Once a field is past its prime and waterflooding or gas injection has reached its limits, other solutions need to be considered to increase recovery further.
Heavy oil reservoirs are notoriously difficult to produce and primary recovery is often low even with CHOPS (Cold Heavy Oil Production with Sand).
Oil wet reservoirs can pose specific challenges for recovery in particular when fractured. Carbonates in particular are often both oil wet and fractured.
CO2 injection is a very successful tertiary recovery method which can however suffer from gravity override and early breakthrough due to thief zones. This leads to the need to recycle CO2 which has a negative impact on the economics of the process.