In situ combustion, or fire flooding, is a unique EOR process because a portion ofthe oil-in place is oxidized and used as a fuel to generate heat.

In the in situ combustion process, the crude oil in the reservoir is ignited andthe fire is sustained by air injection.

The process is initiated by continuous injection of air into a centrally locatedinjection well.

Ignition of the reservoir crude oil can either occur spontaneously after air hasbeen injected over some length of time of it requires heating.

Chemical reaction between oxygen in the injected air and the crude oil generatesheat even without combustion.

Depending on the crude composition, the speed of this oxidation process may besufficient to develop temperatures that ignite the oil.

If not, ignition can be initiated by: downhole electric heaters;

preheating injection air; or

preceding air injection with oxidizable chemicals.

a schematic view of several distinct zones formed in an oil reservoir during the

forward combustion process

Numerous laboratory experiments and field oil recovery data indicate generated heat andthe vaporized hydrocarbon gases from the in situ combustion procedure will displace100% of the oil from the reservoir that is contacted by the process.

There are three forms of in situ combustion processes:

forward combustion;

reversed combustion;

wet combustion.

Forward Combustion

The term “forward combustion” is used to signify the fact that the flame front isadvancing in the same direction as the injected air.

There are seven zones that havebeen recognized during the forwardcombustion process, these are:

The burned zone

The burned zone is the region thatis already burned.

This zone is filled with air and maycontain a small amount of residualunburned organic materials;otherwise, it is essentially composedprimarily of clean sand that iscompletely free of its oil or cokecontent.

Because of the continuous airinjection, the burned zonetemperature increases from theinjected air temperature at theinjector to the temperature at thecombustion leading edge.

Combustion front zone

Ahead of the burned-out zone is combustion front region with a temperature variationranging from 600 oF to 1200 oF.

It is in this region that oxygen combines with fuel and high-temperature oxidationoccurs.

The coke zone

Immediately ahead of the combustion front zone is the coke region.

The coke region represents the zone where carbonaceous material has been deposited asa result of thermal cracking of the crude oil.

The coke residual fractions are composed of components with high molecular weight andboiling point temperatures.

These fractions can represent up to 20% of the crude oil.

Vaporizing zone

Ahead of the coke region is the vaporizing zone that consists of vaporized lighthydrocarbons, combustion products, and steam.

Temperatures across this zone vary from the high temperature of combustion to thatnecessary to vaporize the reservoir connate water

Condensing zone

Further downstream of the vaporizing region is the condensing zone, from which oil isdisplaced by several driving mechanisms.

The condensed light hydrocarbons displace reservoir oil miscibly, condensed steam createsa hot water flood mechanism, and the combustion gases provide additional oil recoveryby gas drive.

Temperatures in this zone are typically 50oF - 200oF above initial reservoir temperature.

Oil bank zone

The displaced oil accumulates in the next zone to form an oil bank.

The temperature in the zone is essentially near the initial reservoir temperature withminor improvement in oil viscosity.

Undisturbed reservoir

Further ahead of the oil bank lies the undisturbed part of the reservoir which has notbeen affected by the combustion process.

The reserve combustion technique has been suggested for application inreservoirs that contain extremely viscous crude oil systems.

The reverse combustion process is first started as a forward combustion processby injecting air in a well that will be converted later to a producer.

After establishing ignition and burning out a short distance in the oil sand, thewell is put on production and air injection is switched to another adjacent well.

The air injection in the adjacent well displaces the oil toward the producing wellpassing through the heated zone while the combustion front travels in theopposite direction toward the air injection well.

However, if the oil around the air injection well ignites spontaneously, the air (i.e.,oxygen supply) is stopped and the process reverts to a forward combustionscheme.

The reverse combustion process has not been successful economically for the followingtwo major reasons:1. Combustion started at the producer results in hot produced fluids that oftencontain unreacted oxygen.

These conditions require special, high-cost tubular to protect against hightemperatures and corrosion.

More oxygen is required to propagate the front compared to forward combustion,thus increasing the major cost of operating an in situ combustion project.

2. Unreacted, coke-like heavy ends will remain in the burned portion of the reservoir.

At some time in the process, the coke will start to burn and the process willrevert to forward combustion with considerable heat generation but little oilproduction.

This has occurred even in carefully controlled laboratory experiments.

Heat utilization in the forward combustion process is very inefficient due to the factthat air has a poor heat-carrying capacity.

Only about 20% of the generated heat during the forward combustion scheme is carriedforward ahead of the combustion front where it is beneficial to oil recovery.

The remaining heat is stored in the burned zone and is eventually lost to the cap andbase rock of the pay zone.

Several variations of the in situ process have been proposed to utilize this lost heat.

Water may be injected simultaneously or alternately with air, resulting in better heatdistribution and reduced air requirements.

In the burned zone, injected water is converted to superheated steam which flowsthrough the flame and heats the reservoir ahead.

This is called COFCAW (combination of forward combustion and water flood) process.

As the superheated system mixed with air reaches the combustion front, only the oxygenis utilized in the burning process.

On crossing the combustion front, the superheated steam mixes with nitrogen from theair and flue gas consisting mainly of CO and CO2.

This mixture of gases displaces the oil in front of the combustion zone and condenses assoon as its temperature drops to about 400oF.

The length of the steam zone is determined by the amount of heat recovered from theburned zone upstream.

Depending on the water/air ratio, wet combustion is classified as:

incomplete when the water is converted into superheated steam and recoversonly part of the heat from the burned zone;

normal when all the heat from the burned zone is recovered;

quenched or super wet when the front temperature declines as a result of theinjected water.

When operated properly, water-assisted combustion reduces the amount of fuelneeded, resulting in increased oil recovery and decreased air requirements to heata given volume of reservoir.

Up to 25% improvement in process efficiency can be achieved.

Determination of the optimum water/ air ratio is difficult because of reservoirheterogeneities and gravity override that can affect fluid movement andsaturation distributions.

Injecting too much water can result in an inefficient fire front, thus losing thebenefits of the process

There are several disadvantages with the in situ combustion process including:

formation of oil-water emulsions which cause pumping problems and reduce well productivity;

production of low-pH (acidic) hot water rich in sulfate and iron that causes corrosionproblems;

increased sand production and cavings;

formation of wax and asphaltene as a result of thermal cracking of the oil;

liner and tubing failure due to excessive temperatures at the production wells.

The in situ combustion process has a tendency to sweep only the upper part of the oil zone;therefore, vertical sweep in very thick formations is likely to be poor.

The burning front produces steam both by evaporating the interstitial water and by combustionreactions.

The steam mobilizes and displaces much of the heavy oil ahead of the front, but when watercondenses from the steam it settles below steam vapors and combustion gases, thus causing theirflow to concentrate in the upper part of the oil zone.

Much of the heat generated by the in situ combustion is not utilized in heating the oil; rather, itheats the oil-bearing strata, interbedded shale and base and cap rock.

Therefore, in situ combustion would be economically feasible when there is less rock material tobe heated, i.e., when the porosity and oil saturations are high and the sand thickness is moderate

AIR REQUIREMENTS

The fuel depositional characteristics of thereservoir oil are the most basic parametersin designing a fire-flood.

Coke deposited as fuel is measured in poundper cubic feet of reservoir rock.

If this value is too low, combustion cannotbe supported.

If it is too high, flame movement is too slowbecause all fuel must be burned before theflame advances.

Fuel deposition also determines the amountof air required to advance the flame througha set volume of reservoir rock.

The amount of air required to burn throughone cu ft of oil sand as a function of oilAPI gravity is also shown in Fig. , whichshows how fuel deposition varies with APIgravity of reservoir oil.

Fuel consumption and air

requirement for in situ combustion

as a function of oil gravity

The main factors which govern the volume of air required for in situ combustionare:

amount of fuel supply (coke) of the oil being burned and

the efficiency of oxygen utilization.

As the combustion zone moves through the reservoir, it continuously emits heat.

The heat moves in the forward direction by conduction, as sensible heat in liquidand gas and as heat of vaporization in vapor.

The hot fluids flush out volatile and mobile substances from the path of thecombustion zone leaving behind residual hydrocarbon (coke), which is the fuelsupply for combustion.

The combustion zone can only move as fast as it depletes the coke.

If the amount of coke deposited by the crude oil is excessive, the rate of advanceof the combustion zone would be slow and the air requirement would be large.

On the other hand, if the oil is paraffin-base and of high API gravity it could becompletely flushed out and combustion would not be sustained in the absence ofcoke

Therefore, asphaltic and naphthenic-base crude oils are normally the bestcandidates for the in situ combustion process.

Efficiency of oxygen utilization depends on the following parameters:

carbon/hydrogen ratio (C/H) of the coke;

amount of CO produced; and

amount of O2 which appears in the exhaust.

When the C/H ratio of the crude oil is low, more air will be required since ittakes more O2 to oxidize hydrogen than to oxidize carbon.

The amount of CO produced is a measure of the relative amount of carbon whichis not completely oxidized.

Since the amount of 02 required to produce CO2 is twice that required to produceCO, it follows that the larger the amount of CO produced the less the air required.

Finally, the air requirement must be increased in order to supply an amount of O2which is equal to the amount that appears in the exhaust.

The volume of air required to bum one pound of hydrocarbon can be estimated by the followingequation:

Once the volume of air required per cubic foot of fuel supply is determined, the air-oil ratio (AOR) maybe calculated by the following equation:

Where F

The factor F corrects for the density difference which might exist between the reservoir rock and thelaboratory sand pack, due to porosity difference.

If the porosity of the system is low, a large fraction of the oil present may be required for fuel.

Nelson and McNiel developed a procedure for the setting up of a full scale in situ combustionproject.

The method requires experimental data where the particular crude oil and reservoir conditions ofporosity and saturation are duplicated as nearly as possible.

Where this is done in a burning tube in the laboratory, observations are made of rates andamounts of air injected, the amounts of oil and water produced, and the amount and compositionof the gases produced.

Calculation of Air Requirements

The following laboratory data will be required: D = internal diameter of the burning tube, ft L = length of the pack burned, t Ø = porosity, fraction Vg = produced gas volume on dry basis, SCF N2i = nitrogen in air injected, fractional volume 02i = oxygen in air injected, fractional volume N2p = nitrogen in produced gas, fractional volume 02P = oxygen in produced gas, fractional volume C02p = carbon dioxide in produced gas, fractional volume COp = carbon monoxide in produced gas, fractional volume

The combustion equation is as follows, where no attempt has been made to balance theequation since the carbon-hydrogen ratio will not be known:

It has been observed that the amount of nitrogen injected equals the amount of nitrogenproduced, since nitrogen does not enter into the reaction. Therefore,

The quantities of other materials, either injected or produced according to combustionequation, then become:

*(One mole of any gas occupies the same volume at standard conditions: 379 cu ft/lb)

Water formed by combustion

Hydrogen in the fuel burned,

Since the fuel consumed at the flame front is essentially 100 percent carbon andhydrogen, the total weight of the fuel used is:

The pounds of fuel used per cubic foot of sand burned in the laboratory tube then is,

On a reservoir basis, the pounds of fuel used per acre-foot of sand burned would be,

It is now possible to calculate the relationships between the fuel burned, the total airinjected, and the volume of sand burned:

The air injected per ft3 of reservoir burned is,

The actual volume of air per acre-foot that will be required to bum a given pattern inthe field will be determined by the product of the volumetric sweep efficiency and theunit air consumption.

Nelson and McNiel have used an areal sweep efficiency of 62.6 per cent for the five-spot pattern.

A safety factor can be imposed upon the calculations so that the air requirements willnot be underestimated by assuming a 100 per cent vertical sweep efficiency.

The area sweep efficiency of 62.6 per cent will be used, thus resulting in an overall, orvolumetric sweep efficiency, of 62.6 per cent.

The air required in MMSCF per acre-ft in a ive-spot looding patern becomes:

Calculation of Injection Rates and Pressures

The reservoir properties and operating economics of the project will determinethe rate of advance of the combustion front, which in turn specifies the airinjection rates and pressures during the life of the project.

For the maximum rate of return on investment, the rate of advance of thecombustion front should be as high as possible.

But this rate should not be so high that the producing wells cannot produce allof the displaced oil.

The lower limit is set by the minimum rate at which a flood front must movebefore heat transfer results in a temperature at which combustion cannot occur.

Practical considerations usually dictate a burning rate of 0.125 to 0.5 ft/day,where the formation is approximately 20 to 30 ft in thickness.

Thinner sand sections would require a somewhat higher minimum rate, due to theproportionately higher heat loss from the narrow lame front

Assuming minimum burning front rate of 0.125 ft/day, the volume of air required per sqft of burning front per day:

The air flux will be different from point-to-point along the combustion front, dependingupon

the relative location of the producing and

injection wells, and the position of the combustion front.

Where the air flux is insufficient to support combustion, the flame goes out and only theheat wave is displaced forward until it is dissipated.

The limited sweep efficiency which results can be related to a dimensionless flow term,id, which is calculated from the maximum air flux required to sustain combustion.

The dimensionless flow term is:

where a is the length of the shortest streamline in feet between the injection andproducing well, and h is the average pay thickness between wells in feet

Therefore, maximum daily air injection rate is

In a five-spot well system, the areal sweep efficiency at break through, corresponding toin values of 3.39, 4.77, 6.06 and infinity, are 50%, 55%, 57.5%, and 62.6%, respectively.

Practical considerations will usually limit the areal sweep efficiency to approximately55%, and a corresponding id, value of 4.77.

Assuming that the maximum rate of flood advance for efficient displacement, vi, is to be0.5 ft/day, the air flux rate in SCF/day, duing the radial phase of the displacement, canbe simply calculated rom the following equation:

where r1 is the radial distance in feet to the burning front at the end of the increasingair injection rate period, and rf is the radial distance to the burning front.

Calculations of air injection rates, for a range of values of rf are calculated until themaximum air rate specified by has been reached.

Therefore,

The time in days required for the increasing rate period of the ire-flood may becalculated as follows:

Since the increase in air injection is essentially linear during this phase, the volume ofair injected to this point in MMSCF is:

If, for the purpose of balancing a burning operation where a number of five-spotpatterns are involved, a similar period exists at the end of the flood where decreasingair injection rates are used, then a volume of air , V3 , will be injected over time, t3.

In this instance, V1 will equal V3, and t1 will equal t3.

The volume of air injected at the constant rate ia in MMSCF will be:

the time in days for this part of the project being:

The time for the overall burning project in the pattern would be the sum of the timesduring the three phases

For the five-spot well network, whether confined or not, the following adaptation ofDarcy,s law for compressible flow during the radial flow period, should indicate themaximum pressure required:

Calculation of Oil Recovery

The mechanism of oil displacement by a combustion front is complex.

Laboratory and field data to indicate that in addition to the oil displaced by the front tothe producing wells, a substantial amount of oil is recovered due to heating of theformation, with subsequent reduction of oil gravity, which then can move under apotential gradient to the producing wells.

The actual oil recovery, as a fraction of the oil displaced due to the sweep of theburned zone, may be rather small, since the volumetric efficiency is a product ofthe areal and vertical sweep efficiencies, Ea and Ev respectively.

As is often the case, the vertical sweep efficiency might approximate 55% whilehorizontal sweep efficiency might be in the range of 55%, for an overallefficiency of 30 per cent.

Then, the actual oil recovered from the swept zone will be the difference betweenthe original oil content of the region swept by the combustion zone, and the oilconsumed as fuel.

Since coke, tar, and/or pitch used as fuel will have a specific gravity ofapproximately one, the following equation may be readily developed for thebarrels of oil displaced per acre-ft of reservoir burned:

Cores, taken from regions where a combustion front had passed through a section of areservoir in the field, indicate that roughly half of the oil in the regions not contactedby the front may have been produced by a combination of gravity drainage and gasdrive.

For preliminary design purposes, a recovery of 40 per cent of the oil from the regionscontiguous to, but not swept by the flame front, may be assumed.

The equation for the oil displaced in barrels per acre-ft of unburned reservoir is:

Total recovery from the burned and unburned zones would be:

with the overall recovery eficiency as a per cent being:

Assuming that the water saturation in the unburned zone is immobile and unchangedduring the flood, the total water production due to in situ combustion can be calculatedas the sum of the water originally present in the burned zone, plus the water formedduring combustion.

The total water produced in barrels per acre-ft of reservoir rock in the well patten is:

Calculation of Producing Rates

Usually, oil production rates will be small during the air injection phase, prior to ignitionof the oil at the injection well.

Production rates will remain small during the increasing air injection phase, unless wellspacing is very close.

Production increases rapidly as the heat of the combustion front reaches the vicinity ofthe producing well, since the oil viscosity will be substantially reduced.

As the displacing gas-to-oil mobility ratio improves, the gas drive producing mechanismwill be increasingly effective in moving oil to the producing wells.

As the combustion front advances, the gas driving mechanism is complemented by thepresence of a cold and finally a hot water displacement of the oil.

For the purpose of design, it will usually suffice to assume that the oil production willbe proportional to the air-injection rate.

The oil recovered in barrels per MMSCF of air injected may then be calculated

The water producing rate could be expected to be zero during the initial air injectionphase and ignition phases of the in situ combustion project, if the initial watersaturation is immobile.

As the combustion front advances, a water bank will be formed which will result inincreased water producing rates after the flush production from the oil bank has beenobtained.

To facilitate a determination of a water producing rate, it will be assumed, however, thatthe water-oil ratio is constant.

The barrels pf water produced per MMSCF of air injected is:

The following upper and lower limits on reservoir properties for a forward combustioninjection project are those which are indicated by laboratory data and field results tothis date, and may change as our understanding of the mechanism of in situ combustionbecomes more complete:

1. Oil Content

Since as much as 300 bbl/acre-ft of hydrocarbons may be consumed by thecombustion front, at least 600 barrels should be present in the reservoir initially.

In other words, the oil saturation should be high, and the water saturation low.

2. Pay Thickness.

The pay thickness should be at least 5 ft thick.

Thinner pay sections are indicated to have high vertical heat losses, withsubsequent danger of the temperature dropping below that necessary to maintain acombustion front.

Preferably, the pay thickness should not exceed 50 feet as with increased thickness,air requirements sufficient to maintain a combustion front movement rate of atleast 0.25 feet per day become excessive, with respect to the practical limitationimposed by compression equipment.

3. Depth. Depth should be greater than 200 feet.

In general, depths less than 200 feet would severely limit the pressure at which aircould be injected.

Operating in deep reservoirs results in high well costs, as well as substantial aircompression expense, and economic considerations will impose a practical depth limit.

This may be on the order of 2500 to 4500 feet.

4. Oil Gravity and Viscosity. In general, oils of gravity greater than 40°API do not deposit sufficient coke (fuel) tomaintain a combustion front.

Oils of gravity less than 10°API are usually too viscous to flow ahead of thecombustion front, where a low reservoir temperature prevails under the influence offorward combustion.

5. Reservoir Permeability. Where oil viscosity is high (a reservoir containing 10°API oil), a permeability greaterthan 100 md would be necessary, especially if the reservoir is of shallow depth andinjection pressures were correspondingly limited.

A 30 to 35 API gravity oil at 2500 feet might respond to in situ combustion, withpermeabilities as low as 25 to 50md

6. Size of Reservoir.

A reservoir should be sufficiently large so that if a small scale pilot flood issuccessful, an economically successful full scale operation can be instituted.

Depending upon sand pay thicknesses, the minimum size of such a project might be100 acres.

7. Reservoir Confinement.

Where possible, the reservoir chosen for in situ combustion operations should have nogas cap or water zone within the area of operation.

Advantages

The process is not strictly limited to high viscosity crudes, Reservoir producing oil up to40°API have been fire flooded.

High displacement efficiency can be achieved, although some oil is burned and notproduced.

The injection fluid, air, is readily available

Disadvantages

1. The in situ combustion process has a tendency to sweep only the upper part of the oil zone;therefore, vertical sweep in very thick formations is likely to be poor.

2. Much of the heat generated by in situ combustion is not utilized in heating the oil; rather , itheats the oil-bearing strata, interbedded shale and base and cap rock. Therefore , in situcombustion would be economically feasible when there is less rock material to be heated

3. Many operators feel viscous, low-gravity crudes are best suited for in situ combustion becausethey provide the needed fuel for combustion. However, the required air-oil ratio for viscouscrudes is high.

4. Installation of in situ combustion requires a large investment.

5. Serious operational problems have been reported for in situ combustion operations. Some ofthese problems are: formation of oil-water emulsions,

production of low pH (acidic) hot water,

increased sand production and cavings which cause plugging of well liner,

plugging of the producing wellbore due to thermal cracking of the oil,

production of environmentally hazardous gases such as carbon monoxide and hydrogensulphide, and

liner and tubing failure due to excessive temperatures at the production wells