Wellhead equipment is used to connect the tubing and casing to an oil or gas pipeline. The casing is a permanently-installed pipe for lining the well hole to provide pressure containment and prevent collapse. The wellhead is a flanged device that attaches to the casing. It is used to suspend the casing strings and help seal the well.
Components of Wellhead Equipment
Wellhead equipment includes components such as casing heads, casing spools, tubing head spools, tees, and crosses. The casing head is part of the connection between the casing and the wellhead itself. Typically, multiple pieces of wellhead equipment are integrated into a wellhead assembly called a "Christmas tree". This wellhead assembly allows tubes to be placed into the well while valves and chokes are used to control the flow of oil and/or gas at the surface.
Wellheads and wellhead equipment may be located on offshore platforms, in sub-sea locations, or onshore. Wellheads are cemented in place and, typically, kept in place; however, with exploration wells, wellhead equipment may be recovered for subsequent re-use. Wellhead equipment is designed to support the casing and tubing strings, which may also be suspended from the Christmas tree. In this way, wellhead equipment provides a seal between the different strings and permits access to the annuli between them. The main components of a wellhead system include the casing head, casing spools, casing hangers, pack-offs and isolation seals, bowl protectors, test plugs, mud-line suspension systems, tubing heads, tubing hangers and tubing head adapters.
Basic requirements for materials, dimensions, test procedures, and pressure ratings for wellheads and wellhead equipment are defined on API Spec 6A: Specification for Wellhead and Christmas Tree Equipment from the American Petroleum Institute (API).
Wellheads are manufactured by different methods and for different purposes. Factors that affect a wellhead’s configuration include pressure, temperature, location, well depth, well size, well purpose, expected well life, well control equipment interface, and drilling method. In terms of applications, wellhead equipment is used for casing suspension and to provide pressure isolation when multiple casings are used. Wellheads also provide a way to attach blow-out preventers (BOPs) during drilling. In addition, wellhead equipment is used to provide a method for attaching a tree for well-control during production, injection, or other operations. Finally, these products are used to provide well access and a means of pump attachment.