The casing head is a wellhead device component installed at the upper end of the surface casing string, used to suspend various layers of casing outside the surface casing and seal the annular space of the casing. Its main functions are as follows:

1. The hanger suspends part or all of the weight of each layer of casing except for the surface casing;

2. Connect wellhead devices such as blowout preventers;

3. Forming a pressure seal between the inner and outer casing columns;

4. Provide an outlet for releasing pressure that may accumulate between two casing strings;

5. In emergency situations, fluid such as well killing fluid or fire extinguishing agent can be pumped into the well from the side hole of the casing head;

6. Special assignments:

A. If the cementing quality is poor, additional cement can be injected from the side hole;

B. During acid fracturing, pressure balancing fluid can be injected from the side hole.