Casing Head Introduction

What is a Casing Head:

The casing head serves as a critical connection between the casing and the wellhead. It is threaded to connect with the surface casing at the lower end, while the upper end connects to the wellhead (or BOP) through a flange or clamp.

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Applications, Characteristics, and Classification of Casing Heads:

1. Purpose:

a. Supports the weight of all casing layers except for the surface casing through the hanger;

b. Bears the weight of the wellhead equipment;

c. Forms a pressure seal between inner and outer casing strings;

d. Provides an outlet for any pressure that may build up between the two casing strings, or allows the pumping of liquid (such as kill drilling fluid, water, or an efficient fire extinguishing agent) into the well during an emergency;

e. Facilitates special operations in drilling and production technology, such as:

(1) Refilling the well with cement via the side port if the well is not fixed;

(2) Injecting pressurized liquid from the side hole during acid fracturing to balance the tubing pressure.

2. Characteristics:

a. Casing connections can be made through either threaded or slip connections, with quick and convenient hanging of the casing;

b. The casing hanger features a composite seal structure combining rigidity and rubber, and may also utilize a metal seal to enhance sealing performance;

c. Anti-wear sleeves and pressure test extraction tools are available to facilitate the removal of anti-wear sleeves and the testing of casing heads;

d. The upper flange comes equipped with a pressure test and secondary grease injection device;

e. Configurations for casing head side wing valves can be tailored to user requirements;

3. Classification:

a. Based on the number of casing layers:

Single-stage casing head (see Figure 1), double-stage casing head (see Figure 2), and triple-stage casing head (see Figure 3);

b. Based on the casing hanger's structure type:

Slip type casing head, screw-type casing head;

c. Based on the body connection mode:

Flange type casing head, clamp type casing head, and independent screw type (which involves a threaded connection between the upper end of the hanging casing string and the lower end of the tubing head body);

d. Based on body structure:

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Single casing head (with a single hanger installed in one body) and combined casing head (with multiple hangers installed in one body);

Relevant Standards for the Design, Manufacture, and Inspection of Casing Heads

1. The design and manufacture of casing heads must adhere to the natural gas industry standard of the People's Republic of China SY / T - specifications for wellhead equipment and Christmas tree, along with API Spec 6A 19th edition specifications published by the American Petroleum Institute;

2. Inspections of casing heads should comply with the specifications for wellhead equipment and Christmas trees (SY / T-) and the 19th edition of API Spec 6A (API Spec 6A) published by the American Petroleum Institute;

Structure and Sealing Principle of Casing Heads

1. Structure of Casing Heads:

The casing head comprises a four-way pipe, casing hanger, jacking screw assembly, flange-type parallel gate valve, connector, and pressure display mechanism;

2. Working Principle of the Rubber Seal of Casing Heads:

The sealing system of the casing head consists of a casing head body, BT rubber seal ring, casing hanger, dovetail rubber seal ring, V-shaped top thread seal ring, and sealing gasket ring. The casing hanger seat rests on the step of the casing head body. The weight of the casing creates contact between metal surfaces, resulting in a rigid passive seal. The seal between the casing hanger and the casing relies on a threaded connection.

The casing head four-way pipe includes a BT seal that corresponds with the outer diameter of the casing hanger (or the casing), along with grease injection and pressure test holes. During operation, high-pressure sealing grease must be injected from the grease injection valve to activate the BT seal. If a seal leak occurs, sealing grease should be injected through both the grease injection valve and the pressure test valve to maintain effective sealing. The grease injection pressure must not exceed the rated working pressure of the flange; the casing seal must not exceed the rated allowable collapse pressure of the casing. A pressure test hole is provided for external sealing tests of the casing hanger.

The flange features a jacking wire that locks the anti-abrasion sleeve to protect the sealing surface. After the casing hanger is seated, it can lock the casing hanger securely. If there is leakage around the top wire, tightening the pressure cap can restore the seal's effectiveness.

Each side of the casing head four-way flange connects one end to a flanged gate valve (or blind flange) and the other end to a flat valve, screw flange, joint, stop valve, and pressure gauge. The pressure gauge allows observation of the annulus pressure between the two casing layers.

3. Working Principle of Metal and Rubber Seals of Casing Heads:

The metal and rubber sealing assembly for the casing head consists of a casing head body, BT-shaped rubber seal ring, upper metal seal assembly, lower metal seal assembly, casing hanger, dovetail-shaped rubber seal ring, top wire V-shaped seal ring, and sealing gasket ring. The casing hanger seat rests on the step of the casing head body. The weight of the casing ensures contact between metal surfaces thus forming a rigid passive seal. The seal between the casing hanger and the casing is threaded (see Figure 5); the position size of the upper metal seal ring is measured in the upper flange hole before installing the upper flange, determining the necessary thickness of the adjustment ring to create a defined interference within the upper flange hole. When the upper flange connects with the casing head cross, the adjustment ring is compressed by the casing head cross, deforming the upper metal sealing ring and thereby sealing the casing hanger.

The sealing principle of the lower metal seal assembly of the casing hanger hinges on ensuring the geometric dimensions of each component through the design of the metal seal ring, casing head cross, and casing hanger. The pressure ring of the lower metal seal assembly descends, and the U-ring of the metal seal ring assembly deforms under the influence of a 45° conical surface rotation.