External Casing Packer ECP Applications & Mechanism
ACP / ECP annular casing packers (also known as external casing packer) are popular because they provide improved sealing and centralization. They are used to:

- Prevent gas migration.
- Reduce the gas-to-oil production ratio.
- Reduce or prevent water production.
- Isolate production or injection zones.
- Avoid squeeze cementing.

External Annular Casing Packer ECP / ACP Applications

ACPs vary in size and valve type depending on their purpose.
https://www.drillingmanual.com/external-annular-casing-packer-acp-ecp/