Combination Effect of in-Situ Combustion and Exhaust Gases Recirculation on 1D Combustion Tube: Numerical Approach

ABSTRACT Despite the widespread use of renewable and green energy, the demand for fossil fuels is also rising due to increasing global energy demand. Therefore, unconventional solutions, with safe environmental impacts, are being pursued to solve this problem. Instead of getting rid of the exhaust gases in the surroundings, one solution might be to inject them with the oxidizer into the oil reservoir, to initiate an in-situ combustion (ISC) process to enhance oil recovery. A numerical study of a 1-D combustion tube has been conducted and validated to simulate the in-situ combustion process using enriched air as the oxidizer. The effects of injecting exhaust gases with the oxidizer are studied. Different ratios of oxygen to nitrogen are used in the enriched air as well as different ratios of exhaust gases. If enriched air which is mostly oxygen, i.e. 95% O2 +5%N2, is used, it is found that replacing 10% of the enriched air with exhaust gases can increase the oil recovery factor (ORF) from 94.7% to 94.9% and replacing 20% can improve oil recovery to 95.1%. For another enriched air, 60% O2 +30% N2, it is found that replacing portions of the enriched air with exhaust gases will reduce the oil recovery factor. In all previous cases, it was found that replacing the proportions of enriched air with exhaust gases reduces the amount of fuel burned and increases hydrogen production.


Introduction
At a time of ever-increasing global energy demand the world is focused on producing energy with reduced pollutant emissions.We are trending toward clean energy production, or at least lower polluting fuels.Parallel to this trend, researchers are struggling to search for traditional energy sources in order to fulfil the growing energy demand.Fossil fuels are still widely used around the world and will be used into the future.However, oil, which is the main fossil fuel used for energy production, is running out.This is observed through the widespread use of enhanced oil recovery (EOR) techniques as oil reservoirs mature.Enhanced oil recovery techniques come after natural extraction has stopped.In-situ combustion (ISC) is considered one of the most effective thermal EOR techniques, due to the huge amount of heat released inside the reservoir.In ISC, a small portion of the reservoir oil is burned by injecting an oxidizer.As a result, heat is released that reduces the oil viscosity, and the exhaust gases displace the low-viscosity oil into the oil production wells (Leaute 1994;Li et al. 2021;Mahinpey, Ambalae, and Asghari 2007;Schleicher 1965;Shu 1983).Insitu combustion is considered a combination of several EOR techniques, such as water flooding, steam injection, hot gases injection and chemical reactions (Aziz and Settari 1979;Rabiu Ado 2017).Various chemical reaction models have been created and updated for Athabasca bitumen (Babushok and Dakdancha 1993;Coates, Lorimer, and Ivory 1995;Guntermann, Gudenau, and Mohtadi 1982;Hajdo, Hallam, and Vorndran 1985;Kapadia et al. 2013;Kapadia, Kallos, andGates 2011, 2013).Improvements have been made to improve ISC performance, such as using Toe-To-Heel air injection as reported in Ado (2020), where a horizontal production well is used instead of a vertical one, but the injection well is still vertical.The Toe-To-Heel system has proven to be a better oil recovery system compared to a conventional system.Another improvement is to inject ammonia with air has been done byHart (2017), in this way ammonia reacts with the hydrocarbon producing a surfactant.Chen et al. (2022) studied the Low temperature oxidation coking mechanism and coke characterization for Tahe ultra-heavy oil via the comparative analyses on the derived oxidized and pyrolytic cokes.Also, Chen et al. (2019 the oxidation kinetic triplets and thermodynamic parameters in specific temperature subranges were respectively investigated for Tahe heavy crude oil under non-isothermal condition.Canas et al. (2016) used a steam assisted gravity drainage technique with ISC, which provides a better performance.Decomposition characteristic analyzing of the actual carbonate rock under non-isothermal condition by thermogravimetric method (TG-DTG) have been done by Chen et al. (2019, to estimate whether the risk of obvious collapse occurred during the ISC process resulting from the fractures and carbonate decomposition.Also, Chen et al. (2022) employed advanced techniques such as (thermogravimetry/differential scanning calorimetry/pressurized differential scanning calorimetry, TG/DSC/PDSC) to characterize the tight light oil with different heating-rates and experimental pressures to investigate the influences of heating-rate and pressure on the oxidation behavior and kinetic characteristic.
The combination of ISC and exhaust gases injection has several advantages if this works, such as the higher oil production rate, due to the higher heat released, and low emissions, and that is because ISC is already occurred inside the reservoir.Another important merit is that this process is sequestrating amount of exhaust gases, especially CO 2 .But there are also several demerits combined with that process, for example ISC is not easy to control, also not suitable to all reservoirs and other demerits.
The novelty of this work is the combination of ISC and exhaust gases injection.Injecting exhaust gases, which are water vapor and carbon dioxide, at the same time with the oxidizer to initiate an ISC process, is studied using a numerical model.Two enriched air configurations are used, namely 95% O 2 /5% N 2 and 60% O 2 /40% N 2 .Moreover, different exhaust gases portions are used to replace the same portions of enriched air, to keep the total injected flow rate constant.The effect of injecting exhaust gases on (i) the oil recovery factor, (ii) heat release, (iii) the portion of burned oil and (iv) the amount of hydrogen generated is reported.

The numerical model
A numerical model to simulate a 1-D heavy oil combustion tube experiment was developed and validated to represent the ISC process and study some parameters.The combustion tube experiment used for simulation was conducted by the ISC Research Group at the University of Calgary and reported by Yang and Gates (2009).The numerical model has been used to study the ISC process by injecting different ratios of exhaust gases.The mathematical model for ISC is adapted from references (Aziz and Settari 1979;CMG, S 2017;Hamdy et al. 2020;Rabiu Ado 2017) which include all the governing equations used in this study, which have been solved using computer modeling group ltda.CMG, S (2017) Mass balance The mass conservation for each species i which exist in a number of phases π, is given using the following equation: where Sj is the saturation of phase j and xij is mole fraction of component i in phase j.

Energy Conservation
The conservation of energy used in the model is governed by the equation: Where: � : P π j¼1 ρ j h j u j is the advection or convection term, while ∇.(λ∇T) is the conduction heat transfer, and λ represent the thermal conductivity.
P π j¼1 θ j h � j is the heat source/sink, h � j is the enthalpy of phase j at injection or production,θ j is the mass production rate per unit volume, and q hc is the heat loss source/sink term.

Darcy's Law
The relationship between the flow rate (flow velocity) and pressure gradient can be obtained using Darcy's law as reported in Aziz and Settari (1979).
where: v j represent the velocity.μ is the fluid viscosity.K is the absolute permeability.k r represents the relative permeability.
∇φ j is the phase potential gradient.

Geometry, initial and inlet conditions
The simulated combustion tube is 1.83 m in length and is 0.1 m in diameter as shown in Figure 1.As mentioned inYang and Gates ( 2009)the combustion tube is saturated with Athabasca heavy oil and water and gas in ratios of 70%, 12% and 18%, respectively.The experiment lasted for 0.82 day after injecting the oxidizer at an initial temperature of 90°C.
All of the oil and reservoir properties such as oil composition, molecular weight, critical pressure and temperature, viscosity correlations, heat capacity, permeability, porosity, and other data required for the simulation are taken from Yang and Gates (2009).A constant volume flowrate of 1.344 m 3 /day was used throughout the experiment, where the exhaust gases replace the same amount of enriched air, to keep the same volume flowrate for direct comparison.According toYang and Gates ( 2009), enriched air with 95% oxygen content was used in this experiment.These values of oxygen and nitrogen were used in the present simulation to validate the model and initiate the study.
The chemical reactions used in the present model and their kinetics are listed in Table 1, and they have been collected from the literature.The chemical reactions contain pyrolysis, low-temperature oxidation (LTO), high-temperature oxidation (HTO), and hydrogen generation and consumption reactions.

Results and discussion
Two classes of enriched air, which are 95%O 2 /5%N 2 and 60% O 2 /40% N 2 , were used to investigate the effect of replacing a portion of the oxidizer with exhaust gases, on the ISC process and performance in this study.The effect of replacing different portions of enriched air with portions of exhaust gases, which are water vapor and carbon dioxide, on the oil recovery factor, combustion tube temperature profile, burned oil and hydrogen generation have been studied.These results are summarized in Table 2 and are presented in detail in Figures 5-12.The present study was performed after validating the numerical model results against published experimental data.Comparing with previous results from the literature, such as (Hamdy et al. 2020;Rabiu Ado 2017), they studied only ISC and increased the mount of oxygen in the mixture, but in the present work, exhaust gases have been injected with the enriched air, which means different conditions.They concluded that increasing the oxygen ratio in the oxidizer will increase the oil production rate continuously, and that is due to increasing the heat release during the ISC process, and for the present work, the results are coming soon in the following paragraphs.

Model validation
To validate the numerical model, a temperature profile resulted from the present model is compared in Figure 2 with the related temperature profile measured experimentally [18], after 0.3 day from starting the experiment.It can be seen that the present model gives acceptable results compared to the experiments, which indicates that the numerical model is  reliable.Deviations between the numerical results and experimental data may be due to several reasons, such as the heterogeneity of the porous tube and the difficulty in simulating the ISC process, which contains thousands of chemical reactions.

Effect of exhaust gases on peak temperature
Figures 3 and 4 together with Figures S1 and S2 in the Supplementary material (SM) indicate temperature profiles along the combustion tube after various times from the start, using two different enriched air oxygen and nitrogen ratios, with different values of exhaust gases.In the presented temperature profiles, the peaks occur at the maximum temperatures that represent the leading edge of the front, and their locations give an indication of the high-temperature oxidation (HTO) reaction region.The temperature profiles represent the temperature profiles after 0.2, 0.35, 0.5, and 0.75 day from the start of the experiment.In the ISC, the higher temperature means higher heat release and lower oil viscosity, resulting in higher oil recovery.
Figure 3 illustrate the temperature profile along the combustion tube in the case of using only an enriched air volume flow rate with oxygen to nitrogen ratios of 95% and 5%.The maximum temperature reached more than 600°C in 0.2, 0.35 and 0.5 day.That is due to the higher oxygen percentage in the enriched air, which is 95% without any exhaust gases.Figure S1(a) indicates the temperature profiles in the case of replacing 10% of the enriched air, 95% O 2 +5% N 2 , with exhaust gases i.e., water vapor and carbon dioxide.The maximum temperature in this case is lower than the former case.That is because injecting exhaust gases with the enriched air absorbs a quantity of heat.Moreover, the peak locations are different at similar times as shown in Figure 3 and Figure S1(a), which means that injecting exhaust gases leads to lower flame speeds.All of these observations are also evident in Figures S1(b) and S1(c).c) indicate the same previous sequence of replacing portions of enriched air with exhaust gas, but the enriched air in this case has an oxygen to nitrogen ratio of 60%O 2 +40% N 2 .The peak temperature in these cases is lower than the previous ones, due to the decrease in oxygen content, which decreases the quantity of heat released.In these figures, the peak temperature decreases from around 550-500°C in the case of using only enriched air compared to replacing 30% of it with exhaust gases.Moreover, the flame speed is lower than the previous cases with the higher oxygen content, 95%O 2 /5% N 2 .As a conclusion, the flame speed decreases with increasing exhaust gas concentrations in the injected mixture.

Effect of exhaust gases on oil recovery factor production
The main objective of the ISC process is to increase the amount of recovered oil, which is reflected in the oil recovery factor (ORF).The ORF is the amount of produced oil relative to the original oil in place (OOIP).Figure 5 illustrate the ORF in the case of using enriched air with an oxygen to nitrogen content of 95%O 2 /5% N 2 .The bars show the percent ORF by injecting 100% enriched air, 90% enriched air + 10% exhaust gases, 80% enriched air + 20% exhaust gases and 70% enriched air + 30% exhaust gases, respectively.In the case of injecting 100% enriched air the ORF is 94.7%.Replacing 10% of the enriched air with exhaust gases, the ORF increases to 94.9% and replacing 20% increases the ORF to 95.1%.Meanwhile, increasing the replacement ratio to 30%, the ORF reduces be 91.7%.Even though the amount of oxidizer decreases with the replacement of 10% to 20% enriched air by exhaust gases, contrary to expectation, the ORF increases.That is because the oxygen content in the enriched air is still high, i.e., 95%, and the heat released from combustion is still high, but it is less than in the case of using 100% enriched air, and this can be observed in Figure 6.This higher heat release heats the exhaust gases, which are used to displace the low-viscosity oil.Meanwhile, increasing the replacement ratio to 30%, the ORF decreases to 91.7%.This may be due to the high amount of the exhaust gases which absorb a high portion of the heat released in combustion, in addition to decreasing the amount of heat released by combustion due to decreasing the amount of the oxidizer.
Figure 6 illustrates the amount of heat released from combustion for the different ratios of exhaust gases used.Increasing the exhaust gases ratio decreases the heat released.This is due to a reduction in the amount of oxidizer as mentioned before.Replacing 20% of the enriched air with exhaust gases, will save 4.2% heat, while simultaneously increasing the ORF from 94.7% to 95.1%.This points to three benefits from this process; (i) an increased ORF, (ii) a decrease in the combustion heat release by decreasing the amount of oxidizer and (iii) sequestering a portion of the exhaust gases, thus reducing emissions.
Figure 7 shows that, in case of using enriched air with different oxygen to nitrogen ratios, i.e., 60%O 2 +40% N 2 , the ORF decreases to 82.7%, compared to 94.7% while using enriched air with an oxygen to nitrogen ratio of 95% O 2 +5% N 2 .Increasing the exhaust gases content directly decreases the ORF.This is due to the associated decrease in the heat released by combustion in addition to absorbing a portion of the heat by the exhaust gases.Increasing the exhaust gas ratios from 0.0% to 30% decreases the ORF by around 23.3%.In this case only two benefits could be achieved; firstly by reducing the heat released by combustion by reducing the amount of oxidizer and secondly by sequestering a portion of exhaust gas.As mentioned previously, the amount of heat released decreases with increasing exhaust gas concentration.Figure 8 indicates the amount of heat released using different ratios of oxidizer and exhaust gases, with an oxidizer content of 60% O 2 /40% N 2 .By increasing the exhaust gases content from 0.0 to 30%, 16.3% of the heat released could be saved, although the ORF decreases by 23.3%, so it is an optimization process.

Effect of exhaust gases on the burned oil
As a result of decreasing the amount of injected oxidizer, the heat release decreases and so the amount of burned oil also decreases.Figure 9 shows the volume of oil burned with every case using an oxidizer content of 95% O 2 /5% N 2 .The amount of burning oil decreases by 9.6% when the ratio of exhaust gases increases from 0 to 30%.However, using an oxidizer content of 60% O 2 +40% N 2 , this ratio increases to around 19.4%, Figure 10.

Effect of exhaust gases on hydrogen production
Figures 11 and 12 show that in all the previous cases with different enriched air contents and different exhaust gases, increasing the exhaust gas concentration will increase hydrogen production.That is because injecting exhaust gases will decrease the temperature, where thermal cracking and low temperature oxidation dominate, resulting in significant dominance of the coke reactions 2 and 10, which accelerates hydrogen generation via reactions 3 and 16.

Conclusions
A numerical model to model 1-D combustion tube experiment has been developed.The insitu combustion process with different oxidizer has been simulated to investigate the effect of adding portions of exhaust gases to the oxidizer.Two enriched air mixtures with different oxygen to nitrogen content were used.It is concluded that replacing up to 20% of the enriched air with exhaust gas can increase the oil recovery factor using enriched air with high oxygen content.Using oxidizer with contents of 60% O 2 /40% N 2 , increasing the exhaust gases ratios from 0.0% to 30% decreases the ORF by around 23.3%.The exhaust gases absorb a portion of the heat released by combustion.Replacing 20% of the enriched air with exhaust gases, will decrease the heat released by 4.2% when using 95% O 2 /5% N 2 .Increasing the exhaust gases content in the oxidizer increases hydrogen production and decreases the amount of burned oil.

Disclosure statement
No potential conflict of interest was reported by the author(s).

C s
Total solid concentration EG Exhaust gases EA Enriched air h j The enthalpy of phase j h* j The enthalpy of phase j at injection or production k Permeability k r Relative permeability ORF Oil recovery factor q hc The heat loss source/sink term.r ik The rate of k th reaction S j The saturation of phase j and x ij is mole fraction of component i in phase j.

Figure 1 .
Figure 1.Grid blocks for the simulated combustion tube.

Figure 2 .
Figures 3 and 4 together with FiguresS1 and S2in the Supplementary material (SM) indicate temperature profiles along the combustion tube after various times from the start, using two different enriched air oxygen and nitrogen ratios, with different values of exhaust gases.In the presented temperature profiles, the peaks occur at the maximum temperatures that represent the leading edge of the front, and their locations give an indication of the high-temperature oxidation (HTO) reaction region.The temperature profiles represent the temperature profiles after 0.2, 0.35, 0.5, and 0.75 day from the start of the experiment.In the ISC, the higher temperature means higher heat release and lower oil viscosity, resulting in higher oil recovery.Figure3illustrate the temperature profile along the combustion tube in the case of using only an enriched air volume flow rate with oxygen to nitrogen ratios of 95% and 5%.The maximum temperature reached more than 600°C in 0.2, 0.35 and 0.5 day.That is due to the higher oxygen percentage in the enriched air, which is 95% without any exhaust gases.FigureS1(a) indicates the temperature profiles in the case of replacing 10% of the enriched air, 95% O 2 +5% N 2 , with exhaust gases i.e., water vapor and carbon dioxide.The maximum temperature in this case is lower than the former case.That is because injecting exhaust gases with the enriched air absorbs a quantity of heat.Moreover, the peak locations are different at similar times as shown in Figure3and FigureS1(a), which means that injecting exhaust gases leads to lower flame speeds.All of these observations are also evident in FiguresS1(b) and S1(c).FigureS1(b) presents the temperature profile in the case of

Figure 5 .
Figure 5. Oil recovery factor for different enriched air-exhaust gases ratios in case of using enriched air with 95% O 2 /5% N 2 .

Figure 6 .
Figure 6.Enthalpy produced from in-situ combustion for different enriched air-exhaust gases ratios in case of using enriched air with 95% O 2 +5% N 2 .

Figure 7 .
Figure 7. Oil recovery factor for different enriched air-exhaust gases ratios in case of using enriched air with 60% O 2 +40% N 2 .

Figure 8 .
Figure 8. Enthalpy produced from in-situ combustion for different enriched air-exhaust gases ratios in case of using enriched air with 60% O 2 /40% N 2 .

Figure 9 .
Figure 9. Volume of burned oil for different enriched air-exhaust gases ratios in case of using oxidizer with contents of 95%O 2 /60% N 2 .

Figure 10 .
Figure 10.Volume of burned oil for different enriched air-exhaust gases ratios in case of using oxidizer with contents of 60%O 2 /40% N 2 .

Figure 11 .
Figure 11.Hydrogen production for different enriched air-exhaust gases ratios in case of using oxidizer with contents of 95% O 2 +5% N 2 .

Figure 12 .
Figure 12.Hydrogen production for different enriched air-exhaust gases ratios in case of using oxidizer with contents of 60% O 2 /40% N 2 .

Table 1 .
Chemical reactions and kinetics parameter used simulation.