COFCAW

COFCAW (combination of forward combustion and waterflooding) is a thermal enhanced oil recovery method for heavy oil and tar sand reservoirs in which an in-situ combustion front (created by injecting air or enriched air into the reservoir to ignite and sustain burning of a fraction of the reservoir oil, 3 to 10 percent of OOIP) is combined with a simultaneous or alternating water injection program into the same well or adjacent wells, using the injected water to recover heat from the burned-out zone behind the combustion front and carry it forward to the unburned oil ahead, effectively converting the dry forward combustion process into a steam-assisted displacement where in-situ generated steam (formed as injected water contacts the hot burned zone, typically 350 to 650 degrees Celsius) mobilizes and displaces oil ahead of the thermal front at higher recovery efficiency than dry forward combustion alone; the COFCAW process was developed and field-tested in the 1960s and 1970s to address the primary limitation of forward in-situ combustion (ISC): large quantities of heat deposited in the already-burned zone behind the front cannot be transported forward to the cold oil bank, resulting in excessive fuel consumption (3 to 10 percent of reservoir oil burned), low thermal efficiency, and poor sweep due to gravity override and viscous fingering of combustion gases through the heavy oil. In the Western Canada Sedimentary Basin, COFCAW has been investigated and piloted primarily in the Lloydminster and Provost areas where Cretaceous Mannville Group heavy oil pools (API gravity 8 to 16 degrees, in-situ viscosity 1,000 to 100,000 mPa-s) are too shallow (200 to 500 m) or too thin (5 to 15 m net pay) for SAGD but too viscous for cold production; while SAGD has largely displaced COFCAW for thick WCSB bitumen pools, in-situ combustion variants remain of interest where high SAGD capital costs or the absence of economic-quality water for steam generation limit the SAGD option, particularly in remote northern Alberta and Saskatchewan where natural gas for once-through steam generators is expensive and air injection represents a lower-cost thermal drive.

• Forward combustion mechanics and the role of water injection in the COFCAW process: In forward in-situ combustion, air injected into the reservoir sustains burning of a coke-like fuel (formed by the thermal cracking and distillation of reservoir oil in the heated zone adjacent to the combustion front) that propagates the combustion front in the same direction as air injection; the combustion zone temperature of 350 to 650 degrees Celsius generates flue gases (CO2, N2, and water vapor) that displace oil ahead while simultaneously upgrading the crude oil by thermal cracking (reducing viscosity and increasing API gravity by 2 to 6 degrees). In dry forward combustion without water injection, the swept zone behind the combustion front rapidly cools as heat is deposited in the formation matrix (sandstone thermal conductivity 1.5 to 3.0 W/m-K conducts heat radially away from the combustion tube), wasting 40 to 60 percent of the generated heat in the overburden and underburden rather than transporting it toward the producer; COFCAW addresses this by injecting water at the air injector or in a separate pattern, converting residual heat in the burned zone to steam at in-situ conditions (injection water contacts rock at 200 to 400 degrees Celsius, flashing to steam at the local pressure) that migrates forward and condenses at the thermal front where oil mobilization occurs. The water-to-air ratio in COFCAW is critical: insufficient water fails to recover heat from the burned zone; excessive water quenches the combustion front and extinguishes burning, requiring re-ignition; WCSB pilot experience and laboratory combustion tube studies indicate optimal water-to-air ratios of 1.0 to 3.0 litres of water per standard m3 of air injected for Mannville heavy oil reservoirs.
• COFCAW pilot performance and recovery factors in WCSB Mannville heavy oil pools: Field experience with COFCAW and dry in-situ combustion in WCSB Mannville heavy oil is limited compared to Cold Lake CSS and Athabasca SAGD but includes the Marguerite Lake pilot (Imperial Oil, 1970s, Cold Lake area, Clearwater Formation at 450 m depth) where dry forward combustion was tested in a 5-spot pattern before being converted to CSS; the Medicine Hat COFCAW pilot (Petro-Canada, 1980s, Sparky/General Petroleum Formation at 350 m depth near Provost, Alberta) where a 3-well pattern operated for 3 years with incremental oil recovery of 18 percent OOIP attributed to the combined combustion-steam drive; and various small field tests in Lloydminster-area pools where in-situ combustion was initiated by downhole electric heaters or injected pyrophoric compounds and sustained by air injection for 6 to 24 months. Recovery factors for COFCAW in these WCSB pilots ranged from 15 to 30 percent OOIP in the directly swept pattern area, higher than the 8 to 15 percent OOIP for dry forward combustion in comparable reservoirs, reflecting the heat recovery improvement from water injection; however, all WCSB COFCAW pilots suffered from the common field challenge of air injectivity (the high-permeability channels through heterogeneous Mannville sands cause air to bypass the combustion front and break through to producers without sustaining burning) and corrosion from combustion flue gases in producer wellbores.
• Air injection, ignition methods, and combustion monitoring in WCSB ISC and COFCAW programs: Air injection for WCSB in-situ combustion programs uses high-pressure air compressors (typically 3 to 10 MPa injection pressure for Mannville reservoirs at 200 to 500 m depth) capable of sustained injection rates of 50,000 to 200,000 standard m3 per day per injector; air compressor capital cost of $2 to $5 million per installed injection station and operating cost of $0.50 to $1.50 per standard m3 of air (electricity, maintenance, fuel for compression) is the largest operating cost driver in WCSB ISC programs. Ignition of the in-situ combustion front in WCSB pilots uses downhole electric heaters (5 to 50 kW, operated for 2 to 8 weeks to raise the near-wellbore temperature above the oil auto-ignition temperature of 180 to 280 degrees Celsius for Mannville crude), pyrophoric material (liquid phosphorus sulfide or similar compound injected ahead of the air) that ignites spontaneously on contact with reservoir oxygen, or direct oxygen injection at higher concentration than air to accelerate ignition kinetics. Combustion monitoring in WCSB ISC programs uses produced gas analysis (CO2 and O2 mole fractions in producer gas, with CO2 above 10 percent and O2 below 2 percent confirming active combustion in a swept zone connected to the producer), produced fluid temperature (sustained temperatures above 150 degrees Celsius at the producer indicating thermal front breakthrough), and oxygen utilization efficiency (the fraction of injected oxygen consumed by combustion reactions versus slip to producers).
• COFCAW versus SAGD and CSS applicability in WCSB shallow thin-pay heavy oil reservoirs: The principal competitive situation for COFCAW in WCSB thermal recovery program selection is against SAGD and CSS for shallow (200 to 500 m), thin-pay (5 to 15 m) Mannville heavy oil pools in the Lloydminster, Provost, and Wainwright areas where SAGD minimum pay requirement of 20 m net continuous sand is not met and CSS first-cycle SOR of 3 to 5 tonnes CWE per m3 is economically marginal at heavy oil prices below $50 per barrel; the economic advantage of COFCAW over SAGD in these reservoirs is the elimination of surface steam generation facilities (OTSG capital cost $15 to $30 million per installation), replaced by air compressors at $5 to $10 million capital, reducing threshold production volume for project economics. The disadvantages of COFCAW relative to SAGD and CSS that have limited its WCSB adoption include: operational complexity of sustaining a combustion front in heterogeneous reservoir sands over multi-year timeframes; corrosion of producer wellbore tubulars from hot combustion gases (H2S and CO2 in flue gas combined with water at 200 to 300 degrees Celsius require 13Cr or higher alloy steel tubing at $80,000 to $150,000 premium per well); and regulatory complexity of air injection operations governed by AER Directive 040 (Schemes for Improved Recovery) which requires detailed air injection safety plans, explosion hazard analysis of produced gas handling facilities, and monitoring protocols to prevent air accumulation in producer wellbores above the explosive limit of 5 percent combustion gas in air.
• Laboratory combustion tube testing and numerical simulation for WCSB COFCAW design: Laboratory combustion tube tests (1 to 3 m long, 5 to 10 cm diameter pressure vessels packed with reservoir sand and live crude oil at reservoir temperature and pressure) are the primary design tool for WCSB ISC and COFCAW programs, measuring the fuel consumption (kg of coke deposited and burned per m3 of reservoir swept, typically 15 to 40 kg/m3 for Mannville heavy oils), air requirement (standard m3 of air per m3 of reservoir swept, typically 150 to 400 m3/m3), combustion front velocity (0.01 to 0.1 m/day), and the temperature, pressure, and gas composition profile through the combustion tube at steady state. Numerical simulation of WCSB COFCAW programs uses thermal reservoir simulators (CMG STARS, Schlumberger ECLIPSE Thermal, or Landmark VIP-THERMAL) with kinetic models for combustion reactions (low-temperature oxidation, cracking, coke deposition, and high-temperature combustion) calibrated to combustion tube data; simulator outputs include oil recovery factor versus time, air injection volume requirement, and the spatial distribution of temperature, oil saturation, and combustion gases in the reservoir to optimize well spacing, air injection rate, and water-to-air ratio for field implementation.

COFCAW Pilot Improving Recovery in WCSB Mannville Heavy Oil Pattern

A WCSB Lloydminster area operator piloted COFCAW in a 4-hectare 5-spot pattern in the Sparky Formation (8 m net pay, 400 m depth, 14 API gravity oil, 8,500 mPa-s viscosity at reservoir temperature). A downhole electric heater ignited the combustion front after 3 weeks of heating at 450 degrees Celsius measured downhole; air injection at 65,000 standard m3 per day was maintained for 18 months before water injection at 45 m3 per day was initiated at the air injector. Produced gas CO2 peaked at 14 percent and O2 dropped to 1.5 percent at 8 months, confirming active combustion in 2 of 4 producers. After 30 months of COFCAW operation, cumulative oil production from the pattern was 4,800 m3, representing 22 percent OOIP of estimated in-place volume; the pre-pilot waterflood from the same pattern had produced 6 percent OOIP over 5 years. Air-to-oil ratio was 380 standard m3 per m3 of incremental oil, within the economic range at Alberta power costs of $0.07 per kWh. Corrosion in one producer required 13Cr tubing replacement at month 20, adding $95,000 to operating costs but allowing continued production for the remaining pilot period.

Related Terms

In-situ combustion (ISC) is the parent process for COFCAW; forward ISC burns 3-10% of reservoir oil as fuel; COFCAW adds water injection to recover heat from the burned zone and increase sweep efficiency. Steam-assisted gravity drainage (SAGD) has largely displaced COFCAW in thick WCSB bitumen pools; COFCAW is evaluated where SAGD minimum pay of 20 m is not met or steam generation facilities are too costly. Enhanced oil recovery (EOR) encompasses both COFCAW and SAGD as thermal methods; COFCAW uses air rather than steam, eliminating water treatment and boiler requirements in remote WCSB locations. Cyclic steam stimulation (CSS) competes with COFCAW for shallow thin-pay WCSB Mannville heavy oil; CSS SOR of 3-5 tonnes CWE per m3 is marginal at low oil prices, making air injection an alternative where gas for steam generation is expensive. Air injection provides the oxidant for WCSB ISC and COFCAW; compressor capital of $2-5 million per station and $0.50-1.50 per standard m3 operating cost are the primary economic drivers versus steam generation.