Detailed explanation of casing head

The Casing head is an important connector between CASING and wellhead equipment. The casing head is an important connector between casing and wellhead equipment. Its lower end is connected with surface casing through thread, and its upper end is connected with wellhead equipment or BOP through flange or clamp.
Characteristics of casing head
Casing connection can be either threaded connection or slip connection. Hanging casing is fast and convenient;
The casing hanger adopts a rigid and rubber composite sealing structure, and metal sealing can also be used to enhance the sealing performance of the product;
The anti wear sleeve and pressure test extraction tool are designed to facilitate the removal of the anti wear sleeve and the pressure test of the casing head;
The upper flange is designed with pressure test and secondary grease injection device;
The casing head side valve configuration is designed according to user requirements;
Role of casing head
The casing head is installed at the upper end of the surface casing string, and is used to suspend the casing of each layer outside the surface casing and the wellhead equipment components in the annular space of the sealing casing. Its main functions are as follows
1. Suspend part or all weight of each layer of casing except surface casing through hanger;
2. Connect BOP and other wellhead devices;
3. Forming a pressure seal between the inner and outer casing strings;
4. Provide outlet for releasing pressure that may accumulate between two casing strings;
5. In case of emergency, fluid can be pumped into the well from the side hole of casing head, such as killing fluid or fire extinguishing agent;
6. Special operations: a. If the cementing quality is poor, multiple side holes can be re injected with cement; b. During acid fracturing, pressure balance fluid can be injected from the side holes.
Classification of casing head
1. According to the number of layers of suspended casing
According to the number of hanging casing layers, it can be divided into single-stage casing head, two-stage casing head and three-stage casing head; Single stage casing head is generally used in production wells in low pressure shallow formation; The two-stage casing head is applicable to most areas with clear formation pressure, and is widely used; The third stage casing head is generally used in high-pressure deep wells or exploration wells
2. According to the structure type of casing hanger
According to the structure type of casing hanger, it can be divided into slip type casing head, mandrel type (threaded) casing head and integral type (welded) casing head;
3. According to the connection mode between bodies
It can be divided into flange type casing head, clamp type casing head and independent thread type (casing head with thread connection at the upper end of suspended casing string and the lower end of TUBING head body) according to the connection mode between the bodies;
4. According to the structure type of the body
According to the structural type of the body, it can be divided into single casing head (one hanger is installed in one body) and combined casing head (multiple hangers are installed in one body);
Structure and sealing of casing head
1. Structure of casing head
The casing head is composed of four-way, casing hanger, jacking assembly, flange type parallel gate valve, connector, pressure display mechanism, etc;
2. Working principle of rubber seal
The casing head seal is composed of the casing head body, BT rubber seal ring, casing hanger, dovetail rubber seal ring, top thread V-shaped seal ring, and sealing gasket ring. The casing hanger is mounted on the casing head body step. Because of 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 thread seal.
The casing head spool has a BT seal matching the outer diameter of the casing hanger (or the outer diameter of the casing) 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. If the seal leaks, the sealing grease shall be injected into the grease injection valve and 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, the rated allowable collapse pressure of the casing shall not be exceeded. The pressure test hole is used for external seal test of casing hanger.
The flange is provided with a jack screw for locking the anti-wear sleeve (protecting the sealing surface). The casing hanger can be locked after the casing hanger is seated. If there is leakage at the jackscrew, tighten the pressure cap to make the seal effective. The flanges at both sides of the spool of the casing head are connected to the flat valve (or blind flange) at one end and the flat valve, threaded flange, connector, stop valve and pressure gauge at the other end. The annular pressure between the two casing layers can be observed through the pressure gauge.
3. Metal and rubber seal of casing head
The metal and rubber seals of the casing head are composed of the casing head body, BT rubber seal ring, upper metal seal component, lower metal seal component, casing hanger, dovetail rubber seal ring, top thread V-shaped seal ring, and sealing gasket ring. The casing hanger is mounted on the casing head body step. Because of the hanging weight of the casing, the contact between metal and metal will produce a rigid passive seal. The casing hanger and casing are sealed by thread (see the figure above); Before installing the upper flange, measure the position and size of the upper metal sealing ring in the upper flange hole, and then determine the thickness of the adjusting ring, so that a certain amount of interference will emerge from the adjusting ring in the upper flange hole. When the upper flange is connected with the casing head cross, the adjusting ring is compressed through the casing head cross to deform the upper metal sealing ring, thus achieving the function of sealing 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, the casing head cross and the casing hanger. The 45 ° cone on the jackscrew rotates to make the lower metal sealing component press ring down, so that the U-shaped ring of the metal sealing ring component is deformed, thus achieving the role of the casing hanger.
Casing head hanger
1. Mandrel hanger
The mandrel type (i.e. threaded type) hanger is equivalent to a CASING COUPLING. It is convenient to cut the casing (both W and WE types are required), and the casing can be suspended without even opening the BOP to solve the problem of wellhead suspension seal. However, the casing depth shall be calculated to avoid bottoming. It is generally used in relatively stable production wells. Generally, the mandrel (threaded) hanger is used with the threaded casing head, and the corresponding slip hanger can also be replaced. The sealing principle of the mandrel hanger with metal and rubber seals on the casing head is the same as that of the mandrel hanger with metal and rubber seals on the Tubing head.
2. Slip hanger
a. Structure of slip hanger:
Slip hanger is mainly composed of slip tooth (quartered), slip seat (quartered), rubber seal (quartered), support seat (quartered), etc.
b. Operating principle of slip hanger:
Hold the sleeve by the slip hanger of the two halves and put it into the casing head hole as a whole to make the slip teeth firmly stick to the casing. Under the effect of the casing self weight and the taper of the slip teeth, the more wedged the slip teeth are, the more tight the slip teeth become to the casing. The rubber seals deform under the action of their own weight, forming a seal on the casing and casing head body.
c. Reverse slip hanger
The inverted slip hanger seat is fixed at the lower part of the casing head body with bolts. After the casing is installed into the BT sealing ring, the slip is put down, so that the slip tooth firmly sticks to the casing. Under the effect of the self weight and taper of the casing, the more wedged the slip tooth is, the tighter the slip tooth is, so that the more tightly the slip tooth sticks to the casing.
Slip hanger
1. WE slip hanger:
Generally speaking, WE slip hanger is used for surface layer, which is characterized by large size and low sealing annulus pressure (compared with W type). Another situation is that the casing can not be lifted and lowered when it is cemented (this is a prerequisite for the W-type hanger), and WE can be installed and sealed passively (the casing is not moved, the hanger is movable, and the seal is activated manually rather than by the hanging weight) for remedy. If the hanging weight, sealing and other factors are not strictly considered, the WE type and W type with the same size and the same manufacturer can generally be interchanged.
2. W-type casing hanger
Compared with the WE type, it relies on the suspended weight of the drill string to activate the seal. Its characteristics are: the largest suspended tonnage, better sealing, more stable, and generally used for the suspension of technical casing.
3. WD hanger
It is only used for surface casing head, and the lower part is connected with surface casing.