Casing Head Introduction

What is a casing head:

The casing head is an important connection between casing and wellhead. Its lower end is connected with the surface casing through the thread, and the upper end is connected with the wellhead (or BOP) through the flange or clamp.

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Application, characteristics, and classification of casing head:

1. Purpose:

a. Support the weight of each layer of casing other than the surface casing through the hanger;

b. Bear the weight of wellhead equipment;

c. Pressure seal can be formed between inner and outer casing strings;

d. Provide an outlet for the pressure that may accumulate between the two casing strings, or pump liquid (kill drilling fluid, water or efficient fire extinguishing agent, etc.) into the well in case of emergency;

e. Special operations in drilling and production technology can be carried out, such as:

(1) if the well is not fixed, it can be refilled with cement from the side port;

(2) when acid fracturing is carried out by taking stimulation measures, the pressure liquid can be injected from the side hole to balance the tubing pressure.

2. characteristics:

a. Casing connection can be either threaded connection or slip connection. Hanging casing is fast and convenient;

b. The casing hanger adopts the composite seal structure of rigidity and rubber, and can also adopt the metal seal, which enhances the sealing performance of the product;

c. Anti-wear sleeve and pressure test take-out tool are designed to facilitate the take-out of anti-wear sleeve and pressure test of casing head.

d. The upper flange is designed with a pressure test and secondary grease injection device.

e. The configuration of casing head side wing valve is designed according to the user's requirements;

3. classification:

a. According to the number of hanging casing layers:

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

b. According to the structure type of casing hanger:

Slip type casing head, screw-type casing head;

c. According to the connection mode between bodies:

Flange type casing head, clamp type casing head, and independent screw type (the casing head with thread connection between the upper end of hanging casing string and the lower end of tubing head body);

d. According to the structure of the body:

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Single casing head (a hanger is installed in one body), combined casing head (multiple hangers are installed in one body);

Relevant standards for design, manufacture, and inspection of casing head

1. The design and manufacture of the casing head shall be in accordance with the natural gas industry standard of the people's Republic of China SY / t- specification for wellhead equipment and Christmas tree and API Spec 6A 19th edition specification for wellhead equipment and Christmas tree equipment published by American Petroleum Institute;

2. During the inspection of casing head, the specification for wellhead equipment and Christmas tree (SY / t-) and the 19th edition of API Spec 6A (API Spec 6a) published by American Petroleum Institute shall be implemented;

Structure and sealing principle of casing head

1. Structure of casing head:

The casing head is composed of four-way pipe, casing hanger, jacking screw assembly, flange type parallel gate valve, connector, pressure display mechanism, etc;

2. Working principle of rubber seal of casing head:

The casing head seal consists of 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 is hung on the step of the casing head body. Due to the hanging weight of the casing, the contact between metal and metal produces a rigid passive seal. The seal between the casing hanger and the casing is a threaded seal.

The casing head four-way pipe has a BT seal matching with the casing hanger outer diameter (or casing outer diameter) and corresponding grease injection and pressure test holes. When using, high-pressure sealing grease must be injected from the grease injection valve to make the BT seal work. In case of leakage of the seal, the sealing grease shall be injected into the grease injection valve and the pressure test valve respectively to make the seal continue to take effect. The grease injection pressure shall not exceed the rated working pressure of the flange; if the casing is sealed, it shall not exceed the rated allowable collapse pressure of the casing. A pressure test hole is used for the external sealing test of the casing hanger.

The flange is equipped with jacking wire to lock the anti-abrasion sleeve (to protect the sealing surface). After the casing hanger is seated, it can lock the casing hanger. If there is leakage at the top wire, tighten the pressure cap to make the seal effective.

The flange at both sides of the casing head four-way, one end is connected with a flanged gate valve (or blind flange), the other end is connected with a flat valve, screw flange, joint, stop valve, and pressure gauge. Through the pressure gauge, the annulus pressure between two casing layers can be observed.

3. Working principle of metal and rubber seal of casing head:

The metal and rubber seal of the casing head assembly 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 is hung on the step of the casing head body. Due to the hanging weight of the casing, the contact between metal and metal will produce a rigid passive seal. The seal between the casing hanger and the casing is a threaded seal (see Figure 5); the upper metal seal ring is to measure the position size of the upper metal seal ring in the upper flange hole before installing the upper flange and then determine the thickness of the adjustment ring to make the adjustment ring emit a certain amount of interference in the upper flange hole. When the upper flange is connected with the casing head cross, the adjusting ring is compressed by the casing head cross to deform the upper metal sealing ring, so as to achieve the function of sealing the casing hanger.

The sealing principle of the lower metal seal assembly of the casing hanger is to ensure the geometric dimension of each part through the design of the metal seal ring, the casing head cross, and the casing hanger, and to make the pressure ring of the lower metal seal assembly go down and the U-ring of the metal seal ring assembly deforms by using the 45 ° conical surface rotation on                    

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