1-Dual Role of Catalytic Agents on in-Situ Combustion Performance

8/12/2019 1-Dual Role of Catalytic Agents on in-Situ Combustion Performance

1/9March 2010 SPE Journal 137

Dual Role of Catalytic Agents on In-SituCombustion Performance

I.Y. Akkutlu,SPE, University of Oklahoma;Y.C. Yortsos,SPE, University of Southern California;and G.D. Adagl-Demirdal,SPE, Encana

Copyright 2010 Society of Petroleum Engineers

This paper (SPE 115506) was accepted for presentation at the SPE Annual TechnicalConference and Exhibition, Denver, 2124 September 2008, and revised for publication.

Original manuscript received for review 24 June 2008. Revised manuscript received forreview 18 February 2009. Paper peer approved 19 February 2009.

Summary

Unlike other thermal recovery methods, air injection and in-situcombustion generates significant amounts of heat in the reservoir.However, the process is subject to acute heat loss rates from thereaction zones because of high temperature gradients; conse-quently, the reaction temperatures may be reduced considerably,leading to a deteriorated combustion performance and debilitatedfield operations. The goal of this paper is to determine underwhich reservoir conditions the combustion temperatures couldbe maintained at sufficient levels. Previous investigators havepartially addressed this issue using kinetic and combustion tubeexperiments. In the absence of heat losses, it has been repeat-edly shown that catalytic agents (naturally occurring clays, metaloxides, and some water-soluble metallic additives) improve the

self-sustainability limit of combustion front in crude oil and sandmixtures. In general, this has been attributed to the dual role ofthese agents on the combustion performance, namely the catalyticand fuel deposition effects. It is currently a common belief thatappropriate introduction of such materials in a reservoir environ-ment could enhance the performance of combustion process and,hence, improve the recoveries. An investigation of their dual roleon combustion requires that the mechanisms of combustion arewell understood in their presence. Complex physical and chemi-cal nature of the problem at the pore-scale has prevented detailedinvestigations using physical and numerical models, however.Here, we approach the problem analytically using a sequential-reaction [high-temperature oxidation/low-temperature oxidation(HTO/LTO)] combustion front propagation model, based on largeactivation energy asymptotics, and introducing the reaction kinet-

ics and fuel deposition effects to the model systematically byvarying the related variables and parameters. Coherent propagationof the reaction regions are then investigated using reaction regiontemperatures, propagation velocity, and the oxygen consumptionefficiency. General characteristics of an ideal catalytic agent arediscussed in terms of its potential to improve in-situ combustion.It is found that the front propagation can be improved under thereservoir conditions only if both the catalytic and fuel depositioneffects of the agents are present. The work is important for ourunderstanding of in-situ combustion processes and can be usedfor development of screening criteria to identify high-performancecatalytic agents in the laboratory using conventional apparatus.

Introduction

Thermal recovery is the principal approach to reduce viscosity ofheavy crude oil by applying heat into a subsurface oil reservoir.The necessary thermal energy can be generated at the surface andintroduced into the reservoir by means of injecting steam, hotwater, or gas. It may also be generated in-situ by injecting air andoxidizing heavy components of the crude oil in place. The latterapproach for heavy oil recovery has long been recognized as theair injection and in-situ combustion (Prats 1982).

Air injection and in-situ combustion processes have severaladvantages when compared with the other thermal heavy oil recovery

techniques. Among them, the most important one is related toinstant availability and low-cost of the injection fluid regardlessof the reservoir location. Furthermore, and perhaps even moredistinctively, air injection processes bypass a necessity to minimizethe wellbore heat losses, and the accompanying injector insulationcosts, realized during the other thermal recovery operations. Dur-ing air injection, unlike the high-temperature injection fluids, noheat losses take place until the injected air reaches the reservoir andconfronts the hydrocarbon deposits and in-situ generated fuel.

During any air injection process, propagation of a self-sustain-ing combustion front in the reservoir is necessary for improved oilrecoveries, however. It is desired to maintain a stable high-tempera-ture combustion front that travels away from the injector deeperinto the reservoir toward the production well, uniformly sweeping

the in-place fluids. The fuel necessary for high-temperature oxi-dation reaction, and the associated front self-sustainability, is acarbon-rich residue resulting from a series of complex in-situ proc-esses: low-temperature oxidations, cracking and coking (pyrolysis)reactions, in-situ generated steam-based distillation, and, finally,multiphase displacement. These sequentially take place ahead ofthe moving combustion front, and their relative locations in thereservoir are dictated by a nonuniform temperature profile monoto-nously decreasing further away from the combustion front.

The produced oil is often upgraded and, hence, lighter thanthe original oil in place because of the in-situ consumption of theheavier fractions at the combustion front and because of in-situprocesses (e.g., distillation) taking place ahead of it. Hence, theair injection processes may have an added benefit of recoveringin-situ upgraded heavy oils.

Despite the potential advantages, air injection processes have notbeen widely used in the field primarily because of operational andtechnical difficulties. Earlier, it is reported that, among the 1.3 mil-lion B/D of oil produced by thermal methods, only 2.2% is producedby in-situ combustion (Moritis 2002). One major technical difficultythat limits their practical application is related to control and optimi-zation of combustion front and of its complex interaction with thepreceding phenomena under the reservoir conditions. The precedingphenomena take place at the reservoir scale under restrictive influ-ence of a nonuniform temperature profile, distribution of whichis dictated by the combustion front. On the other hand, the issuesrelated to combustion front performance and optimizationin termsof the amount of in-situ generated heat, front temperature and propa-gation speedare closely tied to in-situ deposition and availabilityof the hydrocarbon fuel generated by the preceding phenomena. Ifthe deposited fuel amount is not sufficient, as is often the case withlighter oil reservoirs, the front may propagate in an unstable combus-tion mode susceptible to detrimental reservoir conditions (e.g., highreservoir heat losses) and it could become extinct. To the contrary,if the deposited fuel amount is largethe case with some heavy oilreservoirs, bitumen deposits, and oil shalesthe front propagationvelocities are expected to be significantly low, which slows downthe operations and brings an uneconomically high air compressionand injection costs for the production (Alexander et al. 1962). Thus,the air injection processes depend upon not only a detailed analysisof the reservoir and fluid properties showing the existence of opti-mum conditions in place but also upon a better understanding ofthe physical and chemical phenomena and their coupling under thereservoir conditions.

A series of experimental investigations have been performedsince the 1970s mainly focusing on application of the air injection


2/9138 March 2010 SPE Journal

processes to an extended range of reservoir rock matrix and fluidcharacteristics. The investigations also involved the so-called cata-lytic agents, namely naturally occurring minerals such as clays andmetal oxides, and certain water-soluble metallic salts, with thepotential to play a catalytic role on the combustion reactions. Inthe chemical industry, metals have been known for their catalyticability in hydrocarbon oxidation and cracking reactions. Manystudies appeared in the literature of chemistry investigating theeffects of metallic additives on the oxidation characteristics ofcrude oils. They were also considered as the agents to control andoptimize the in-situ combustion processes.

Previous experimental works showed that the overall reactionmechanism of crude oils in porous media is caused by a sequenceof several oxidation reactions that occur at different temperature

ranges. On the thermograms, these have been classified as theregions of low-temperature (LTO), medium-temperature (MTO),and high-temperature (HTO) oxidation reactions. In the absence ofoxygen, there also exist pyrolysis and thermal cracking reactionsduring the in-situ combustion, although significantly high tem-peratures and longer times may be required for their dominance.Mamora (1993) considered the experimental occurrence of LTOand HTO reactions during a series of kinetic tube experiments andconcluded that the LTO reactions are mainly responsible for fuelgeneration. On the basis of this premise, he developed a two-reac-tion hydrocarbon oxidation kinetic model.

Earlier, Burger and Sahuquet (1972) performed kinetic experi-ments and observed that the oxidation reactions could occur atlower temperatures and the area under the high-temperature peakincreases in the presence of catalytic agents. They interpretedthat the former observation was because of an increase in oxida-tion reaction rate and the latter was because of an increase in thedeposited fuel amount.

Fassihi (1981) ran kinetic experiments using 27API crude oil. Hecompared the effluent gas data for the clean sand with the one addedcopper. The results showed that the activation energy of HTO regiondecreases approximately 50% and activation energy of MTO regionincreases approximately 50%. No significant change is observed inthe LTO region other than a higher Arrhenius constant.

Drici and Vossoughi (1985) studied the effects of metal oxideson combustion characteristics of the crude oil by using differen-tial scanning calorimetry (DSC) and thermogravimetric analysis(TGA). The results showed that, as the metal oxides concentrationincreases, the amount of heat released in the LTO reaction region

gradually increases. The combustion peak temperature shifts toa lower temperature and became smaller and smoother, whichreflects a more homogeneous composition of the solid residue.They also noted that the change in the reaction rate constant must

be considered simultaneously with the activation energy of thereaction when the catalytic effects are investigated.

De los Rios et al. (1988) and Shallcross et al. (1991) performedkinetic experiments with various metallic additives and developedan analytical model to estimate the kinetic parameters of the oxida-tion reaction regions, namely LTO, MTO, and HTO. They foundthat iron and tin salts enhance fuel deposition and increase oxygenconsumption while copper, nickel, and cadmium salts show noapparent effect. For each oxidation reaction, the activation energiesare estimated using their model. For most of the catalytic agents,their results showed that the activation energy of HTO and MTOreactions increases while that of the LTO reaction decreases.

Castanier et al. (1992) carried out 13 combustion tube runswith metallic additives. In the experiments, four different types of

oil were used. The results showed that tin, iron, and zinc enhancecombustion efficiency, while copper, nickel, and cadmium have littleor no effect. Increase in fuel deposition, oxygen utilization effi-ciency, and front velocity are found in the presence of the formermetals. In addition, zinc is found to be less effective compared totin and iron. For the light oil case, they observed a sustained com-bustion using iron additive while combustion had failed withoutany additive.

Holt (1992) used iron nitrate and zinc nitrate on Cymric fieldlight and heavy oil and observed a catalytic effect of the additives.For the Cymric heavy oil, results of the kinetic experiments showedthat addition of 1% iron nitrate increased fuel amount by 20%. Onthe other hand, 1% zinc nitrate increased fuel concentration by only5%.It is noted that the generated fuels require different amountsof air to burn unit masses. They also repeated the same runs forthe Cymric light oil without any additives. Sustained combustioncould not be achieved, however, because of a lack of fuel deposi-tion. On the other hand, they observed efficient combustion withthe addition of 1% (mole) iron nitrate solution.

He et al. (2005) explored the effect of water-soluble metallicadditives on in-situ combustion using combustion tube and kineticscell. Cymric light and heavy oil are used during the experiments.For the tests, sand, silica powder, or kaolinite are mixed with waterand oil. In the cases of test with metallic additive, 0.5 g of the addi-tive is added to the water. The results of their experiments are sum-marized in Tables 1and 2.It is observed that the additive improvesperformance in all cases, including changing activation energies,greater oxygen consumption, low temperature threshold, and morecomplete oxidation. For Cymric light crude oil, the catalytic effect

is obvious in the LTO where fuel deposition is increased to sustaincombustion. For Cymric heavy crude oil, metallic additives arefound to have an effect on HTO. Kaolinite also has an effect oncrude oil combustion even without a metallic additive.

TABLE 1KINETIC RUNS FOR CYMRIC LIGHT CRUDE OIL*

Kaolitineor Silica

MetallicAdditive

Peak T,LTO (C)

Peak T,HTO (C)

E/R,LTO

E/R,HTO

Max. O2LTO (%)

Max. O2HTO (%)

Kaolitine None 275 385 9647 10669 3 2.8

Kaolitine Fe3+

275 355 8535 11522 5.1 3.8

Silica powder None 287 410 8331 10681 2.4 2.6

Silica powder Fe3+

280 370 7886 9775 2.9 2.7

* He et al. 2005

TABLE 2KINETIC RUNS FOR CYMRIC HEAVY CRUDE OIL*

Kaolitineor Silica

MetallicAdditive

Peak T,LTO ( C)

Peak T,HTO (C)

E/R,LTO

E/R,HTO

Max. O2LTO (%)

Max. O2HTO (%)

Kaolitine None 270 395 9090 12319 12.6 12.2

Kaolitine Fe3+

280 355 9085 9427 13.7 18.7

Silica powder None 265 410 7293 10489 6.3 18.4

Silica powder Fe3+

260 380 7098 10640 7 18.4

* He et al. 2005



In summary, results of the experimental studies have shownthat the introduction of catalytic agents to certain types of crudeoil/sand mixtures could significantly (1) change the oxidation reac-tion kinetics inside the combustion front (i.e., catalytic effect) and(2) influence the fuel deposition. However, we do not have a clearunderstanding of the mechanisms that lead to the dualcatalyticand fuel depositioneffects. For example, it is not known howexactly the fuel is deposited. Is it the specific surface area of theporous medium ahead of the combustion front or the stiochiometryof fuel generation that is changed in the presence of these materi-als? Further, with the changing fuel availability, does the specificsurface area of the deposited fuel also change such that an addi-

tional catalytic effect comes into play for an optimal combustionfront propagation?

It is also not well understood how significant the dual effectsare on the combustion front propagation, how the so-called catalyticeffect should appear during the propagation and to what extent itinfluences the combustion performance. The main objective should,therefore, be an investigation on the combustion front performancein the presence of dual effects of catalytic agents under the reservoirconditions.

Because of the complex nature of the crude oil componentsand their numerous nonisothermal reactions, the task is, however,a difficult one in the laboratory or using a numerical approach.In this work, a self-sustaining combustion front propagation isconsidered analytically using a sequential-reaction (fuel-generat-

ing, LTO; fuel-burning, HTO) front propagation model. Given thecomplexity of problem, the approach is quite simple, althoughit makes rigorous analysis of the in-situ fuel generation andconsumption processes along with their nonlinear interactions.The reaction kinetics (i.e., frequency factors, activation energies,deposited hydrocarbon, and generated fuel specific surface areas),hydrocarbon amount and fuel generation stoichiometry are inputparameters of the model. During the investigation, these param-eters are systematically changed as indication of the effects causedby the presence of catalytic agents and the impact on the frontpropagation are investigated under detrimental reservoir conditions(i.e., the conditions that may lead combustion to the extinctionlimit), such as insufficient hydrocarbon and fuel amounts or largeheat loss rates. The role of the catalytic agentswhether catalyticor notthat could play in optimizing the combustion performance

is identified.The theoretical approach essentially builds on a recent descrip-

tion of the single-reaction combustion front propagation usinglarge activation energy asymptotics (Akkutlu and Yortsos 2003).The work was extended to the sequential oxidation reactions(Akkutlu and Yortsos 2004), and the coherence of the sequentialreaction regions propagating in a porous medium under typicalreservoir conditions (Adagulu and Akkutlu 2007). Further, in theabsence of agents, the authors investigated the interactions betweenthe reaction regions, delineating the effects of fuel generation andcombustion kinetics on the propagation. It was found out thatthe reaction regions have the ability to travel closely spaced and,consequently, minimize the effects of reservoir heat losses on thecombustion. This mechanism has been shown to thermally supportthe combustion front under deleterious reservoir conditions.

Combustion Front Propagation Model

Consider air injection into a linear homogeneous reservoir underthe influence of heat transfer to the surroundings. A steady propa-gation of sequentialHTO and LTOreaction regions separatedby a finite distance * develops. Because of oxidation, the tworeaction regions experience discontinuities in heat and mass fluxesand interact with each other through temperature and reactant(oxygen/fuels) concentration fields. Using considerable analysisreported previously (Akkutlu and Yortsos 2004), dimensionlesstemperatures of the propagating reaction regions are

fHH

hH

L

hH

hH DH

q qV= + + ( )

11

21

exp * . . . . . . . . . . . . (1)

and

fLL

hL

H

hL

hL DL

q qV= + + +( )

11

21

exp * , . . . . . . . . . . . (2)

where subscriptsHandLcorrespond to the HTO and LTO reactions,respectively. q =qH +qL=(QHofH+QLofL)/[(1)cssTo] representsthe ratio of the heat generated by the combustion process to the abso-lute heat content of the matrix. This quantity is directly proportionalto the amounts of hydrocarbon deposited fLand fuel-generated

fH

for the LTO and HTO reactions, respectively. hn2 = 1 + 4h/VDn2

represents the influence of external heat losses on temperatures ofthe reaction regions. Steady propagation velocities of the reaction

regions, on the other hand, are given by the following equations:

V A YDH H H fH

H

fH

2 =

exp . . . . . . . . . . . . . . . . . . . . . . . . . . (3)

and

V A YDL L L fL

L

fL

2 =

exp . . . . . . . . . . . . . . . . . . . . . . . . . . . (4)

Hence, the velocity equations hold nonlinear dependency onthe temperatures of the reaction regions and involve, among oth-ers, two dimensionless quantitiesA

n

=s

kn

asn

pYi

/qn

En

vi

2 and Yn

=(1 nVDn)/(1 + gnVDn). The former reflects a combination ofphysicochemical properties, including the kinetics (i.e., frequencyfactor k, deposited hydrocarbon, and generated fuel surface areasasL, asH, and activation energy E), of the reaction; whereas the lat-ter accounts for the concentration of oxygen left unburned by thereaction region. Here, we note that An is inversely proportionalto the square of air injection rate, vi

2 [i.e., (volumetric air/unittime)/cross-sectional area]. The latter is also referred to as injectionrate, expressed in m/d, and it is defined at the inlet (i.e., injection)conditions in the presence of heat losses; see Akkutlu and Yortsos(2003) for derivations and further details of Eqs. 3 and 4.

Investigation of the combustion front propagation thus requiressimultaneous solution of four coupled-algebraic equations (i.e.,Eqs. 1 through 4), in the presence of reservoir heat losses. Condi-

tion of coherence asserts that these reaction regions travel with thesame speed (i.e., VDH=VDL); thus, in addition to the temperaturesof reaction regions and their separation distance, this commonvelocity must also be determined. Details of the computationalprocedure can be found in Adagulu and Akkutlu (2007).

Results and Discussion

The presence of catalytic agents are expected to lead to changes inthe quantities n(Arrhenius number of reaction n) and Anof Eqs.3 and 4 because of their dependence on the frequency factor, k,specific surface area, as, and activation energy,E. Later, we elabo-rate on these dependencies as an indication of the catalytic rolethe agents play on the combustion front propagation. The valuespresented in Table 3are used to generate the base case scenario.

Activation Energies. An increase in the value of activation energymeans that chemical transformation has to overcome a higherenergy barrier related to the energy of the covalent bonds. Therequired amount of reaction energy is provided by the kineticenergy of translational motion of the colliding oxygen and hydro-carbon molecules (i.e, two-body collisions). If, initially, heat isreleased and local temperature becomes larger, the mean kineticenergy of the molecules will increase, and, thus, a greater numberof colliding molecules will soon have sufficient amount of kineticenergy to overcome the energy barrier. Thus, in accordance withthe Arrhenius dependency, the reaction rate increases. If the reac-tion is accelerated, more heat will be released, the temperature ofthe medium will further increase, and, consequently, the reactionwill be further accelerated. Therefore, an increase in the activation

energy of a reaction should lead a steadily propagating reactionregion to higher temperatures.



We observe that this explanation is strictly valid for theenergy and temperature relationship of an HTO reaction regionin the presence of a preceding LTO region. Table 4 shows allthe possible variations in the activation energies of the oxidationreactions at a constant air injection rate along with their corre-sponding changes in the HTO front temperature estimated usingthe model. Additionally, four cases are observed where the HTOtemperature is increased: Cases 2, 4, 6, and 8. Only Case 8 amongthemwhen the contrast in the HTO/LTO activation energies isincreasedhas the potential to increase the HTO temperaturesignificantly. Interestingly, this is also the case observed duringthe kinetics experiments of He et al. (2005), when Fe+is added to

silica/Cymric heavy oil and to kaolinite/Cymric light oil mixtures(see Tables 1 and 2).

Fig. 1shows the effect of variations in the activation energiesin accordance with Case 8 on the propagation dynamics of thecoherent reaction regions, namely the reaction region temperatures,propagation velocity, and separation distance. When comparedwith the base-case solutions (dashed lines), it is clear in Fig. 1athat the contrast in activation energies improves the HTO tempera-ture, in particular at high air injection rates. However, the catalyticeffect (i.e., the shaded region in Fig. 1a) becomes smaller as theinjection is decreased because of the presence of reservoir heatlosses. This decrease in catalytic effect is because of a decrease inthe difference between the rate of heat generation and the rate ofheat losses. Here, it is also observed that the changes in activationenergies have negligible influence on the estimated LTO tempera-tures, which is nearly a constant for a large range of air injection,neither on the coherent front propagation speed (Fig. 1b). It is,however, clearly indicated in Fig. 1c that, at high injection rates,the reaction regions approach each other, eventually overlapping,when the rate is approximately 25300 m/d. Consequently, thereexist no solutions corresponding to coherent propagation at higherinjection rates.

Results indicate that the experimentally observed improvementin self-sustainability of the combustion process may be attributedto a catalytic effect originating from a contrast in the HTO andLTO activation energies in the presence of catalytic agents. Com-bustion tube runs of He et al. (2005) also support this observation.Interestingly, however, their kinetic-cell results corresponding toCase 8 predicts 3040C lower HTO temperatures than the base

values. The latter experimental observation then appears to be incontradiction with the improvement observed in the combustion

tube data. This may point to a pitfall during the interpretation ofkinetic cell data, which are based on zero-dimensional observationsand, therefore, do not reflect the 1D propagation dynamics of thesequential reaction regions.

Frequency Factors. An increase in the reaction frequency factorcauses the reaction rate to proportionally increase, which, conse-quently, is expected to accelerate the reaction region propagationand its temperature. Similarly, considering Cases 1 through 8, the

influence of frequency factors on the HTO temperature is inves-tigated using the sequential reaction model with a 5% change inthe frequency factors. Table 5 shows that the HTO temperaturedoes not necessarily obey the anticipated trends. This points outto a complex interplay of the reaction regions during the in-situcombustion process. In addition, it is found that the changes inHTO temperature are much smaller than those caused by variationsin the activation energies.

Specific Fuel Surface Areas. Based on the formulation of themodel (i.e., Eqs. 3 and 4), the influence of hydrocarbon fuel sur-face areas should be identical with the frequency factors. Thus,HTO temperature changes observed in Table 5 are equally validfor the hydrocarbon and fuel surface area changes. For example,a 5% increase in the deposited hydrocarbon surface area and a 5%

decrease in the generated fuel surface area (i.e., Case 7) lead to a2.3C increase in the HTO temperature.

Combined Effects of the Activation Energies and FrequencyFactors. The estimated temperatures in Tables 4 and 5 are suchthat, in all the cases considered, the temperature change fromthe variations in the activation energies consistently offsets thosecaused by variations in the frequency factors. Thus, it becomes animportant issue for the discussion whether the activation energiesand the frequency factors of the reactions vary independent ofeach other in the presence of catalytic agents. Often, a variationin the activation energies only has been considered as an indica-tion of the catalytic activity. However, Drici and Vossoughi (1985)showed that consideration of the variations in the energy alonecan be misleading; instead, the combined (compensation) effectof the frequency factor and activation energy of a reaction shouldbe considered throughout an investigation. In this study, a normal(m > 0) compensation effect is considered for the HTO and LTOreactions in the presence of agents, i.e., the frequency factor andactivation energy relationship of the oxidation reaction follows apositive trend:

logk mE c= + , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (5)

where m =6.453 105kmole/kJ is given by Drici and Vossoughi(1985) and valid for various metal oxides, and c=2.38 is takenhere so that the base-state values of activation energies and fre-quency factors are recovered using Eq. 5. Fig. 1 (solid blue lines)shows the compensation corresponding to Case 8 in terms of the

propagation dynamics of the reaction regions. It clearly showsthat the compensation effect indeed eliminates the previously

TABLE 3TYPICAL PARAMETER VALUES USED IN THECOMBUSTION FRONT ANALYSIS

Parameter Value

o

fH 19.0 kg/m3

EH 7.35 104kJ/kmole

QH 3.95 104kJ/kg fuel

To 373.15 K

p 1.0 atm

Yi 1.0 kg/kg

vi 100.0 m/day

R 8.314 kJ/kmole-K

k 227 kW-m/atm-kmole

as 1 .41 105m

2/m

3

8.654 104kW/m-K

cg gi 1.2338 kJ/m3-K

(1 )cs s 2.012 103kJ/m

3-K

H 2 m

0.078 kW/m2-K

3.018

1.0

TABLE 4INFLUENCE OF 5% CHANGE IN ACTIVATIONENERGIES ON HTO TEMPERATURE ( vi= 100 m/d)

Case EH EL HTO Temperature (C)

1 base 22.33

2 base 4.45

3 19.81

4 base 19.42

5 base 7.66

6 6.38

7 26.71

8 27.13



observed catalytic effect from the contrast in the activation energiesonly. The estimated HTO temperatures and velocities are nearlyidentical to the base-state solution at low and moderate air injec-tion rates; at high rates, the estimates become even less than thepredicted base values.

In summary, the results distinctly illustrate that the changes inkinetic parameters has the potential to influence the combustionfront propagation significantly, in particular, through the propertiesof HTO region. It is found that any selected catalytic agent shouldmodify the activation energies of the oxidation reactions such thatthe contrast in HTO/LTO activation energies is further increased.The results also clearly show the importance of variations in the

hydrocarbon and fuel specific surface areas as the sole sourcesof catalytic effect in the presence of normal compensation effect.

100

200

300

400

500

0 100 200 300 400 500

Injection Velocity, m/day

RxnRegionTemperatures,

HTO

LTO

base

compensation effect

catalytic effectdue to activationenergies only

case 8

Injection velocity, vi, m/day

Temperatures,

oC

100

200

300

400

500

0 100 200 300 400 500

Injection Velocity, m/day

HTO

LTO

Base

Compensation effect

Catalytic effectdue to activationenergies only

Case 8

Injection Velocity, vi,m/d

Temperature,

oC

(a)

(b)

(c)

Injection Velocity, v ,m/d

Injection Velocity, vi,m

Compensation effect

Compensation effect

Base

Base

Case 8

EFront

Velocity,m/d

SeparationDista

nce,cm

7

6

5

4

3

2

1

00 100 200 300 400 500

40

30

20

10

0

0 100 200 300 400 500

i

Fig. 1Catalytic effects on combustion front propagation. Reaction region (a) temperatures, (b) front velocity, and (c) separa-

tion distance.

Further experimental investigation is required in the light of thesetheoretical observations.

Fuel Deposition Effect. Here, we note that the surface areas of thedeposited hydrocarbon and the generated fuel are not the same as thesurface area of the porous medium, although they may be correlatedto the latter. Earlier, it is argued that the catalytic agents may changethe specific surface are of the porous medium and, consequently,vary the hydrocarbon deposition and the fuel generation and,hence, the total heat content of the reservoir. Therefore, dynamicsof the coherent reaction regions is expected to be influenced by thedimensionless quantities qnin Eqs. 1 through 4,An(this time because

of its dependency on qn) and Yn (because of dependencies on thestoichiometric coefficients nand gn) in Eqs. 3 and 4.



Sensitivity of the HTO temperature on the hydrocarbon and fueldensities is given in Table 6.Cases 9 and 10 consider the possibil-ity of increased hydrocarbon deposition ahead of the LTO reactionregion. In Case 10, the amount of generated fuel is increased pro-portionally, however; hence, the stoichiometry of fuel generationis not changed and is the same as that in the base case. Noticethat, relative to the temperature variations from activation energiesgiven in Table 1, the HTO temperature increases only slightly, eventhough the air injection rate is kept three times larger. Regardless,Table 6 also shows that the temperature increase of Case 10 isapproximately two times larger than that of Case 9.

Fig. 2shows the combustion behavior corresponding to Case9 when the hydrocarbon deposition ahead of the LTO is doubled.Estimated values of the HTO temperature are much lower thanthe base values at air injection velocities below 400 m/d; at higherinjection rates, however, there exists a small region of increasedcombustion performance from increased hydrocarbon deposition.At higher (lower) air injection (heat loss) rates, it is expected thatthe region of improved combustion performance is much larger.

Fig. 2 also shows that, unlike the catalytic effect, the hydrocar-bon deposition causes dramatic changes in the LTO temperature.At low and moderate air injection (35200 m/d) its estimated val-ues increase such that the LTO temperature becomes even largerthan the HTO temperature, (see the LTO-dominated region in Fig.

2). Hence, in this region, control over the combustion performance

shifts from HTO and is dictated by the LTO region. This observa-tion is in agreement with the experimental results of Drici andVossoughi (1985), where, using thermal analysis techniques, the

authors predicted a shift of a large amount of heat from a high toa low temperature range because of an increase in the solid surfacearea in the presence of agents. Nevertheless, the results here showthat the overall influence of this shift may not be strong enoughto improve front propagation.

Fig. 3 shows combustion behavior from fuel deposition inaccordance with Case 10. In this case, the deposited hydrocarbonamount ahead of the LTO region is 50% larger than the base valueand the generated HTO fuel amount increases proportionally.Hence, the total heat content of the reservoir is the same as inCase 9 (i.e., two times larger than the base-state), and, therefore,Figs. 2 and 3 are thermally calibrated and comparable. Estimatedchanges in the separation distance are shown in Fig. 4for the base,Case 9 and Case 10. At high air injection rates, it is clear that theincreased fuel amounts significantly affect both reaction region

temperatures, which, consequently, improves the combustion per-formance. As the injection rate decreases, however, the separationdistance of the reaction regions increase, which promotes the heattransfer to the surroundings, and, consequently, the enhancementon the combustion performance disappears. Note that the predicteddistances with Case 10 are significantly larger than those with Case9 and the base state at large air injection rates. As the injectionrate is decreased, the distance increases nonlinearly and, at the airinjection rate of 230 m/d, it becomes infinitely large, namely thereaction regions are fully separated (i.e., *). At lower injec-tion rates, there still exist solutions corresponding to the coherentpropagation of reaction regions with a finite distance; however,the predicted temperatures in this region are close to the base-statevalues. The estimated coherent propagation velocity of the reactionregions with Cases 9 and 10 does not show significant variationsfrom the base values either.

In summary, results in this section shows that the fuel deposi-tion effect plays a significant role on the combustion dynamics.However, the presence of a catalytic agent with a potential togenerate such an effect does not warrant an improvement in

TABLE 6INFLUENCE OF 5% CHANGE IN HYDROCARBONFUEL DENSITIES ON HTO TEMPERATURE (vi= 300 m/d)

Case fH fL HTO Temperature ( C)

9 base 1.64

10 3.52

TABLE 5INFLUENCE OF 5% CHANGE IN FREQUENCYFACTORS OR HYDROCARBON SURFACE AREAS ON HTO

TEMPERATURE (vi= 100 m/d)

Case kHor asH kLor asL HTO Temperature ( C)

1 base 1.88

2 base 0.45

3 1.42

4 base 1.77

5 base 0.42

6 1.367 2.30

8 2.23

100

200

300

400

500

0 100 200 300 400 500

Injection Rate, vi, m/d

ReactionR

egionTemperature,

oC

HTO (Base)

LTO (Base)

LTO (Case 9)

HTO (Case 9)

LTO-dominated

Region

Fig. 2Hydrocarbon deposition effect (Case 9) on combustionfront propagation. Temperatures of the reaction regions vs.

air injection rate. The hydrocarbon amount ahead of the LTOregion is two times larger than the base value.

100

200

300

400

500

0 100 200 300 400 500

Injection Rate, vi,m/d

ReactionRegionTemperature,

C HTO (Base)

LTO (Base)

HTO (Case 10)

LTO (Case 10)

Fig. 3Hydrocarbon deposition effect (Case 10) on combus-tion front propagation. Temperatures of the reaction regions vs.

air injection rate. The hydrocarbon amount ahead of the LTOregion is 50% larger than the base value.



the combustion performance. On the contrary, it may lead to

detrimental effects such as incoherence of the reaction regionsand low-temperature-dominated front propagation. In light of theobservations in the Activation Energies subsection through theFuel Deposition Effect subsection, we consider a special casewhere combustion front propagation is under the influence of anideal catalytic agent with combined catalytic (Cases 8) and fueldeposition (Case 10) effects.

Dual Effects on Combustion Front Propagation. Next, thecombined catalytic (Cases 8) and fuel deposition (Case 10) effects(i.e., the dual effects) of the agents are considered without the com-pensation. The results could be equally considered as the catalyticeffect with compensation where the changes in reaction rates arefrom variations in the specific surface areas (Case 7). We look forsolutions in a parameter space that demonstrate the dual effects

of catalytic agents. Fig. 5ashows that the dual effects noticeablyimprove the combustion performance nearly at every injectionrate. The HTO temperature reaches values larger than 500C inthe presence of reservoir heat losses, and the LTO temperaturesincrease approximately 30%, reaching values as high as 320C.The estimated separation distances in Fig. 5b also points to a sig-nificant improvement: The reaction regions propagate coherentlywith an average distance of 1 m at air injection rates as low as100 m/d. The coherent propagation velocity of the reaction regionsdecreases in Fig. 5c, however insignificantly.

It is also found that the dual effects maintain a preferentialdominance on the combustion front dynamics. The catalytic effectplays a role on improvement of the combustion performance atlow injection rates, where, under the influence of reservoir heatlosses, the HTO temperature drops significantly; on the other hand,the fuel deposition effect is more pronounced on the combustionperformance at high air injection rates. Regardless of their pref-erential influences, the combustion front propagates at relativelylow temperatures and velocities as the air injection rate (reservoirheat losses) is decreased (increased).

Note that, in Fig. 5a, the combustion front appears not to beinfluenced significantly by the presence of dual effects at injectionrates less than 100 m/d. Disappointingly, in this region, the tem-perature drop is so large and overall influence of our ideal agentis so small that the front is expected to have no significant effecton recovery under the reservoir conditions. Important issues thatneed to be addressed for application of air injection methods arethen (1) identification of the optimum reservoir conditions and(2) selection of suitable catalytic agents that could prevent these

observed drastic temperature drops at low injection and high heatloss rates.

Indeed, a better understanding of the dual effects under thereservoir conditions calls for a detailed analysis of the combustionfront propagation model in the complete parameter space. Thetask, however, is a rather challenging one because the problem islarge (i.e., the number of parameters varying in the presence ofcatalytic agents) and involves nonlinearities inherent to the oxida-tion reactions, their interactions. For the purpose of analysis, thefront propagation solutions are presented in 2D Cartesian coordi-nates reflecting percent change in the kinetics and fuel depositionparameters. Hence, x-y coordinates correspond to the combinedkinetic effects (Case 8) and to the fuel deposition effects (Case 10),respectively. For example, the combustion front propagation solu-tion obtained for point (x =0.4,y =0.6) in the dual effect parameterspace is that particular solution for which the activation energiesand frequency factors of the reactions are changed 40% from theirbase values according to Case 8; the deposited hydrocarbon andgenerated fuel surface areas increased 40% according to Case 7;

and the deposited hydrocarbon and fuel amounts increased 60%according to Case 10.

0

50

100

150

200

250

300

0 100 200 300 400 500


SeparationDistance,cm

Case 10

Case 9Case 10

Base

Fig. 4Hydrocarbon deposition effects (Cases 9 and 10) oncombustion front propagation. Separation distance of HTO andLTO reaction regions vs. air injection rate.

100

200

300

400

500

ReactionRegionTemperature,

oC

HTO (Base)

LTO (Base)

HTO (Cases 8 and 10)

LTO (Cases 8 and 10)

(a)

0

50

100

150

200

250

300

0 100 200 300 400 500


Sepa

rationDistance,cm

Base

(Cases 8 and 10)

(b)

0

1

2

3

4

5

6

7

0 100 200 300 400 500


VelocityofReactionRegions,m/d

(c)

Base

(Cases 8 and 10)

0 100 200 300 400 500

Injection Rate, vi , m/d

Fig. 5Dual effects (Case 8 and Case 10) on combustion frontpropagation.



Fig. 6shows coherence of the reaction regions developing in alarge portion of the dual effect space. Outside of the area of coher-ence, the regions are either fully separated (*) or overlap witheach other (*0). Obviously, these are not desired during in-situcombustion, and they appear when a catalytic agent would influencepropagation due to only deposition or kinetics, respectively. Next,we search for the existence of optimum local conditions inside thearea of coherence. Our primary interest is to determine whether theestimated propagation velocity, oxygen consumption efficiency, andHTO temperature could be maximized as the reaction regions main-tain a finite separation distance. Typically, the distance is 50150 cm;it increases abruptly, however, as the boundary of frontal separationis approached. When the kinetic and fuel deposition effects increaseproportionally (i.e., as we move along the SW-NE diagonal) thesystem can maintain higher temperatures and propagation velocitieswhile its oxygen consumption efficiency increases. There exists aclear and smooth gradation of the latter properties until roughly point(x =0.35,y =0.35) is reached; at larger values, however, nonlineari-ties appear, in particular, in the case of propagation velocity underthe kinetics effects of a catalytic agent. Consequently, no localizedarea yields solutions that may lead to an ideal combustion perform-ance. Thus, in the presence of catalytic agents, an in-situ combustionfront can reach extremely high temperatures, consuming nearly allof the injected oxygen, but it has to compromise on its propagationspeed. Consider, for example, the case where a catalytic agent ledto 80%, or higher increase in the kinetics and deposition parameters.The improvement on the HTO temperature and oxygen consump-tion efficiency would be significant. The temperature now reachesvalues as high as 600650C in the presence of reservoir heat losses;

whereas, the predicted propagation velocity stays relatively low, inthe range of 2.43.0 m/d.

ConclusionsIn-situ combustion front propagation involves diffusive processesand complex chemical reactions. Traditional approach to analysisof the combustion fronts is based on classification of the crude oilsaccording to their oxidation characteristics. At least two distincttemperature ranges, HTO and LTO, have previously been found toaffect the propagation characteristics. These reaction regions havea spatially narrow width within which heat release and reactionrates vary significantly. The narrow width calls for an approach inwhich these reaction regions are treated as surface of discontinui-ties in the appropriate variables. The model reduces the complexnonlinear problem of combustion front propagation with a fuelgenerating reaction to a system of coupled algebraic equations.

The reaction regions could self-sustain and propagate in thereservoir within a distance from each other that could vary sig-nificantly. There exist two limits to their propagation: The frontscould either coincide (with a distance nil) or they could becomeinfinitely separated and de-coupled. The extent of these limits inthe injection velocity space varies significantly with the kineticsand stoichiometry of the fuel generating reaction.

Dual (catalytic/fuel deposition) effects of catalytic agents oncoherent propagation of the HTO and LTO reaction regions areinvestigated. It is theoretically shown that the reservoir heat lossesare detrimental to in-situ combustion process; at low air injectionrates, temperature of the combustion front drops drastically. In thepresence of clays/additives, this deleterious influence of heat lossespersists. Their presence, however, may have a significant effect onthe combustion front performance at high injection rates.

A contrast in the activation energies of the oxidation reactions

improved the combustion performance. A normal compensation effect,however, eliminated this improvement. According to the formulation

Fig. 6 Coherent propagation of the HTO and LTO reaction regions in the space of dual effects.



of the model, in the presence of the latter, a positive catalytic effectappears to be possible only if the reaction rates change because ofvariations in the specific surface areas of the hydrocarbon fuels. Inthe literature, often the product kas is considered as the frequencyfactor, however. If the compensation effect is because of k/E rela-tionship as described here, then the combustion process could beenhanced by the variations in specific fuel surface areas.

Investigation regarding fuel densities showed that hydrocarbondeposition markedly influences the LTO region temperature; the LTOtemperature could become comparable and even higher than the HTOregion temperature as the hydrocarbon deposition increases ahead(i.e., LTO dominated in-situ combustion processes). A significant

improvement on the overall combustion performance is possible, how-ever, only when the HTO fuel is generated in direct proportions to thehydrocarbon deposition ahead of the LTO region. The latter points outthe significance of HTO fuel generation stoichiometry on the in-situcombustion dynamics. When the HTO fuel generation is favorableand, in particular, at high (low) air injection (heat loss) rates, a strongcombustion enhancement effect could be observed in the presence ofclays, metallic minerals, and water-soluble additives because of therole they play in modifying specific surface area of the solid grainsand, hence, on deposition of the hydrocarbons.

The results emphasize the importance of reservoir selectionbefore any air injection and in-situ combustion process and callsfor a consideration (screening) of the additives with the purposeof increased control over the in-situ combustion front propagation

during the air injection processes.

Nomenclature

asn =specific hydrocarbon/fuel surface areas/unit volume, m2/m3

An =dimensionless quantity described in Eqs. 3 and 4, sknasnpYi/qnEnvi

2

css =effective heat capacity of solid matrix, kJ/m3-KE

n =activation energy of reaction n, kJ/kmole h =dimensionless heat transfer coefficient, ( ) h v c H i s s / 1

2

h =convective heat transfer coefficient, kW/m2-K H =reservoir thickness, m kn =frequency factor of reactionn , kW-m/atm-kmole q =ratio of the heat generated by the combustion process to the

absolute heat content of the matrix

qn =ratio of the heat generated by reaction nto the absolute heatcontent of the matrix

Qn =heat of reaction n, kJ/kg hydrocarbon reacted R =universal gas constant, kJ/kmole-K To =initial reservoir temperature, K vi =volumetric injection rate per cross-sectional area, m3/m2-dayVDn =dimensionless propagation velocity of reaction region n Yi =inlet oxygen concentration, kg/kg s =effective thermal diffusion coefficient, m2/s n =Arrhenius number of reaction n,E/RTo h =influence of external heat losses on reaction region tempera-

tures, (1 + 4hn/ VDn2)1/2

fn =dimensionless temperature of reaction region n

n =dimensionless stoichiometric coefficient for oxygengn =dimensionless stoichiometric coefficient for gas products * =dimensionless separation distance between reaction regionsofn =hydrocarbon mass density, kg/m3

=porosity, fraction

Acknowledgments

The third author was supported by the Natural Science and Engi-neering Research Council of Canada.

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I. Ycel Akkutlu is a professor and graduate liaison of theMewbourne School of Petroleum and Geological Engineeringat the University of Oklahoma. He holds MS and PhD degreesfrom the University of Southern California. His current researchinterests are molecular simulation and multiscale theoreticaldescription of fluid flow, heat/mass transport, and reactions inporous media. His work finds applications in reservoir engineer-ing, particularly in the areas of unconventional gas recoveryand improved oil recovery/enhanced oil recovery. Akkutlu hasserved on the editorial board of SPE Journaland on the SPE-ATCERecovery Mechanisms and Flow in Porous Media subcommit-

tee since 2007.Yannis C. Yortsosis the Chester Dolley Professor ofPetroleum Engineering and Professor of Chemical Engineeringat the University of Southern California. Since June 2005, he alsoserves as dean of the USC Viterbi School of Engineering, hold-ing the Zohrab A. Kaprielian Chair in Engineering. Yortsos holdsa BS degree from the National Technical University of Athens,Greece, and MS and PhD degrees from the California Instituteof Technology, all in chemical engineering. Yortsos researchinterests are in various aspects of fluid flow and transport inporous media, with specific applications to the recovery ofsubsurface fluids, such as oil and gas. Since fall 2006, he hasalso served as the editor-in-chief of the SPE journals. G. DeryaAdagl-Demirdalis a reservoir engineer with Encana, Calgary.Previously, she worked for the Turkish Petroleum Corporation(TPAO) in the reservoir and production groups in Ankara, Turkey.Adagl-Demirdal holds a BS degree from the Middle East

Technical University, and MS degree from the University ofAlberta, both in petroleum engineering.

1-Dual Role of Catalytic Agents on in-Situ Combustion Performance

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