reverse combustion, Oil & Gas Glossary

What Is Reverse Combustion?

Reverse combustion is a variant of in-situ combustion enhanced oil recovery in which ignition is initiated at a production well while air is injected at a separate injection well, causing the combustion front to propagate toward the air injection source (upstream through the reservoir) rather than toward the producer, creating a forward-moving heated zone that mobilises heavy oil ahead of the fire front for collection at the producer.

Key Takeaways

In reverse combustion, air flows toward the producer while the combustion front advances toward the injector, opposite to forward combustion direction.
The combustion front propagates upstream against the airflow direction, fed by the oxygen arriving from the injector.
Temperatures ahead of the combustion front in reverse combustion are lower than in forward combustion, reducing thermal cracking of oil.
Reverse combustion is more applicable to very heavy oils and tar sands where forward combustion generates insufficient heat to initiate spontaneously.
Operational complexity and the risk of uncontrolled channelling have limited widespread commercial deployment relative to forward (normal) in-situ combustion.

How Reverse Combustion Works

In-situ combustion burns a fraction of the heavy oil or coke deposited in the reservoir pore space to generate heat that reduces oil viscosity, enabling the oil to flow to the producer. In the more common forward combustion (also called normal combustion), air is injected at an injection well and ignition occurs at or near the injection well. The combustion front advances in the same direction as the airflow, toward the production well. The high-temperature burning zone pyrolyses oil immediately behind it, leaving a char deposit, while the heated, thermally cracked oil moves ahead of the front toward the producer.

In reverse combustion, ignition is initiated at the production well end. Air injected at the injector flows through the reservoir toward the production well where the fire was started. The combustion front then migrates upstream, against the airflow, consuming the lighter fractions of the oil and advancing toward the injector. The very heavy oil and tar ahead of the combustion front (between the fire and the producer) is heated and mobilised by the fire's thermal output but is not directly in the fire zone; it drains and flows to the producer under the combined effects of gravity and the reduced viscosity from thermal heating. This mechanism was hypothesised to be advantageous for ultra-heavy bitumen where oil viscosity is so high that no mobilisation can occur in the cold zone ahead of a forward combustion front.

Reverse Combustion Applications Across International Jurisdictions

In Canada, reverse combustion has been investigated for Athabasca oil sands recovery in Alberta's oil sands region, where bitumen viscosities exceed 1 million centipoise at reservoir temperature and conventional in-situ combustion struggles to initiate because the cold oil between the injector and the incipient combustion zone cannot be mobilised to feed the fire. Pilot tests in the Athabasca and Cold Lake areas have explored reverse combustion as a potential initiating mechanism for in-situ combustion projects in formations too viscous for forward combustion to self-sustain. AER regulation of in-situ combustion operations requires environmental impact assessments and air quality monitoring for any burning scheme in Alberta.

In the United States, reverse combustion was tested in experimental pilots in California heavy oil fields (San Joaquin Valley) and in shallow Midcontinent heavy oil reservoirs during the 1960s and 1970s when in-situ combustion was actively explored as a thermal EOR method. BSEE does not regulate onshore EOR method selection, which falls under state jurisdiction in the US. In Romania, the Suplacu de Barcau field operated one of the world's most successful commercial in-situ combustion projects for decades using forward combustion; reverse combustion variants were investigated in laboratory studies at Petrom's research facilities to address channelling problems in heterogeneous intervals. In the Middle East, research interest in air injection EOR for heavy oil recovery from Arab Formation carbonates has explored reverse combustion as a potential mechanism for high-API-gravity carbonate systems where conventional forward combustion faces challenges from formation heterogeneity.

Fast Facts

The concept of reverse combustion was proposed and investigated extensively in laboratory flow-tube combustion experiments by researchers at Stanford, the Bureau of Mines, and various oil company research laboratories between the 1950s and 1980s. The fundamental challenge that has prevented widespread commercial deployment is the difficulty of controlling the combustion front direction in heterogeneous reservoir rock: in highly permeable zones, the fire tends to channel and potentially reach the air injection well rather than uniformly sweeping the reservoir toward the producer as the theory predicts for homogeneous rock.

Reverse Combustion Versus Forward In-Situ Combustion

Forward in-situ combustion is the established commercial method, used in Romania, Canada, India, and the United States in varying degrees of success. In forward combustion, the injection well is the ignition point, the fire front moves with the airflow toward the producer, and the oil ahead of the fire is heated and mobilised. Temperatures in forward combustion exceed 400-600°C at the fire front, thermally cracking and vaporising lighter fractions. In reverse combustion, the fire moves against the airflow, temperatures ahead of the front are lower because the cold injected air has not yet been preheated, and the oil thermal cracking is less severe. This gentler thermal environment was considered an advantage for preserving oil quality, but the operational advantages proved insufficient to overcome the channelling and control challenges that limit reverse combustion's commercial viability relative to forward combustion and the competing SAGD and cyclic steam stimulation methods that now dominate Canadian heavy oil thermal operations.

Tip: When evaluating literature data on in-situ combustion EOR performance, carefully distinguish between forward and reverse combustion references because their produced fluid compositions, energy efficiencies, and operational requirements are fundamentally different. Reverse combustion produces a higher proportion of unburned heavy oil at lower temperatures; forward combustion produces lighter, partially thermally cracked oil but burns more formation oil to sustain the front. Economic comparisons that mix forward and reverse combustion performance data without distinguishing the two mechanisms will produce misleading conclusions about the viability of either method.

Reverse combustion is also referenced as:

Counter-current combustion — the flow-mechanics description used in reservoir engineering papers to indicate that the combustion front propagates counter to the direction of airflow
Reverse in-situ combustion — the full-form name used when precision about the in-situ combustion category is needed to distinguish from surface combustion or other thermal methods
Upstream combustion — descriptive term used in some SPE technical papers to indicate that the fire front moves upstream relative to the injected air direction

Frequently Asked Questions

Why is reverse combustion not more widely used commercially?

Several technical challenges have prevented wide commercial deployment of reverse combustion. The combustion front propagating against the airflow is inherently unstable in heterogeneous reservoirs: high-permeability streaks allow air to channel to the production well much faster than in the surrounding lower-permeability matrix, creating conditions where the combustion front in the high-permeability streak can potentially reach the injection well without sweeping the rest of the reservoir. Additionally, sustained ignition at the production well requires maintaining the combustion temperature in the face of the inflowing cold air stream from the injector, which tends to quench the fire. The combination of air channelling risk and ignition difficulty has led most operators to prefer forward combustion, cyclic steam, or SAGD over reverse combustion for commercial heavy oil recovery.

What is the difference between in-situ combustion and SAGD?

Both are thermal heavy oil recovery methods but they use fundamentally different heat sources. In-situ combustion (forward or reverse) generates heat by burning a fraction of the oil in the reservoir through air injection, with no external energy input required to sustain combustion once the fire is established. SAGD (steam-assisted gravity drainage) generates heat by injecting steam from a surface boiler into a horizontal well pair, relying entirely on external energy (fuel combustion at surface to generate steam) to deliver thermal energy to the reservoir. SAGD is the dominant commercial method for Alberta oil sands because it provides more predictable, controllable heating and higher recovery factors under the conditions found in Athabasca, Cold Lake, and Peace River oil sands; in-situ combustion offers potential energy efficiency advantages for specific reservoir conditions but has seen less commercial success in the oil sands context.

Why Reverse Combustion Matters in Oil and Gas

The world's largest hydrocarbon resource base by volume is the extra-heavy oil and bitumen in Canada's oil sands (estimated at 170 billion barrels recoverable) and Venezuela's Orinoco Belt. Current commercial recovery from these resources uses steam-based methods that are energy-intensive and generate significant greenhouse gas emissions from steam generation. In-situ combustion, if commercially deployable at scale, would use a fraction of the energy of steam methods because the heat is generated within the reservoir from the oil itself, not from external fuel combustion. Reverse combustion research represents one pathway toward unlocking the efficiency advantages of in-situ combustion for the ultra-heavy oil resources where forward combustion has not succeeded, and renewed interest in lower-carbon-footprint thermal recovery methods is driving fresh academic and industry research into combustion-based EOR alternatives.

Reverse Combustion: Definition, In-Situ Combustion EOR, and Heavy Oil Recovery