How does in situ combustion recover heavy oil from a reservoir?

In situ combustion is a thermal recovery method used to extract heavy oil by burning a portion of the oil directly within the reservoir. This process generates heat in place, which reduces the oil's viscosity, allowing it to flow more easily toward production wells.12

The operation involves injecting large quantities of compressed air into the reservoir to ignite the oil and sustain a burning front. Several variations exist:12

• Forward combustion: Air is injected to advance the burning front, heating and displacing oil and formation water toward production wells.1
• Modified forward combustion: Cold water is injected alongside air to recover residual heat from behind the combustion front, which reduces the amount of air required and limits the amount of in-place oil consumed to between 5 and 10 percent.1
• Reverse combustion: A short-term forward burn is initiated at a production well before switching air injection to adjacent wells. This is specifically used for extremely viscous oil that cannot move through the cold zones found ahead of a standard forward-combustion front.1

In situ combustion is generally preferred for deeper reservoirs, whereas steam-based methods are typically used for shallower ones.12