What is the function of wellhead casing head
　　The wellhead casing head is installed on the upper end of the surface casing string. The wellhead casing head is used to hang the casing of each layer outside the surface casing and to seal the wellhead components in the annular space of the casing. The main functions of the wellhead casing head are as follows
　　1. The wellhead device suspends part or all of the weight of casing except surface casing through hanger;
　　2. The wellhead device is connected with BOP and other wellhead devices;
　　3. The wellhead equipment forms a pressure seal between the inner and outer casing strings;

　　4. The wellhead device provides an outlet for releasing the pressure that may accumulate between two casing strings;
　　5. In case of emergency, the wellhead equipment can pump fluid 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 not good, the side hole can be used to inject cement; B. during acid fracturing, the pressure balance fluid can be injected from the side hole.