1.What is a casing head?
The casing head is an important connection between casing and wellhead equipment.Its lower end is connected to the surface casing pipe through threads, and its upper end is connected to the wellhead device or blowout preventer through flanges or clamps.
2. Characteristics of casing head
(1) The casing connection can be either threaded or slip connected, and hanging the casing is fast and convenient;(2) The casing hanger adopts a rigid and rubber composite sealing structure, and a metal seal can also be used to enhance the sealing performance of the product;
(3) It is designed with a wear protection sleeve and a pressure test removal tool to facilitate the removal of the wear protection sleeve and pressure testing of the casing head;
(4) The upper flange is designed with pressure testing and secondary grease injection devices;
(5) The valve configuration on the side of the casing head is designed according to user needs;
3.The function of casing head
The casing head is installed at the upper end of the surface casing string. It is used to hang all layers of casing other than the surface casing and to seal the wellhead device components of the casing annular space. Its main functions are as follows:(1) Hang part or all of the weight of each layer of casing except the surface casing through a hanger;
(2) Connect wellhead devices such as blowout preventers;
(3) Form a pressure seal between the inner and outer casing strings;
(4) Provide an outlet to release the pressure that may accumulate between the two sets of pipe strings;
(5) In an emergency, fluid, such as well killing fluid or fire extinguishing agent, can be pumped into the well through the side hole of the casing head;
(6) Special operations: a. If the cementing quality is not good, cement can be replenished through multiple side holes. b. During acidification fracturing, pressure balancing fluid can be injected from the side holes.
4. Classification of casing heads
(1) According to the number of hanging casing layersAccording to the number of suspended casing layers, it can be divided into: single-stage casing head, double-stage casing head, and three-stage casing head; single-stage casing head is generally used in low-pressure shallow formation production wells; double-stage casing head It is suitable for most areas with relatively clear formation pressure and is widely used; three-stage casing heads are generally used in high-pressure deep wells or exploration wells;
(2) According to the structural type of casing hanger
According to the structural type of the casing hanger, it can be divided into slip-type casing heads, mandrel-type (threaded) casing heads, and integral (welded) casing heads;
(3)According to the connection method between bodies
According to the connection method between bodies, it is divided into flange type casing head, clamp type casing head, and independent thread type (casing head in which the upper end of the hanging casing string and the lower end of the tubing head body are connected by threads);
(4) According to the structural type of the body
According to the structural type of the body, it is divided into single-piece casing head (one body is equipped with one hanger) and combined casing head (one body is equipped with multiple hangers).
5. Structure and sealing of casing head
The casing head consists of four-way, casing hanger, jackscrew assembly, flange-type parallel gate valve, connectors, pressure display mechanism, etc.(1) Structure of casing head
The casing head is composed of four-way, casing hanger, jackscrew assembly, flange-type parallel gate valve, connectors, pressure display mechanism, etc.;
(2) Working principle of rubber seal
The pipe head seal consists of the casing head body, BT-shaped rubber sealing ring, casing hanger, dovetail-shaped rubber sealing ring, jackscrew V-shaped sealing ring, and sealing gasket ring. The casing hanger seat is hung on the step of the casing head body. Due to the suspended weight of the casing, the contact between metal and metal creates a rigid passive seal, and the seal between the casing hanger and the casing is a thread seal.
The casing head spool has a BT seal that matches the outer diameter of the casing hanger (or casing outer diameter) and corresponding grease injection and pressure test holes. When used, high-pressure sealing grease must be injected from the grease injection valve before it can be used. BT seal works. If leakage occurs in the seal, sealing grease should be injected from the grease injection valve and pressure test valve respectively to make the seal continue to be effective. The grease injection pressure does not exceed the rated working pressure of the flange; if the casing is sealed, it does not exceed the rated allowable collapse pressure of the casing. The pressure test hole is used for the external sealing test of the casing hanger.
There is a jackscrew on the flange, which is used to lock the wear sleeve (protect the sealing surface), and lock the casing hanger after the casing hanger is installed. If leakage occurs at the top screw, the pressure cap can be tightened to make the seal effective. There are flanges on both sides of the four-way casing head. One end is connected to the flat valve (or blind flange), and the other end is connected to the flat valve, threaded flange, joint, stop valve and pressure gauge. The two casing layers can be observed through the pressure gauge. annular pressure between.
(3) Metal and rubber seal of casing head
The metal and rubber seal of the casing head consists of the casing head body, BT-shaped rubber sealing ring, upper metal sealing component, lower metal sealing component, casing hanger, dovetail-shaped rubber sealing ring, top wire V-shaped sealing 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 metal-to-metal contact creates a rigid passive seal. The seal between the casing hanger and the casing is a thread seal; before installing the upper flange, the upper metal sealing ring is measured by measuring the position and size of the upper metal sealing ring in the upper flange hole, and then determining the thickness of the installation adjustment ring, so that The adjusting ring has a certain amount of interference in the upper flange hole. When the upper flange is connected to the casing head four-way, the adjusting ring is compressed through the casing head four-way to deform the upper metal sealing ring, thereby achieving the function of sealing the casing hanger.
The sealing principle of the lower metal sealing component of the casing hanger is to ensure the geometric dimensions of each part through the design of the metal sealing ring, casing head spool, and casing hanger, and use the 45° cone rotation on the top wire to The pressure ring of the lower metal sealing assembly is moved downward to deform the U-shaped ring of the metal sealing ring assembly, thereby achieving the function of a sealing casing hanger.
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