Wet Combustion (In-Situ)

Wet combustion is a variant of in-situ combustion (ISC) enhanced oil recovery in which water is co-injected with compressed air into the reservoir, generating steam in the hot burned zone behind the combustion front and driving that steam forward ahead of the front where it heats the oil bank and reduces viscosity; it achieves superior heat utilization efficiency compared to dry combustion by recovering thermal energy from the burned-out coke zone rather than leaving it as wasted subsurface heat, and reduces the air-oil ratio (AOR) requirements that make dry in-situ combustion projects economically marginal at high air compression costs.

What Is Wet Combustion?

In-situ combustion (ISC), sometimes called fire flooding, is a thermal EOR process in which oxygen-containing gas (typically compressed air) is injected into a heavy oil or bitumen reservoir where a combustion front is ignited and sustained by the burning of a small fraction of the reservoir oil (converted to coke by the heat). The combustion front generates temperatures of 350-600 deg C, dramatically lowering oil viscosity in adjacent zones and driving a thermally mobilized oil bank toward production wells.

Dry in-situ combustion (air injection only) has a fundamental thermodynamic inefficiency: the high-temperature burned zone behind the combustion front contains enormous thermal energy, but since the only injected fluid is air, that heat largely remains stranded in the subsurface and is never transferred to the oil bank ahead of the front. Wet combustion addresses this by co-injecting water with the air, converting the stranded heat into steam that migrates forward to pre-heat the oil bank, significantly improving the overall thermal efficiency of the ISC process.

How Wet Combustion Works

The wet combustion process can be divided into distinct zones moving from the injection well toward the production well. At the injection face, compressed air (or air plus water) contacts the hot reservoir and maintains the combustion front by supplying oxygen. Immediately behind the front is the burned zone: fully oxidized rock at temperatures of 400-600 deg C, depleted of combustible fuel. In dry combustion, this zone cools slowly by conductive heat transfer with no active heat transport. In wet combustion, injected water flows into this burned zone, contacts the hot rock, and vaporizes to steam at formation pressure.

The generated steam migrates forward through the partially heated zones ahead of the combustion front, condensing where it encounters cooler rock and oil. This steam condensation front delivers a thermal banking effect, creating a hot water zone that provides viscosity reduction and partial solvent stripping of lighter oil fractions from the residual oil. The combination of steam condensate drive, thermal viscosity reduction, and solvent banking ahead of the combustion front produces higher oil recovery than dry combustion alone.

Three process variants are distinguished by water-air ratio. In normal wet combustion, the water-air ratio is maintained at a level that recovers most of the burned-zone heat as steam without extinguishing the combustion front; temperatures in the burned zone drop from 500-600 deg C to 200-300 deg C. In super wet combustion, excess water injection fully quenches the combustion front; the process transitions to a hot waterflood driven by steam condensate, which can achieve high sweep efficiency but loses the direct viscosity reduction of active combustion. Partially quenched combustion is an intermediate regime where alternating periods of dry and wet injection are used to control front temperature and steam generation.

The COFCAW process was developed to use waterflooding as a secondary recovery mechanism combined with forward combustion. In COFCAW, water is injected at rates designed to maintain a hot waterflood bank ahead of the combustion front. The process achieves high displacement efficiency in the swept zone because the combined action of steam, hot water, and combustion gases strips and mobilizes oil that would otherwise be left as residual in the post-combustion zone. Surface equipment requirements for COFCAW include high-capacity air compressors, water injection pumps, heat exchangers for produced water recovery, and separation facilities designed for the hot, high-volume produced fluids characteristic of thermal floods.

Wet Combustion Across International Jurisdictions

In Canada, wet combustion and in-situ combustion research has been driven by the enormous heavy oil and oil sands resources of the WCSB, particularly the Athabasca, Cold Lake, and Lloydminster heavy oil deposits. The AER regulates ISC projects under its enhanced recovery approval process, requiring full reservoir simulation, combustion tube test validation, and environmental impact assessment. Petrobank Energy (now Touchstone Exploration) piloted the THAI (Toe-to-Heel Air Injection) process, a variant of ISC combined with a horizontal production well, at Whitesands, Alberta in the 2000s; earlier COFCAW tests in Alberta's heavy oil pools were conducted by Alberta Research Council and Husky Energy in the Lloydminster area.

In the United States, the Department of Energy and predecessor agencies funded numerous in-situ combustion and wet combustion pilot projects from the 1950s through the 1990s under the Enhanced Oil Recovery program. Notable projects include the Bellevue field in Louisiana (one of the longest continuously operating ISC projects in the world), the Brea-Olinda field in California, and the Horse Creek field in Montana. The DOE National Energy Technology Laboratory (NETL) maintains a comprehensive database of all US ISC field pilots. The majority of US ISC projects have been in heavy oil reservoirs in California, Texas, and Louisiana where thermal EOR methods are most applicable.

In Norway, the Norwegian Research Centre (NORCE) and the University of Bergen have conducted laboratory-scale wet combustion and ISC research applicable to North Sea carbonate and clastic reservoirs; however, no full-field ISC projects have been implemented on the NCS due to the reservoir characteristics (light oil, high reservoir pressure) being generally unsuited to in-situ combustion relative to SAGD or miscible gas injection alternatives. The Sodir maintains research collaboration with international institutions studying ISC applicability to future NCS enhanced recovery scenarios.

In the Middle East, high reservoir temperatures and relatively low-viscosity crudes in most producing formations of Saudi Arabia, Kuwait, and the UAE make conventional in-situ combustion less applicable than in heavy oil basins. However, the heavy oil accumulations of Kuwait's Wafra field and Oman's Qarn Alam carbonate reservoir have been evaluated for various thermal EOR methods including in-situ combustion variants. Kuwait Oil Company and Petroleum Development Oman (PDO) have conducted feasibility studies and small-scale pilots for ISC-type processes in these high-viscosity carbonate settings, with Saudi Aramco's research division maintaining ongoing interest in wet combustion for future application to their significant heavy oil resources.

Synonyms and Related Terminology

Wet combustion is also called forward wet combustion, COFCAW (combination of forward combustion and waterflooding), or wet in-situ combustion. The non-water version is called dry combustion or dry forward combustion. Related terms include in-situ combustion, thermal EOR, SAGD, air-oil ratio (AOR), combustion front, and enhanced oil recovery (EOR).

FAQ

Q: What is the main advantage of wet combustion over dry combustion?
A: The primary advantage is significantly higher thermal efficiency. In dry combustion, the heat generated in the burned zone stays in the rock behind the front, wasting most of the energy. In wet combustion, injected water picks up that heat and carries it forward as steam to the oil bank, potentially increasing heat utilization from around 20-30% in dry combustion to 60-70% in wet combustion. This translates to lower air compression costs per barrel of oil recovered, which is often the key economic driver for ISC projects.

Q: Can wet combustion extinguish the combustion front?
A: Yes, if the water-air ratio is too high (super wet conditions), the injected water can quench the combustion front by absorbing heat faster than combustion generates it, effectively extinguishing active burning. The process then transitions to a hot waterflood at formation temperature. This is not necessarily a failure if significant reservoir heating has already been accomplished, but recovering an extinguished ISC front requires reducing water injection and re-igniting, which is complex and costly.

Why Wet Combustion Matters

Wet combustion matters because it represents one of the most thermally efficient in-situ methods for recovering viscous heavy oil and bitumen that cannot be economically produced by primary or conventional secondary methods. The world's heavy oil resources exceed conventional crude reserves by a factor of three or more; developing viable thermal EOR processes for these resources is a long-term strategic priority. Wet combustion's ability to extract stranded heat from the burned zone and convert it to useful steam pre-heating reduces the air compression energy penalty that makes dry in-situ combustion marginal in many applications, potentially making ISC competitive with SAGD in resources where natural gas for steam generation is expensive or unavailable.