Chapter 10 - Wellhead Assembly

Abstract

An oil and gas well wellhead assembly is used for hanging downhole tubing string and casing string; sealing tubing-casing and casing-casing annuluses; injecting steam, gas, water, and chemicals; and acidizing and fracturing. It is a key piece of equipment for safe production. The wellhead assembly consists mainly of casing head, tubing head, and Christmas tree. A “Christmas tree” is an assembly consisting of valves and fittings and is used for controlling oil- and gas-well fluid and providing outlets and inlets for produced fluid and washing fluid, and so on. It includes all the equipment above the tubinghead spool flange. A Christmas tree assembly can consist of various combinations to meet the requirements of any special usage. Different Christmas trees have different uses including oil production (flowing and artificial lift), gas production (natural gas and various acidic gases), water injection, thermal production, and fracturing and acidizing, and they form a series with different grades of pressures under which they are used. A tubinghead consists of a tubinghead spool and a tubing hanger. It is mainly used for hanging tubing string and sealing tubing string and tubing-casing annulus. The tubinghead is installed between the Christmas tree and the casinghead. The upper flange plane of the tubinghead spool is the datum level for calculating the distance between the master bushing and the tubing hanger and the well depth data. The tubinghead spool is installed on the casinghead, which is used for hanging intermediate casing and production casing, sealing casing annulus, and providing a connection for installing upper devices including blowout preventer and tubinghead.