Dual Role of Catalytic Agents on In-Situ

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

Summary techniques. Among them, the most important one is related to

Unlike other thermal recovery methods, air injection and in-situ instant availability and low-cost of the injection fluid regardless
combustion generates significant amounts of heat in the reservoir. of the reservoir location. Furthermore, and perhaps even more
However, the process is subject to acute heat loss rates from the distinctively, air injection processes bypass a necessity to minimize
reaction zones because of high temperature gradients; conse- the wellbore heat losses, and the accompanying injector insulation
quently, the reaction temperatures may be reduced considerably, costs, realized during the other thermal recovery operations. Dur-
leading to a deteriorated combustion performance and debilitated ing air injection, unlike the high-temperature injection fluids, no
field operations. The goal of this paper is to determine under heat losses take place until the injected air reaches the reservoir and
which reservoir conditions the combustion temperatures could confronts the hydrocarbon deposits and in-situ generated fuel.
be maintained at sufficient levels. Previous investigators have During any air injection process, propagation of a self-sustain-
partially addressed this issue using kinetic and combustion tube ing combustion front in the reservoir is necessary for improved oil
experiments. In the absence of heat losses, it has been repeat- recoveries, however. It is desired to maintain a stable high-tempera-
edly shown that catalytic agents (naturally occurring clays, metal ture combustion front that travels away from the injector deeper
oxides, and some water-soluble metallic additives) improve the into the reservoir toward the production well, uniformly sweeping
self-sustainability limit of combustion front in crude oil and sand the in-place fluids. The fuel necessary for high-temperature oxi-
mixtures. In general, this has been attributed to the dual role of dation reaction, and the associated front self-sustainability, is a
these agents on the combustion performance, namely the catalytic carbon-rich residue resulting from a series of complex in-situ proc-
and fuel deposition effects. It is currently a common belief that esses: low-temperature oxidations, cracking and coking (pyrolysis)
appropriate introduction of such materials in a reservoir environ- reactions, in-situ generated steam-based distillation, and, finally,
ment could enhance the performance of combustion process and, multiphase displacement. These sequentially take place ahead of
hence, improve the recoveries. An investigation of their dual role the moving combustion front, and their relative locations in the
on combustion requires that the mechanisms of combustion are reservoir are dictated by a nonuniform temperature profile monoto-
well understood in their presence. Complex physical and chemi- nously decreasing further away from the combustion front.
cal nature of the problem at the pore-scale has prevented detailed The produced oil is often upgraded and, hence, lighter than
investigations using physical and numerical models, however. the original oil in place because of the in-situ consumption of the
Here, we approach the problem analytically using a sequential- heavier fractions at the combustion front and because of in-situ
reaction [high-temperature oxidation/low-temperature oxidation processes (e.g., distillation) taking place ahead of it. Hence, the
(HTO/LTO)] combustion front propagation model, based on large air injection processes may have an added benefit of recovering
activation energy asymptotics, and introducing the reaction kinet- in-situ upgraded heavy oils.
ics and fuel deposition effects to the model systematically by Despite the potential advantages, air injection processes have not
varying the related variables and parameters. Coherent propagation been widely used in the field primarily because of operational and
of the reaction regions are then investigated using reaction region technical difficulties. Earlier, it is reported that, among the 1.3 mil-
temperatures, propagation velocity, and the oxygen consumption lion B/D of oil produced by thermal methods, only 2.2% is produced
efficiency. General characteristics of an ideal catalytic agent are by in-situ combustion (Moritis 2002). One major technical difficulty
discussed in terms of its potential to improve in-situ combustion. that limits their practical application is related to control and optimi-
It is found that the front propagation can be improved under the zation of combustion front and of its complex interaction with the
reservoir conditions only if both the catalytic and fuel deposition preceding phenomena under the reservoir conditions. The preceding
effects of the agents are present. The work is important for our phenomena take place at the reservoir scale under restrictive influ-
understanding of in-situ combustion processes and can be used ence of a nonuniform temperature profile, distribution of which
for development of screening criteria to identify high-performance is dictated by the combustion front. On the other hand, the issues
catalytic agents in the laboratory using conventional apparatus. related to combustion front performance and optimization—in terms
of the amount of in-situ generated heat, front temperature and propa-
Introduction gation speed—are closely tied to in-situ deposition and availability
Thermal recovery is the principal approach to reduce viscosity of of the hydrocarbon fuel generated by the preceding phenomena. If
heavy crude oil by applying heat into a subsurface oil reservoir. the deposited fuel amount is not sufficient, as is often the case with
The necessary thermal energy can be generated at the surface and lighter oil reservoirs, the front may propagate in an unstable combus-
introduced into the reservoir by means of injecting steam, hot tion mode susceptible to detrimental reservoir conditions (e.g., high
water, or gas. It may also be generated in-situ by injecting air and reservoir heat losses) and it could become extinct. To the contrary,
oxidizing heavy components of the crude oil in place. The latter if the deposited fuel amount is large—the case with some heavy oil
approach for heavy oil recovery has long been recognized as the reservoirs, bitumen deposits, and oil shales—the front propagation
air injection and in-situ combustion (Prats 1982). velocities are expected to be significantly low, which slows down
Air injection and in-situ combustion processes have several the operations and brings an uneconomically high air compression
advantages when compared with the other thermal heavy oil recovery and injection costs for the production (Alexander et al. 1962). Thus,
the air injection processes depend upon not only a detailed analysis
of the reservoir and fluid properties showing the existence of opti-
mum conditions in place but also upon a better understanding of
Copyright © 2010 Society of Petroleum Engineers
the physical and chemical phenomena and their coupling under the
This paper (SPE 115506) was accepted for presentation at the SPE Annual Technical reservoir conditions.
Conference and Exhibition, Denver, 21–24 September 2008, and revised for publication.
Original manuscript received for review 24 June 2008. Revised manuscript received for
A series of experimental investigations have been performed
review 18 February 2009. Paper peer approved 19 February 2009. since the 1970s mainly focusing on application of the air injection

March 2010 SPE Journal 137

 TABLE 1—KINETIC RUNS FOR CYMRIC LIGHT CRUDE OIL*
Kaolitine Metallic Peak T, Peak T, E/R, E/R, Max. O2 Max. O2
or Silica Additive LTO (°C) HTO (°C) LTO HTO LTO (%) HTO (%)

Kaolitine None 275 385 9647 10669 3 2.8

3+
Kaolitine Fe 275 355 8535 11522 5.1 3.8
Silica powder None 287 410 8331 10681 2.4 2.6
3+
Silica powder Fe 280 370 7886 9775 2.9 2.7
* He et al. 2005

processes to an extended range of reservoir rock matrix and fluid be considered simultaneously with the activation energy of the
characteristics. The investigations also involved the so-called cata- reaction when the catalytic effects are investigated.
lytic agents, namely naturally occurring minerals such as clays and De los Rios et al. (1988) and Shallcross et al. (1991) performed
metal oxides, and certain water-soluble metallic salts, with the kinetic experiments with various metallic additives and developed
potential to play a catalytic role on the combustion reactions. In an analytical model to estimate the kinetic parameters of the oxida-
the chemical industry, metals have been known for their catalytic tion reaction regions, namely LTO, MTO, and HTO. They found
ability in hydrocarbon oxidation and cracking reactions. Many that iron and tin salts enhance fuel deposition and increase oxygen
studies appeared in the literature of chemistry investigating the consumption while copper, nickel, and cadmium salts show no
effects of metallic additives on the oxidation characteristics of apparent effect. For each oxidation reaction, the activation energies
crude oils. They were also considered as the agents to control and are estimated using their model. For most of the catalytic agents,
optimize the in-situ combustion processes. their results showed that the activation energy of HTO and MTO
Previous experimental works showed that the overall reaction reactions increases while that of the LTO reaction decreases.
mechanism of crude oils in porous media is caused by a sequence Castanier et al. (1992) carried out 13 combustion tube runs
of several oxidation reactions that occur at different temperature with metallic additives. In the experiments, four different types of
ranges. On the thermograms, these have been classified as the oil were used. The results showed that tin, iron, and zinc enhance
regions of low-temperature (LTO), medium-temperature (MTO), combustion efficiency, while copper, nickel, and cadmium have little
and high-temperature (HTO) oxidation reactions. In the absence of or no effect. Increase in fuel deposition, oxygen utilization effi-
oxygen, there also exist pyrolysis and thermal cracking reactions ciency, and front velocity are found in the presence of the former
during the in-situ combustion, although significantly high tem- metals. In addition, zinc is found to be less effective compared to
peratures and longer times may be required for their dominance. tin and iron. For the light oil case, they observed a sustained com-
Mamora (1993) considered the experimental occurrence of LTO bustion using iron additive while combustion had failed without
and HTO reactions during a series of kinetic tube experiments and any additive.
concluded that the LTO reactions are mainly responsible for fuel Holt (1992) used iron nitrate and zinc nitrate on Cymric field
generation. On the basis of this premise, he developed a two-reac- light and heavy oil and observed a catalytic effect of the additives.
tion hydrocarbon oxidation kinetic model. For the Cymric heavy oil, results of the kinetic experiments showed
Earlier, Burger and Sahuquet (1972) performed kinetic experi- that addition of 1% iron nitrate increased fuel amount by 20%. On
ments and observed that the oxidation reactions could occur at the other hand, 1% zinc nitrate increased fuel concentration by only
lower temperatures and the area under the high-temperature peak 5%. It is noted that the generated fuels require different amounts
increases in the presence of catalytic agents. They interpreted of air to burn unit masses. They also repeated the same runs for
that the former observation was because of an increase in oxida- the Cymric light oil without any additives. Sustained combustion
tion reaction rate and the latter was because of an increase in the could not be achieved, however, because of a lack of fuel deposi-
deposited fuel amount. tion. On the other hand, they observed efficient combustion with
Fassihi (1981) ran kinetic experiments using 27°API crude oil. He the addition of 1% (mole) iron nitrate solution.
compared the effluent gas data for the clean sand with the one added He et al. (2005) explored the effect of water-soluble metallic
copper. The results showed that the activation energy of HTO region additives on in-situ combustion using combustion tube and kinetics
decreases approximately 50% and activation energy of MTO region cell. Cymric light and heavy oil are used during the experiments.
increases approximately 50%. No significant change is observed in For the tests, sand, silica powder, or kaolinite are mixed with water
the LTO region other than a higher Arrhenius constant. and oil. In the cases of test with metallic additive, 0.5 g of the addi-
Drici and Vossoughi (1985) studied the effects of metal oxides tive is added to the water. The results of their experiments are sum-
on combustion characteristics of the crude oil by using differen- marized in Tables 1 and 2. It is observed that the additive improves
tial scanning calorimetry (DSC) and thermogravimetric analysis performance in all cases, including changing activation energies,
(TGA). The results showed that, as the metal oxides concentration greater oxygen consumption, low temperature threshold, and more
increases, the amount of heat released in the LTO reaction region complete oxidation. For Cymric light crude oil, the catalytic effect
gradually increases. The combustion peak temperature shifts to is obvious in the LTO where fuel deposition is increased to sustain
a lower temperature and became smaller and smoother, which combustion. For Cymric heavy crude oil, metallic additives are
reflects a more homogeneous composition of the solid residue. found to have an effect on HTO. Kaolinite also has an effect on
They also noted that the change in the reaction rate constant must crude oil combustion even without a metallic additive.

TABLE 2—KINETIC RUNS FOR CYMRIC HEAVY CRUDE OIL*

Kaolitine Metallic Peak T, Peak T, E/R, E/R, Max. O2 Max. O2
or Silica Additive LTO (°C) HTO (°C) LTO HTO LTO (%) HTO (%)

Kaolitine None 270 395 9090 12319 12.6 12.2

3+
Kaolitine Fe 280 355 9085 9427 13.7 18.7
Silica powder None 265 410 7293 10489 6.3 18.4
3+
Silica powder Fe 260 380 7098 10640 7 18.4
* He et al. 2005

138 March 2010 SPE Journal

 In summary, results of the experimental studies have shown qL q ⎡ 1 ⎤
that the introduction of catalytic agents to certain types of crude  fL = 1 + + H exp ⎢ − ( hL + 1) VDL  * ⎥ , . . . . . . . . . . . (2)
oil/sand mixtures could significantly (1) change the oxidation reac- hL hL ⎣ 2 ⎦
tion kinetics inside the combustion front (i.e., catalytic effect) and
(2) influence the fuel deposition. However, we do not have a clear where subscripts H and L correspond to the HTO and LTO reactions,
understanding of the mechanisms that lead to the dual—catalytic respectively. q = qH + qL = (QHofH + QLofL)/[(1−)cssTo] represents
and fuel deposition—effects. For example, it is not known how the ratio of the heat generated by the combustion process to the abso-
exactly the fuel is deposited. Is it the specific surface area of the lute heat content of the matrix. This quantity is directly proportional
porous medium ahead of the combustion front or the stiochiometry to the amounts of hydrocarbon deposited οfL and fuel-generated οfH
of fuel generation that is changed in the presence of these materi- for the LTO and HTO reactions, respectively. hn2 = 1 + 4h/VDn2
als? Further, with the changing fuel availability, does the specific represents the influence of external heat losses on temperatures of
surface area of the deposited fuel also change such that an addi- the reaction regions. Steady propagation velocities of the reaction
tional catalytic effect comes into play for an optimal combustion regions, on the other hand, are given by the following equations:
front propagation?
It is also not well understood how significant the dual effects ⎛  ⎞
are on the combustion front propagation, how the so-called catalytic
2
VDH = AHYH fH exp ⎜ − H ⎟ . . . . . . . . . . . . . . . . . . . . . . . . . . (3)
⎝  fH ⎠
effect should appear during the propagation and to what extent it
influences the combustion performance. The main objective should,
therefore, be an investigation on the combustion front performance and
in the presence of dual effects of catalytic agents under the reservoir
conditions. ⎛  ⎞
Because of the complex nature of the crude oil components
2
VDL = ALYL fL exp ⎜ − L ⎟ . . . . . . . . . . . . . . . . . . . . . . . . . . . (4)
⎝  fL ⎠
and 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 is Hence, the velocity equations hold nonlinear dependency on
considered analytically using a sequential-reaction (fuel-generat- the temperatures of the reaction regions and involve, among oth-
ing, LTO; fuel-burning, HTO) front propagation model. Given the ers, two dimensionless quantities An = sknasnpYi /qnEnvi2 and Yn =
complexity of problem, the approach is quite simple, although (1 − nVDn)/(1 + gnVDn). The former reflects a combination of
it makes rigorous analysis of the in-situ fuel generation and physicochemical properties, including the kinetics (i.e., frequency
consumption processes along with their nonlinear interactions. factor k, deposited hydrocarbon, and generated fuel surface areas
The reaction kinetics (i.e., frequency factors, activation energies, asL, asH, and activation energy E), of the reaction; whereas the lat-
deposited hydrocarbon, and generated fuel specific surface areas), ter accounts for the concentration of oxygen left unburned by the
hydrocarbon amount and fuel generation stoichiometry are input reaction region. Here, we note that An is inversely proportional
parameters of the model. During the investigation, these param- to the square of air injection rate, vi2 [i.e., (volumetric air/unit
eters are systematically changed as indication of the effects caused time)/cross-sectional area]. The latter is also referred to as injection
by the presence of catalytic agents and the impact on the front rate, expressed in m/d, and it is defined at the inlet (i.e., injection)
propagation are investigated under detrimental reservoir conditions conditions in the presence of heat losses; see Akkutlu and Yortsos
(i.e., the conditions that may lead combustion to the extinction (2003) for derivations and further details of Eqs. 3 and 4.
limit), such as insufficient hydrocarbon and fuel amounts or large Investigation of the combustion front propagation thus requires
heat loss rates. The role of the catalytic agents—whether catalytic simultaneous solution of four coupled-algebraic equations (i.e.,
or not—that could play in optimizing the combustion performance Eqs. 1 through 4), in the presence of reservoir heat losses. Condi-
is identified. tion of coherence asserts that these reaction regions travel with the
The theoretical approach essentially builds on a recent descrip- same speed (i.e., VDH = VDL); thus, in addition to the temperatures
tion of the single-reaction combustion front propagation using of reaction regions and their separation distance, this common
large activation energy asymptotics (Akkutlu and Yortsos 2003). velocity must also be determined. Details of the computational
The work was extended to the sequential oxidation reactions procedure can be found in Adagulu and Akkutlu (2007).
(Akkutlu and Yortsos 2004), and the coherence of the sequential
reaction regions propagating in a porous medium under typical Results and Discussion
reservoir conditions (Adagulu and Akkutlu 2007). Further, in the The presence of catalytic agents are expected to lead to changes in
absence of agents, the authors investigated the interactions between the quantities n (Arrhenius number of reaction n) and An of Eqs.
the reaction regions, delineating the effects of fuel generation and 3 and 4 because of their dependence on the frequency factor, k,
combustion kinetics on the propagation. It was found out that specific surface area, as, and activation energy, E. Later, we elabo-
the reaction regions have the ability to travel closely spaced and, rate on these dependencies as an indication of the catalytic role
consequently, minimize the effects of reservoir heat losses on the the agents play on the combustion front propagation. The values
combustion. This mechanism has been shown to thermally support presented in Table 3 are used to generate the base case scenario.
the combustion front under deleterious reservoir conditions.
Activation Energies. An increase in the value of activation energy
Combustion Front Propagation Model means that chemical transformation has to overcome a higher
Consider air injection into a linear homogeneous reservoir under energy barrier related to the energy of the covalent bonds. The
the influence of heat transfer to the surroundings. A steady propa- required amount of reaction energy is provided by the kinetic
gation of sequential—HTO and LTO—reaction regions separated energy of translational motion of the colliding oxygen and hydro-
by a finite distance * develops. Because of oxidation, the two carbon molecules (i.e, two-body collisions). If, initially, heat is
reaction regions experience discontinuities in heat and mass fluxes released and local temperature becomes larger, the mean kinetic
and interact with each other through temperature and reactant energy of the molecules will increase, and, thus, a greater number
(oxygen/fuels) concentration fields. Using considerable analysis of colliding molecules will soon have sufficient amount of kinetic
reported previously (Akkutlu and Yortsos 2004), dimensionless energy to overcome the energy barrier. Thus, in accordance with
temperatures of the propagating reaction regions are the Arrhenius dependency, the reaction rate increases. If the reac-
tion is accelerated, more heat will be released, the temperature of
qH q ⎡ 1 ⎤ the medium will further increase, and, consequently, the reaction
 fH = 1 + + L exp ⎢ − ( hH − 1) VDH  * ⎥ . . . . . . . . . . . . (1) will be further accelerated. Therefore, an increase in the activation
hH hH ⎣ 2 ⎦
energy of a reaction should lead a steadily propagating reaction
and region to higher temperatures.

March 2010 SPE Journal 139

 TABLE 3—TYPICAL PARAMETER VALUES USED IN THE TABLE 4—INFLUENCE OF 5% CHANGE IN ACTIVATION
COMBUSTION FRONT ANALYSIS ENERGIES ON HTO TEMPERATURE ( vi = 100 m/d)

Parameter Value Case EH EL HTO Temperature (°C)

o 3
ρ fH 19.0 kg/m 1 base 22.33
EH
4
7.35 10 kJ/kmole 2 base 4.45
QH
4
3.95 10 kJ/kg fuel 3 19.81
To 373.15 K 4 base 19.42
p 1.0 atm 5 base 7.66
Yi 1.0 kg/kg 6 6.38
vi 100.0 m/day 7 26.71
R 8.314 kJ/kmole-K 8 27.13
k 227 kW-m/atm-kmole
as
5 2 3
1.41 10 m /m
4
8.654 10 kW/m-K tube data. This may point to a pitfall during the interpretation of
cg gi 1.2338 kJ/m -K
3 kinetic cell data, which are based on zero-dimensional observations
3 3 and, therefore, do not reflect the 1D propagation dynamics of the
(1 φ) cs s 2.012 10 kJ/m -K
sequential reaction regions.
H 2m
2
0.078 kW/m -K Frequency Factors. An increase in the reaction frequency factor
causes the reaction rate to proportionally increase, which, conse-
3.018
quently, is expected to accelerate the reaction region propagation
1.0 and 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 in
the frequency factors. Table 5 shows that the HTO temperature
We observe that this explanation is strictly valid for the does not necessarily obey the anticipated trends. This points out
energy and temperature relationship of an HTO reaction region to a complex interplay of the reaction regions during the in-situ
in the presence of a preceding LTO region. Table 4 shows all combustion process. In addition, it is found that the changes in
the possible variations in the activation energies of the oxidation HTO temperature are much smaller than those caused by variations
reactions at a constant air injection rate along with their corre- in the activation energies.
sponding changes in the HTO front temperature estimated using
the model. Additionally, four cases are observed where the HTO Specific Fuel Surface Areas. Based on the formulation of the
temperature is increased: Cases 2, 4, 6, and 8. Only Case 8 among model (i.e., Eqs. 3 and 4), the influence of hydrocarbon fuel sur-
them—when the contrast in the HTO/LTO activation energies is face areas should be identical with the frequency factors. Thus,
increased—has the potential to increase the HTO temperature HTO temperature changes observed in Table 5 are equally valid
significantly. Interestingly, this is also the case observed during for the hydrocarbon and fuel surface area changes. For example,
the kinetics experiments of He et al. (2005), when Fe+ is added to a 5% increase in the deposited hydrocarbon surface area and a 5%
silica/Cymric heavy oil and to kaolinite/Cymric light oil mixtures decrease in the generated fuel surface area (i.e., Case 7) lead to a
(see Tables 1 and 2). 2.3°C increase in the HTO temperature.
Fig. 1 shows the effect of variations in the activation energies
in accordance with Case 8 on the propagation dynamics of the Combined Effects of the Activation Energies and Frequency
coherent reaction regions, namely the reaction region temperatures, Factors. The estimated temperatures in Tables 4 and 5 are such
propagation velocity, and separation distance. When compared that, in all the cases considered, the temperature change from
with the base-case solutions (dashed lines), it is clear in Fig. 1a the variations in the activation energies consistently offsets those
that the contrast in activation energies improves the HTO tempera- caused by variations in the frequency factors. Thus, it becomes an
ture, in particular at high air injection rates. However, the catalytic important issue for the discussion whether the activation energies
effect (i.e., the shaded region in Fig. 1a) becomes smaller as the and the frequency factors of the reactions vary independent of
injection is decreased because of the presence of reservoir heat each other in the presence of catalytic agents. Often, a variation
losses. This decrease in catalytic effect is because of a decrease in in the activation energies only has been considered as an indica-
the difference between the rate of heat generation and the rate of tion of the catalytic activity. However, Drici and Vossoughi (1985)
heat losses. Here, it is also observed that the changes in activation showed that consideration of the variations in the energy alone
energies have negligible influence on the estimated LTO tempera- can be misleading; instead, the combined (compensation) effect
tures, which is nearly a constant for a large range of air injection, of the frequency factor and activation energy of a reaction should
neither on the coherent front propagation speed (Fig. 1b). It is, be considered throughout an investigation. In this study, a normal
however, clearly indicated in Fig. 1c that, at high injection rates, (m > 0) compensation effect is considered for the HTO and LTO
the reaction regions approach each other, eventually overlapping, reactions in the presence of agents, i.e., the frequency factor and
when the rate is approximately 25–300 m/d. Consequently, there activation energy relationship of the oxidation reaction follows a
exist no solutions corresponding to coherent propagation at higher positive trend:
injection rates.
Results indicate that the experimentally observed improvement log k = mE + c , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (5)
in self-sustainability of the combustion process may be attributed
to a catalytic effect originating from a contrast in the HTO and where m = 6.453 × 10−5 kmole/kJ is given by Drici and Vossoughi
LTO activation energies in the presence of catalytic agents. Com- (1985) and valid for various metal oxides, and c = −2.38 is taken
bustion tube runs of He et al. (2005) also support this observation. here so that the base-state values of activation energies and fre-
Interestingly, however, their kinetic-cell results corresponding to quency factors are recovered using Eq. 5. Fig. 1 (solid blue lines)
Case 8 predicts 30–40°C lower HTO temperatures than the base shows the compensation corresponding to Case 8 in terms of the
values. The latter experimental observation then appears to be in propagation dynamics of the reaction regions. It clearly shows
contradiction with the improvement observed in the combustion that the compensation effect indeed eliminates the previously

140 March 2010 SPE Journal

 500
catalytic effect
Catalytic effect
due to
to activation
activation case 88
Case
due

C
energies only base
Base
energies only

Region Temperatures,
400 compensation effect
Compensation effect

oC
HTO
HTO

Temperatures,
Temperature,
300
LTO
LTO

200

Rxn 100
0 100
100 200 300
300 400
400 500
500

(a) Injection Velocity, m/day

Injection velocity, vvii,, m/day
InjectionVelocity, m/d
7
Base
6
Front Velocity, m/d

Compensation effect
4

2 E
1

0
0 100 200 300 400 500

(b) Injection Velocity, vi , m/d

40
Separation Distance, cm

Base
30
Compensation effect

20
Case 8

10

0
0 100 200 300 400 500

(c) Injection Velocity, vi , m

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

observed catalytic effect from the contrast in the activation energies Further experimental investigation is required in the light of these
only. The estimated HTO temperatures and velocities are nearly theoretical observations.
identical to the base-state solution at low and moderate air injec-
tion rates; at high rates, the estimates become even less than the Fuel Deposition Effect. Here, we note that the surface areas of the
predicted base values. deposited hydrocarbon and the generated fuel are not the same as the
In summary, the results distinctly illustrate that the changes in surface area of the porous medium, although they may be correlated
kinetic parameters has the potential to influence the combustion to the latter. Earlier, it is argued that the catalytic agents may change
front propagation significantly, in particular, through the properties the specific surface are of the porous medium and, consequently,
of HTO region. It is found that any selected catalytic agent should vary the hydrocarbon deposition and the fuel generation and,
modify the activation energies of the oxidation reactions such that hence, the total heat content of the reservoir. Therefore, dynamics
the contrast in HTO/LTO activation energies is further increased. of the coherent reaction regions is expected to be influenced by the
The results also clearly show the importance of variations in the dimensionless quantities qn in Eqs. 1 through 4, An (this time because
hydrocarbon and fuel specific surface areas as the sole sources of its dependency on qn) and Yn (because of dependencies on the
of catalytic effect in the presence of normal compensation effect. stoichiometric coefficients n and gn) in Eqs. 3 and 4.

March 2010 SPE Journal 141

 TABLE 5—INFLUENCE OF 5% CHANGE IN FREQUENCY TABLE 6—INFLUENCE OF 5% CHANGE IN HYDROCARBON
FACTORS OR HYDROCARBON SURFACE AREAS ON HTO FUEL DENSITIES ON HTO TEMPERATURE ( vi = 300 m/d)
TEMPERATURE (vi = 100 m/d)
Case ρ οfH ρ οfL HTO Temperature ( °C)
Case kH or asH kL or asL HTO Temperature ( °C)
9 base 1.64
1 base 1.88 10 3.52
2 base 0.45
3 1.42
shifts from HTO and is dictated by the LTO region. This observa-
4 base 1.77 tion is in agreement with the experimental results of Drici and
5 base 0.42 Vossoughi (1985), where, using thermal analysis techniques, the
6 1.36 authors predicted a shift of a large amount of heat from a high to
7 2.30
a low temperature range because of an increase in the solid surface
area in the presence of agents. Nevertheless, the results here show
8 2.23 that the overall influence of this shift may not be strong enough
to improve front propagation.
Sensitivity of the HTO temperature on the hydrocarbon and fuel Fig. 3 shows combustion behavior from fuel deposition in
densities is given in Table 6. Cases 9 and 10 consider the possibil- accordance with Case 10. In this case, the deposited hydrocarbon
ity of increased hydrocarbon deposition ahead of the LTO reaction amount ahead of the LTO region is 50% larger than the base value
region. In Case 10, the amount of generated fuel is increased pro- and the generated HTO fuel amount increases proportionally.
portionally, however; hence, the stoichiometry of fuel generation Hence, the total heat content of the reservoir is the same as in
is not changed and is the same as that in the base case. Notice Case 9 (i.e., two times larger than the base-state), and, therefore,
that, relative to the temperature variations from activation energies Figs. 2 and 3 are thermally calibrated and comparable. Estimated
given in Table 1, the HTO temperature increases only slightly, even changes in the separation distance are shown in Fig. 4 for the base,
though the air injection rate is kept three times larger. Regardless, Case 9 and Case 10. At high air injection rates, it is clear that the
Table 6 also shows that the temperature increase of Case 10 is increased fuel amounts significantly affect both reaction region
approximately two times larger than that of Case 9. temperatures, which, consequently, improves the combustion per-
Fig. 2 shows the combustion behavior corresponding to Case formance. As the injection rate decreases, however, the separation
9 when the hydrocarbon deposition ahead of the LTO is doubled. distance of the reaction regions increase, which promotes the heat
Estimated values of the HTO temperature are much lower than transfer to the surroundings, and, consequently, the enhancement
the base values at air injection velocities below 400 m/d; at higher on the combustion performance disappears. Note that the predicted
injection rates, however, there exists a small region of increased distances with Case 10 are significantly larger than those with Case
combustion performance from increased hydrocarbon deposition. 9 and the base state at large air injection rates. As the injection
At higher (lower) air injection (heat loss) rates, it is expected that rate is decreased, the distance increases nonlinearly and, at the air
the region of improved combustion performance is much larger. injection rate of 230 m/d, it becomes infinitely large, namely the
Fig. 2 also shows that, unlike the catalytic effect, the hydrocar- reaction regions are fully separated (i.e., *→∞). At lower injec-
bon deposition causes dramatic changes in the LTO temperature. tion rates, there still exist solutions corresponding to the coherent
At low and moderate air injection (35–200 m/d) its estimated val- propagation of reaction regions with a finite distance; however,
ues increase such that the LTO temperature becomes even larger the predicted temperatures in this region are close to the base-state
than the HTO temperature, (see the LTO-dominated region in Fig. values. The estimated coherent propagation velocity of the reaction
2). Hence, in this region, control over the combustion performance regions with Cases 9 and 10 does not show significant variations
from the base values either.
In summary, results in this section shows that the fuel deposi-
500 tion effect plays a significant role on the combustion dynamics.
However, the presence of a catalytic agent with a potential to
LTO-dominated
Region generate such an effect does not warrant an improvement in
HTO (Base)
Reaction Region Temperature, o C

500 HTO (Case 10)

400
LTO (Case 9)
Reaction Region Temperature, °C

HTO (Base)
HTO (Case 9)
400

300
LTO (Case 10)

LTO (Base) 300

LTO (Base)

200
200

100 100
0 100 200 300 400 500 0 100 200 300 400 500
Injection Rate, vi , m/d Injection Rate, v i , m/d

Fig. 2 —Hydrocarbon deposition effect (Case 9) on combustion Fig. 3 —Hydrocarbon deposition effect (Case 10) on combus-
front propagation. Temperatures of the reaction regions vs. tion front propagation. Temperatures of the reaction regions vs.
air injection rate. The hydrocarbon amount ahead of the LTO air injection rate. The hydrocarbon amount ahead of the LTO
region is two times larger than the base value. region is 50% larger than the base value.

142 March 2010 SPE Journal

 300 500
Case 10 HTO (Cases 8 and 10)

Reaction Region Temperature, oC

250 HTO (Base)
400
Separation Distance, cm

200 LTO (Cases 8 and 10)

300
LTO (Base)
150

200
100
Case 9
Case 10
50 100
Base 0 100 200 300 400 500

(a) Injection Rate, v i , m/d

0
0 100 200 300 400 500 300
Injection Rate, v i , m/d
250
Fig. 4 —Hydrocarbon deposition effects (Cases 9 and 10) on

Separation Distance, cm
combustion front propagation. Separation distance of HTO and
200
LTO reaction regions vs. air injection rate.
(Cases 8 and 10)
150
the combustion performance. On the contrary, it may lead to
detrimental effects such as incoherence of the reaction regions 100
and low-temperature-dominated front propagation. In light of the
observations in the Activation Energies subsection through the 50
Fuel Deposition Effect subsection, we consider a special case Base
where combustion front propagation is under the influence of an
ideal catalytic agent with combined catalytic (Cases 8) and fuel 0
0 100 200 300 400 500
deposition (Case 10) effects.
(b) Injection Rate, v i , m/d

Dual Effects on Combustion Front Propagation. Next, the 7

combined catalytic (Cases 8) and fuel deposition (Case 10) effects
Velocity of Reaction Regions, m/d

6
(i.e., the dual effects) of the agents are considered without the com-
pensation. The results could be equally considered as the catalytic 5 Base
effect with compensation where the changes in reaction rates are
from variations in the specific surface areas (Case 7). We look for 4
solutions in a parameter space that demonstrate the dual effects
3
of catalytic agents. Fig. 5a shows that the dual effects noticeably (Cases 8 and 10)
improve the combustion performance nearly at every injection 2
rate. The HTO temperature reaches values larger than 500°C in
1
the presence of reservoir heat losses, and the LTO temperatures
increase approximately 30%, reaching values as high as 320°C. 0
The estimated separation distances in Fig. 5b also points to a sig- 0 100 200 300 400 500
nificant improvement: The reaction regions propagate coherently (c) Injection Rate, v i , m/d
with an average distance of 1 m at air injection rates as low as
100 m/d. The coherent propagation velocity of the reaction regions Fig. 5 — Dual effects (Case 8 and Case 10) on combustion front
decreases in Fig. 5c, however insignificantly. propagation.
It is also found that the dual effects maintain a preferential
dominance on the combustion front dynamics. The catalytic effect
plays a role on improvement of the combustion performance at Indeed, a better understanding of the dual effects under the
low injection rates, where, under the influence of reservoir heat reservoir conditions calls for a detailed analysis of the combustion
losses, the HTO temperature drops significantly; on the other hand, front propagation model in the complete parameter space. The
the fuel deposition effect is more pronounced on the combustion task, however, is a rather challenging one because the problem is
performance at high air injection rates. Regardless of their pref- large (i.e., the number of parameters varying in the presence of
erential influences, the combustion front propagates at relatively catalytic agents) and involves nonlinearities inherent to the oxida-
low temperatures and velocities as the air injection rate (reservoir tion reactions, their interactions. For the purpose of analysis, the
heat losses) is decreased (increased). front propagation solutions are presented in 2D Cartesian coordi-
Note that, in Fig. 5a, the combustion front appears not to be nates reflecting percent change in the kinetics and fuel deposition
influenced significantly by the presence of dual effects at injection parameters. Hence, x-y coordinates correspond to the combined
rates less than 100 m/d. Disappointingly, in this region, the tem- kinetic effects (Case 8) and to the fuel deposition effects (Case 10),
perature drop is so large and overall influence of our ideal agent respectively. For example, the combustion front propagation solu-
is so small that the front is expected to have no significant effect tion obtained for point (x = 0.4, y = 0.6) in the dual effect parameter
on recovery under the reservoir conditions. Important issues that space is that particular solution for which the activation energies
need to be addressed for application of air injection methods are and frequency factors of the reactions are changed 40% from their
then (1) identification of the optimum reservoir conditions and base values according to Case 8; the deposited hydrocarbon and
(2) selection of suitable catalytic agents that could prevent these generated fuel surface areas increased 40% according to Case 7;
observed drastic temperature drops at low injection and high heat and the deposited hydrocarbon and fuel amounts increased 60%
loss rates. according to Case 10.

March 2010 SPE Journal 143

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

Fig. 6 shows coherence of the reaction regions developing in a Conclusions

large portion of the dual effect space. Outside of the area of coher- In-situ combustion front propagation involves diffusive processes
ence, the regions are either fully separated (*→∞) or overlap with and complex chemical reactions. Traditional approach to analysis
each other (*→0). Obviously, these are not desired during in-situ of the combustion fronts is based on classification of the crude oils
combustion, and they appear when a catalytic agent would influence according to their oxidation characteristics. At least two distinct
propagation due to only deposition or kinetics, respectively. Next, temperature ranges, HTO and LTO, have previously been found to
we search for the existence of optimum local conditions inside the affect the propagation characteristics. These reaction regions have
area of coherence. Our primary interest is to determine whether the a spatially narrow width within which heat release and reaction
estimated propagation velocity, oxygen consumption efficiency, and rates vary significantly. The narrow width calls for an approach in
HTO temperature could be maximized as the reaction regions main- which these reaction regions are treated as surface of discontinui-
tain a finite separation distance. Typically, the distance is 50–150 cm; ties in the appropriate variables. The model reduces the complex
it increases abruptly, however, as the boundary of frontal separation nonlinear problem of combustion front propagation with a fuel
is approached. When the kinetic and fuel deposition effects increase generating reaction to a system of coupled algebraic equations.
proportionally (i.e., as we move along the SW-NE diagonal) the The reaction regions could self-sustain and propagate in the
system can maintain higher temperatures and propagation velocities reservoir within a distance from each other that could vary sig-
while its oxygen consumption efficiency increases. There exists a nificantly. There exist two limits to their propagation: The fronts
clear and smooth gradation of the latter properties until roughly point could either coincide (with a distance nil) or they could become
(x = 0.35, y = 0.35) is reached; at larger values, however, nonlineari- infinitely separated and de-coupled. The extent of these limits in
ties appear, in particular, in the case of propagation velocity under the injection velocity space varies significantly with the kinetics
the kinetics effects of a catalytic agent. Consequently, no localized and stoichiometry of the fuel generating reaction.
area yields solutions that may lead to an ideal combustion perform- Dual (catalytic/fuel deposition) effects of catalytic agents on
ance. Thus, in the presence of catalytic agents, an in-situ combustion coherent propagation of the HTO and LTO reaction regions are
front can reach extremely high temperatures, consuming nearly all investigated. It is theoretically shown that the reservoir heat losses
of the injected oxygen, but it has to compromise on its propagation are detrimental to in-situ combustion process; at low air injection
speed. Consider, for example, the case where a catalytic agent led rates, temperature of the combustion front drops drastically. In the
to 80%, or higher increase in the kinetics and deposition parameters. presence of clays/additives, this deleterious influence of heat losses
The improvement on the HTO temperature and oxygen consump- persists. Their presence, however, may have a significant effect on
tion efficiency would be significant. The temperature now reaches the combustion front performance at high injection rates.
values as high as 600–650°C in the presence of reservoir heat losses; A contrast in the activation energies of the oxidation reactions
whereas, the predicted propagation velocity stays relatively low, in improved the combustion performance. A normal compensation effect,
the range of 2.4–3.0 m/d. however, eliminated this improvement. According to the formulation

144 March 2010 SPE Journal

of the model, in the presence of the latter, a positive catalytic effect Akkutlu, I.Y. and Yortsos, Y.C. 2004. Steady-State Propagation of In-Situ
appears to be possible only if the reaction rates change because of Combustion Fronts with Sequential Reactions. Paper SPE 91957
variations in the specific surface areas of the hydrocarbon fuels. In presented at the SPE International Petroleum Conference in Mexico,
the literature, often the product kas is considered as the frequency Puebla, Mexico, 7–9 November. doi: 10.2118/91957-MS.
factor, however. If the compensation effect is because of k/E rela- Alexander, J.D., Martin, W.L., and Dew, J.N. 1962. Factors Affecting
tionship as described here, then the combustion process could be Fuel Availability and Composition During In-Situ Combustion. J.
enhanced by the variations in specific fuel surface areas. Pet Tech 14 (10): 1154–1164; Trans, AIME, 225. SPE-296-PA. doi:
Investigation regarding fuel densities showed that hydrocarbon 10.2118/296-PA.
deposition markedly influences the LTO region temperature; the LTO Burger, J.G and Sahuquet, B.C. 1972. Chemical Aspects of In-Situ Com-
temperature could become comparable and even higher than the HTO bustion—Heat of Combustion and Kinetics. SPE J. 12 (5): 410–422;
region temperature as the hydrocarbon deposition increases ahead Trans., AIME, 253. SPE-3599-PA. doi: 10.2118/3599-PA.
(i.e., LTO dominated in-situ combustion processes). A significant Castanier, L.M., Baena, C.J., Holt, R.J., Brigham, W.E., and Tavares, C.
improvement on the overall combustion performance is possible, how- 1992. In Situ Combustion With Metallic Additives. Paper SPE 23708
ever, only when the HTO fuel is generated in direct proportions to the presented at the SPE Latin America Petroleum Engineering Confer-
hydrocarbon deposition ahead of the LTO region. The latter points out ence, Caracas, 8–11 March. doi: 10.2118/23708-MS.
the significance of HTO fuel generation stoichiometry on the in-situ de los Rios, C.F., Brigham, W.E., and Castanier, L.M. 1988. The Effect
combustion dynamics. When the HTO fuel generation is favorable of Metallic Additives on the Kinetics of Oil Oxidation Reactions in
and, in particular, at high (low) air injection (heat loss) rates, a strong In-Situ Combustion. Technical Report SUPRI TR-63, Contract No.
combustion enhancement effect could be observed in the presence of DOE/BC/14126-4, Stanford University Petroleum Research Institute,
clays, metallic minerals, and water-soluble additives because of the Stanford, California (November 1988).
role they play in modifying specific surface area of the solid grains Drici, O. and Vossoughi, S. 1985. Study of the Surface Area Effect on
and, hence, on deposition of the hydrocarbons. Crude Oil Combustion by Thermal Analysis Techniques. J. Pet Tech
The results emphasize the importance of reservoir selection 37 (4): 731–735. SPE-13389-PA. doi: 10.2118/13389-PA.
before any air injection and in-situ combustion process and calls Fassihi, M.R. 1981. Analysis of Fuel Oxidation in In-Situ Combustion Oil
for a consideration (screening) of the additives with the purpose Recovery. PhD dissertation, Stanford University, Stanford, California
of increased control over the in-situ combustion front propagation (April 1981).
during the air injection processes. He, B., Chen, Q., Castanier, L.M., and Kovscek, A.R. 2005. Improved
In-Situ Combustion Performance With Metallic Salt Additives. Paper
Nomenclature SPE 93901 presented at the SPE Western Regional Meeting, Irvine,
asn = specific hydrocarbon/fuel surface areas/unit volume, m2/m3 California, USA, 30 March–1 April. doi: 10.2118/93901-MS.
Holt, R.J. 1992. In Situ Combustion with Metallic Additives. Technical
An = dimensionless quantity described in Eqs. 3 and 4, sknasnpYi /
Report SUPRI TR-87, Contract No. DOE/BC/14600-29, Stanford
qnEnvi2
University, Stanford, California (July 1992).
css = effective heat capacity of solid matrix, kJ/m3-K Mamora, D.D. 1993. Kinetics of In-Situ Combustion. PhD dissertation,
En = activation energy of reaction n, kJ/kmole Stanford University, Stanford, California.
h = dimensionless heat transfer coefficient, h ′ / ⎢⎣(1 −  ) vi2cs s H ⎥⎦ Moritis, G. 2002. California Steam EOR Produces Less; Other EOR Con-
h′ = convective heat transfer coefficient, kW/m2-K tinues. Oil & Gas Journal 100 (15): 43–47.
H = reservoir thickness, m Prats, M. 1982. Thermal Recovery. Monograph Series, SPE, Richardson,
kn = frequency factor of reaction n, kW-m/atm-kmole Texas 7.
q = ratio of the heat generated by the combustion process to the Shallcross, D.C., de los Rios, C.F., Castanier, L.M., and Brigham, W.E.
absolute heat content of the matrix 1991. Modifying In-Situ Combustion Performance by the Use of Water-
qn = ratio of the heat generated by reaction n to the absolute heat Soluble Additives. SPE Res Eng 6 (3): 287–294. SPE-19485-PA. doi:
content of the matrix 10.2118/19485-PA.
Qn = heat of reaction n, kJ/kg hydrocarbon reacted
R = universal gas constant, kJ/kmole-K
To = initial reservoir temperature, K I. Yücel Akkutlu is a professor and graduate liaison of the
vi = volumetric injection rate per cross-sectional area, m3/m2-day Mewbourne School of Petroleum and Geological Engineering
VDn = dimensionless propagation velocity of reaction region n at the University of Oklahoma. He holds MS and PhD degrees
Yi = inlet oxygen concentration, kg/kg from the University of Southern California. His current research
interests are molecular simulation and multiscale theoretical
s = effective thermal diffusion coefficient, m2/s description of fluid flow, heat/mass transport, and reactions in
n = Arrhenius number of reaction n, E/RTo porous media. His work finds applications in reservoir engineer-
h = influence of external heat losses on reaction region tempera- ing, particularly in the areas of unconventional gas recovery
tures, (1 + 4hn / VDn2)1/2 and improved oil recovery/enhanced oil recovery. Akkutlu has
fn = dimensionless temperature of reaction region n served on the editorial board of SPE Journal and on the SPE-ATCE
Recovery Mechanisms and Flow in Porous Media subcommit-
n = dimensionless stoichiometric coefficient for oxygen tee since 2007. Yannis C. Yortsos is the Chester Dolley Professor of
gn = dimensionless stoichiometric coefficient for gas products Petroleum Engineering and Professor of Chemical Engineering
* = dimensionless separation distance between reaction regions at the University of Southern California. Since June 2005, he also
ofn = hydrocarbon mass density, kg/m3 serves as dean of the USC Viterbi School of Engineering, hold-
ing the Zohrab A. Kaprielian Chair in Engineering. Yortsos holds
 = porosity, fraction a BS degree from the National Technical University of Athens,
Greece, and MS and PhD degrees from the California Institute
Acknowledgments of Technology, all in chemical engineering. Yortsos’ research
The third author was supported by the Natural Science and Engi- interests are in various aspects of fluid flow and transport in
neering Research Council of Canada. porous media, with specific applications to the recovery of
subsurface fluids, such as oil and gas. Since fall 2006, he has
References also served as the editor-in-chief of the SPE journals. G. Derya
Adagülü-Demirdal is a reservoir engineer with Encana, Calgary.
Adagulu, G.D. and Akkutlu, I.Y. 2007. Influence of In-situ Fuel Deposition on
Previously, she worked for the Turkish Petroleum Corporation
Air Injection and Combustion Processes. J. Cdn. Pet. Tech. 46 (4): 54–61. (TPAO) in the reservoir and production groups in Ankara, Turkey.
Akkutlu, I.Y. and Yortsos, Y.C. 2003. The dynamics of in-situ combustion Adagülü-Demirdal holds a BS degree from the Middle East
fronts in porous media. Combustion and Flame 134 (3): 229–247. doi: Technical University, and MS degree from the University of
10.1016/S0010-2180(03)00095-6. Alberta, both in petroleum engineering.

March 2010 SPE Journal 145