on the bottom, stuck casing), slip-on CHH has to be used.
Slip-on: The welding must be tested through the “test port” located between the two welding. Not that the plug needs to be removed before the welding.
Sliplock: They can be installed faster than a slip-on weld type. They can be re-installed or removed. This allows us to use it as a temporary tool while drilling the intermediate hole.
Casing hanger
The objectives of the casing hangers are to:
-Suspends intermediate or production casing string
-Centers the casing string in the head(or spool)
-Seals off the casing annulus(it is primary seal)
There are many types of hangers. Two major types of casing hangers: slip-type hangers(wraparound casing slips) and mandrel-type hangers. The wraparound casing can be set without removing BOP under certain conditions.
-Casing head spool
It has a top bowl to accommodate the next casing hanger. It has a bottom bowl with a pack-off seal(or bushing) and a flange for mounting it up on the top of the casing head housing or previous casing head spool. Multiple casings spools can be stacked to hang intermediate or production casing strings.
Its main functions are:
-Seals off(packs off) the previous string.
-Provides for suspending and packing of the next casing string.
-Provides support for well control equipment(BOP stack)
-Provides outlets for annular access.
-Provides for testing BOP while drilling.
-Tubing head spool
It is the top spool on a surface wellhead assembly. It is installed after the last casing string to provide a shoulder to support the tubing string and a seal bore for the tubing hanger or production. When the well is completed, the tree is installed on the top of the tubing head.
Its main functions are:
-Seals off(packs off) the production casing.
-Provides for suspending and packing of the completion string.
-Provides support for well control equipment(BOP stack)
-Provides outlets for annular access.
