External Casing Packer ECP Applications & Mechanism
ACP / ECP annular casing packers (also known as external casing packer) are one of the packers types that is popular because they provide improved sealing and centralization (check also: Permanent Packers). 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.
https://www.drillingmanual.com/external-annular-casing-packer-acp-ecp/