Casing head is an important connecting piece between casing and wellhead equipment. It is installed on the upper end of surface casing string, used to hang casing of different layers outside surface casing and wellhead equipment components to seal casing annular space.
　　The structure of casing head is not complicated, which is composed of casing hanger, cross, flange type parallel gate valve, pressure display mechanism, etc.
　　According 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.
　　The single-stage casing head is generally used in low-pressure shallow formation production wells.
　　The two-stage casing head is suitable for most areas with clear formation pressure and is widely used.

　　The tertiary casing head is generally used in high pressure deep wells or exploration wells.
　　The functions of casing head are as follows:
　　1. Connect BOP and other wellhead devices;
　　2. A pressure seal is formed between the inner and outer casing strings;
　　3. The hanger is used to suspend part or all of the weight of casing except surface casing;
　　4. Provide outlet for releasing pressure that may accumulate between two casing strings;
　　5. In case of emergency, fluid, such as killing fluid or fire extinguishing agent, can be pumped into the well from the side hole of casing head;