In-Situ Combustion

In-situ combustion (ISC) is a thermal enhanced oil recovery method in which air or oxygen-enriched air is injected into a heavy oil or bitumen reservoir and ignited, generating a high-temperature combustion front that moves through the formation, thermally cracking and mobilizing viscous oil ahead of it while consuming a fraction of the heaviest in-place hydrocarbons as fuel.

Key Takeaways

ISC generates temperatures of 350 to 650 degrees C at the combustion front, far exceeding steam-based thermal methods, which enables viscosity reduction, thermal cracking, and in-situ upgrading of heavy crude.
Forward combustion is the dominant variant: air is injected at the injector and the front moves toward producers. Reverse combustion (rare) propagates the front from producer to injector but is difficult to control and rarely used commercially.
The THAI (Toe-to-Heel Air Injection) process combines in-situ combustion with a horizontal producer well, confining the combustion zone and improving combustion efficiency and oil drainage compared to conventional ISC with vertical producers.
Air compression to reservoir injection pressure is a major capital and operating cost for ISC; compressor systems must be sized for sustained high-pressure injection, typically 3 to 15 MPa depending on depth.
Heavy oil reservoirs in Cold Lake, Lloydminster, and Romania's Suplacu de Barcau have hosted the most successful commercial ISC projects; Suplacu is the longest continuously operating ISC field in the world, active since 1964.

Fast Facts

In-situ combustion typically consumes 5 to 15 percent of the original oil in place as fuel to sustain the combustion front. The Suplacu de Barcau field in Romania has been under ISC since 1964, producing heavy crude continuously for over 60 years. ISC can theoretically achieve higher recovery factors than SAGD (40 to 60 percent OOIP in ideal conditions) because it does not require steam generation or water disposal at the same scale. The THAI process was developed by Petrobank Energy (now Touchstone Exploration) in Canada.

What Is In-Situ Combustion?

In-situ combustion is the most thermally intense EOR method available for heavy oil and bitumen. Rather than injecting heat from surface (as with steam), ISC generates heat inside the reservoir by sustaining a combustion reaction using the heaviest fractions of the crude itself as fuel. This eliminates the energy penalty of generating and injecting steam and allows operation in formations where steam injection would be impractical due to depth, pressure, or the lack of water supply.

The process belongs to the broader category of thermal EOR, alongside steam-assisted gravity drainage (SAGD) and cyclic steam stimulation (CSS). Unlike steam, ISC produces a chemically altered oil product: the combustion front thermally cracks high-molecular-weight asphaltenes and resins, producing lighter, lower-viscosity hydrocarbons that flow more readily to producers. This in-situ upgrading effect can increase the API gravity of produced oil by 2 to 8 degrees.

How In-Situ Combustion Works

In a forward dry combustion project, air is compressed to reservoir pressure and injected through one or more injection wells. Initially, ignition is achieved either by spontaneous ignition (if the oil is sufficiently reactive at reservoir temperature) or by a downhole igniter. Once ignited, the combustion front advances outward from the injector, driven by the continuous air supply.

Moving away from the injector in the direction of flow, the reservoir is divided into several distinct zones. Immediately behind the combustion front is a burned zone, which is hot, largely depleted, and acts as a conduit for injected air. Ahead of the front is the coke deposition zone, where temperatures are high enough to crack heavy oil fractions and deposit solid coke that serves as the primary fuel. Further ahead lies the condensation zone, where steam and hot gases from the combustion carry heat forward and condense, creating a hot water bank. Beyond this, thermally cracked and mobilized oil banks ahead of the hot fluid zone are pushed toward producer wells.

Wet combustion adds water injection to the air stream, generating steam in the burned zone that carries additional heat forward more efficiently than hot gas alone. Water-alternating-air injection cycles can improve heat utilization and reduce the total air volume needed per barrel of oil produced.

The THAI (Toe-to-Heel Air Injection) process places horizontal producer wells with open intervals directly beneath the combustion zone. Oil drains by gravity into the horizontal producer as the combustion front advances toe-to-heel. The geometry confines combustion gases and improves air utilization, producing a more predictable and controllable sweep compared to conventional ISC with vertical producers.

In-Situ Combustion Across International Jurisdictions

Canada (AER / WCSB): Alberta's Cold Lake and Lloydminster regions have hosted multiple ISC pilots and small commercial projects, driven by the high viscosity of Lloydminster-type heavy oil (API 10 to 14) and Cold Lake bitumen. The AER regulates ISC under its enhanced recovery directive framework, requiring detailed injection plans, air compression specifications, and monitoring programs for combustion gas production (CO2, CO) at producers. The THAI process was piloted at Whitesands in Alberta by Petrobank, with encouraging recovery results.

United States (BSEE / State Agencies): California's Kern County hosted early ISC pilots in the Midway-Sunset field during the 1950s and 1960s. The Texas Railroad Commission and California DOGGR have both issued permits for ISC field tests. Wider commercial adoption in the US has been limited by the economic preference for steam flooding in shallow California heavy oil and by operational complexity relative to SAGD in colder northern climates.

Romania (NAMR): The Suplacu de Barcau field, operated by Petrom (now OMV Petrom) in northwest Romania, is the benchmark commercial ISC operation globally. With over 60 years of continuous operation and hundreds of producer wells, it has demonstrated that ISC can be sustained economically at field scale in a Miocene heavy oil reservoir. Romanian regulatory oversight sits with the National Agency for Mineral Resources (NAMR), which has supported continued ISC development as part of Romania's heavy oil recovery strategy.

Middle East and International: Saudi Aramco has evaluated ISC for its heavy oil resources in the Wafra and Manifa fields. High reservoir temperatures reduce ignition challenges but increase cooling and material requirements for injection equipment. India's ONGC has conducted ISC pilots in the Balol and Santhal fields of Gujarat, targeting Eocene heavy oil reservoirs, with mixed results due to reservoir heterogeneity and early gas breakthrough.

In-situ combustion is also called fire flooding, in-situ fire flooding, or underground combustion. Related terms include thermal EOR, SAGD, THAI, air injection, cyclic steam stimulation (CSS), heavy oil, bitumen, and in-situ upgrading. Forward combustion and reverse combustion are sub-variants; dry combustion (air only) and wet combustion (air plus water) describe injection fluid composition variants.

Tip: The air-oil ratio (AOR, in cubic metres of air injected per cubic metre of oil produced) is the primary economic indicator of ISC efficiency. AORs below 300 sm3/sm3 are generally considered economic; AORs above 500 sm3/sm3 indicate poor combustion efficiency or air channeling through the burned zone, which sharply increases compression costs and reduces project economics. Monitor the producing gas composition (CO to CO2 ratio) to assess combustion front quality: a high CO2 to CO ratio indicates complete high-temperature oxidation; a rising CO fraction signals incomplete combustion or front deterioration.

FAQ

What is the difference between forward and reverse combustion?
In forward combustion, air is injected and the combustion front moves in the same direction as air flow, away from the injector toward producers. This is the commercial standard because it is self-sustaining once ignited. In reverse combustion, the front moves against the air flow, from producer toward injector. Reverse combustion theoretically maintains higher temperatures and avoids coke plugging near the injector, but it is extremely difficult to control and has not achieved commercial success.

How does ISC compare to SAGD for bitumen recovery?
SAGD is the dominant commercial technology for oil sands bitumen in Alberta due to its predictable performance, existing infrastructure, and relatively straightforward operation. ISC offers potential advantages in energy efficiency (no steam generation required), deeper application, and in-situ upgrading. However, ISC operational complexity, air compression capital cost, and challenges managing combustion gas production have limited its commercial deployment relative to SAGD. THAI-ISC is considered a potential long-term complement to SAGD for deeper or thinner bitumen reservoirs.

Why In-Situ Combustion Matters

Global heavy oil and bitumen resources dwarf the combined conventional oil endowment, but the majority of this resource remains stranded or only partially recovered by primary and steam-based methods. ISC offers a path to higher recovery factors and lower-carbon thermal EOR (the heat source is the oil itself, not natural gas burned to generate steam) that could improve the energy return on investment for heavy oil development. As operators and governments seek to maximize recovery from existing fields while reducing the energy intensity of thermal operations, in-situ combustion and its THAI variant represent technically credible, commercially demonstrated options for heavy oil basins worldwide.