it always stays in the steam plateau region i e the second operational phase in the final phase large amounts of refluxing oil are also carried out as vapor hence operation in this mode is limited to the available oil inventory unless this phase can be prolonged by replenishment of oil to the oil pool from the surface or directly from the transport pipe between the production tubing inlet and the surface in contrast to oil refluxing within the retort this oil flow is called u201c oil recycle u201d it can be u201c internal u201d if the recycle occurs from the piping system in the cased vapor production well or u201c external u201d if the recycle occurs from the surface facility as an alternative to recycling oil the retort could be shut down when the oil pool dries up such a strategy would require an optimized design of the vapor production wells minimizing channeling leading to premature termination of the retort alternatively the retort operation can continue through the recycling of liquid oil into the heater region the recycled oil can even be injected at a temperature above the normal operation of the boiling oil pool to provide supplemental heat input however it is desirable.