High-Pressure Air Injection for Improved Oil Recovery: Low-Temperature Oxidation Models and Thermal Effect

High-Pressure Air Injection for Improved Oil Recovery: Low-Temperature Oxidation Models and Thermal EffectZhenya Chen,* Lei Wang, Qiong Duan, Liang Zhang, and Shaoran Ren

School of Petroleum Engineering, China University of Petroleum, Qingdao 266555, People’s Republic of China

ABSTRACT: High-pressure air injection into light oil reservoirs is an effective technique for improved oil recovery (IOR), andthis technique mainly depends upon a spontaneous low-temperature oxidation (LTO), which occurs at original reservoirtemperatures. During the LTO process of air injection, oxygen in the injected air is consumed and carbon dioxide can beproduced in the reservoir, leading to flue gas flooding and producing a significant amount of heat. In this paper, the mechanismsof the LTO reaction are analyzed and an improved LTO reaction model is established to facilitate reaction mechanism andreservoir simulation studies. A series of high-pressure LTO experiments using typical light oils were carried out in a temperaturerange from 70 to 170 °C to measure the reaction rates and to reveal the exothermic behavior of the LTO reactions. Heat-transferanalysis in the experimental process was conducted to examine the exothermic characteristics of the reaction using differentreactors and at various experimental conditions. A conceptual reservoir simulation model was used to study the thermal effect ofthe LTO reaction during the air injection process in terms of the temperature rise in the reaction zone and its IOR effect.

1. INTRODUCTIONAir injection has been proven to be an effective improved oilrecovery (IOR) technology.1 Traditionally, air injection isapplied to heavy oil reservoirs via an in situ combustionprocess,2,3 and oil recovery is enhanced mainly because ofsteam flooding and viscosity reduction, resulting from a largeamount of heat generated in the oxidation reaction at hightemperatures. For light oil reservoir, the air injection processhas the benefits of both traditional gas injection as well asthermal effects because of the exothermic nature of theoxidation.4−6 In comparison to other gas injection techniques,the low-temperature oxidation (LTO) process of high-pressureair injection is more like nitrogen injection but with anadditional thermal effect, which is considered a major advantagefor improving oil recovery. In recent years, air injection LTOtechniques, including air foam injections, have been applied inseveral light oil reservoirs in China, including Zhongyuanoilfield, Baise oilfield, and Changqing oilfield. Results fromZhongyuan oilfield have shown significant benefits of air and airfoam injection via the LTO process to water-flooded oilreservoirs. Up to 4% original oil in place (OOIP) ofincremental oil production had been achieved by air injectionin the pilot region during 3 years of air injection.7 The fieldobservation reveals that oxygen was effectively consumed andthere was a significant amount of carbon dioxide in theproduced gas, which proves the nature of the LTO reactions.However, whether there is a thermal effect during the LTOprocess is still in speculation. In previous studies, it wasconsidered that heat generation during air injection is notnecessary for IOR in light oil reservoirs; therefore, air injectioncan be considered as a conventional gas injection process ifoxygen can be removed through spontaneous LTO at reservoirtemperature to prevent gas and oil explosion in productionwells.6,14

The mechanisms of the LTO reaction and its reactionmodels have been studied by a few researchers,8,9 and manyexperimental methods and devices have been used to measure

the reaction rate and to observe the exothermic phenomenaduring the LTO reactions, including near-adiabatically con-trolled reactor [accelerating rate calorimetry (ARC)] andisothermally controlled reactors.10,11 In the studies of the in situcombustion process (high-temperature oxidation), LTO wasusually considered as a primary stage of the reaction, which canprovide fuels for the following HTO via a hydrocarbon crackingprocess.8 It has been observed that, during typical LTOreaction experiments at reservoir temperatures (i.e., 70−120°C), hydrocarbons react with oxygen and carbon monoxide isproduced. The reactions may involve a complex process ofhydrocarbon oxidation and carbon oxide generation. Forsimplicity of the reservoir simulation models, a one-stepreaction model was usually used, in which the oxidation ofhydrocarbon molecules was ignored, while a two-step modelmay be better for modeling the reaction, in which oxidation andcarbon oxide generation can be described separately. A lot ofexperiments were carried out to reveal the thermal effect of theLTO.11,12 The experimental results showed that, either using anARC facility or an isothermal reactor, complete or nearlycomplete oxygen consumption can be achieved and a lot ofCO2 (up to 12%) was produced but there was no temperaturerise in the reactor observed when the reaction occurred at arelatively low temperature range. While only at a hightemperature range (over 168 °C), measurable temperaturerising in the reactor or the so-called accelerating reaction couldbe observed.13

In this paper, a small batch reactor is used to perform a seriesof experiments under isothermal conditions at a temperaturerange from 70 to 170 °C to measure the reaction rate andreaction products and the thermal effect of the LTO reaction isrevealed via direct measurement of temperature rising and heat

Received: November 19, 2012Revised: January 17, 2013Published: January 17, 2013

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© 2013 American Chemical Society 780 dx.doi.org/10.1021/ef301877a | Energy Fuels 2013, 27, 780−786

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generation and heat loss analysis of the reactors. A two-stepLTO reaction model is proposed for the reaction simulationstudies. A reservoir simulation model is established toinvestigate the “flue gas flooding” process and the extra thermaleffect on IOR during the air injection process. Using the systemand conclusions from this paper, some important insights canbe obtained, and a good explanation can be offered for somephenomena and speculations in the past. More importantly, itpointed out a way to study the feasibility of the air injectiontechnique.

2. LTO REACTION MODELSIt has been proven that CO2 is the main gas product of light oilLTO reactions.14 Chemical analysis results showed thatcarbonyl was produced in the oil after the LTO reaction.15

The possible reaction products and paths of the LTO processof hydrocarbons can be summarized as shown in Figure 1.16

Referring to the LTO reaction paths shown in Figure 1, thedecarboxylation reaction may be responsible for the furtherreaction of oil oxides, leading to CO2 and CO production.13,17

The LTO experiments (100−200 °C) of light and medium-light oils using ARC also found that a significant amount ofCO2 and CO had been produced. For the observation of thethermal effect, only at high temperatures, an acceleratedreaction that can cause a significant temperature rise in thereactor could be observed. A reservoir simulation study haspredicted that, during air injection of the light oil field project,the reservoir temperature in the reaction zone can be raised toover 200 °C, which could have a positive effect on oilrecovery.18

Figures 2 and 3 describe a simplified mechanism showing thepossible paths leading to the generation of carbon oxides in theLTO process. Several free radical reactions are involved in thismechanism, and CO2 is the final gas product.In summary, the LTO mechanisms can be described in

details as follows: (1) The molecules of hydrocarboncompounds are primarily oxidized into intermediate products,such as carboxylic acid, aldehyde, ketone, alcohol, ether, etc. (2)Either the intermediate products may be directly subjected tothe decarboxylation reaction and generate CO2, CO, and water,or they can be first oxidized into hyperoxides and then undergoa decarboxylation process to produce CO2, CO, and water. (3)During the decarboxylation reaction, large oil components maytransform into lighter components, which causes a higher

activation energy requirement for further oxidation, and finally,this may terminate the reaction at certain temperatures.A two-step reaction model can be given to describe the

oxidation and decarboxylation processes described above.Step 1 (oxidation):

+ → + − Δ− HC H32

O C H O H Ox y x y2 2 2 2 1 (1)

Step 2 (decarboxylation):

→ + + Δ− − − HC H O C H COx y x y2 2 1 2 2 2 (2)

where ΔH is the reaction enthalpy, with a positive valuemeaning that the reaction is exothermic, while a negative valuesuggests an endothermic reaction. Some CO may be producedin the process, but it is usually neglected in the modelingbecause CO can be further oxidized to CO2.

3. EXPERIMENTAL SECTION3.1. Materials. The light oil sample used is from Changqing oil

field, northwest China. The viscosity of the oil is 4.28 mPa s atreservoir conditions (70 °C and 16.2 MPa). The oil density is 0.86 g/cm3 at standard surface conditions, with an average molecular weightof 177. Crushed reservoir cores and washed quartz sands were used tomake oil sands samples. Brine water with a salinity of 20 mg/L wasreconstituted on the basis of the composition of the reservoir aquifer.

3.2. LTO Experiment. A schematic diagram of the experimentalfacility used is showed in Figure 4, and details of the experimentalprocedure for LTO can be found in the literature.19 The volume of thereactor is 112 mL. For a typical experiment, materials loaded into thereactor include 22 mL of oil, 79 mL of quartz sand (with a mesh size of80−120 and packing porosity of 0.4), and 6.5 mL of distilled water.

In the experiments, the reactor loaded with oil and oil sands isimmersed in an isothermally controlled oil bath at a constanttemperature. Compressed air was then charged into the reactor to therequired pressure, and the LTO reaction may spontaneously occur.For measurement, the pressure of the reactor system and temperaturesinside and outside the reactor are recorded. During and/or after thereaction, the gas in the reactor was sampled to measure thecomposition of CO2 and O2.

Figure 1. Graph of LTO reactions of hydrocarbons.

Figure 2. Mechanism of the LTO process during air injection.

Figure 3. Simplified schematic of the LTO process during air injection.

Energy & Fuels Article

dx.doi.org/10.1021/ef301877a | Energy Fuels 2013, 27, 780−786781

The LTO reaction rate can be measured and calculated via thepressure reaction, and it can also be calculated from the consumingrate of O2 and the generating rate of CO2

ν = Δn t/ (3)

where v is the reaction rate (mol/h), Δn is the amount of O2 or CO2(mol), and t is the reaction time (h).

4. RESULTS AND DISCUSSION4.1. Pressure Reduction and Reaction Rate. The curve

of pressure reduction of the reactor system means O2consumption and can also reveal the change of the reactionrate, as shown in Figure 5 for four experiments at differenttemperatures. The pressure curve usually shows a sharppressure drop in the beginning of the experiment, lasting foronly a short period of time (less than 1 h), which could be dueto gas dissolving in the liquid phase. During the reactions, the

consumption of O2 can cause the pressure to decrease, whilethe generation of CO2 can cause the pressure to increase.Initially, oxidation or the oxygen consumption process mayprevail during LTO reactions,20 and that makes pressuredecrease monotonically. As the reaction progressed, the lengthof the carbon chain in the oil compounds may become shorterand shorter because of decarboxylation, which means that theactivation energy required for further oxidation is increased.Meanwhile, as the oxygen concentration in the reactordecreased, the reaction rate can be further reduced.The pressure reduction and average reaction rate calculated

from the pressure drop are shown in Table 1, and thecomposition of produced gases and the calculated CO2conversion ratio are also listed in Table 1.The CO2 conversion ratio (ΔnCO2

/ΔnO2) is calculated as the

ratio of the amount of CO2 produced and the amount of O2consumed. It reflects the percentage of oxygen elementtransformed from O2 to CO2, and it also represents the degreeof the decarboxylation reaction. Please note that, in thecalculation, the dissolved gas in the liquid phase is neglected.

4.2. Temperature Change and Exothermicity. For thereactions shown in Figure 5 (at oil bath temperatures of 140,150, and 160 °C and at 23.5 MPa), the measured temperaturein the center of the reactor is almost equal to that of the oilbath, which was isothermally controlled, and no temperaturerising was detected inside the reactor. However, there was asignificant temperature rise recorded at 170 °C, clearlydemonstrating the exothermic or thermal effect of the reaction.During other experiments, under the reservoir conditions of theoil sample studied (70 °C and 16 MPa), there was notemperature rise observed inside the reactor over 7 days ofreaction, and at higher temperature and pressure conditions (at23.5 MPa and 80−120 °C), no temperature rising was detected,although a significant pressure drop (because of O2consumption) was observed during those reactions. In fact,

Figure 4. LTO experimental schematic diagram: 1, isothermal oil bathtank (temperature-control precision of 0.01 °C); 2, stainless-steelreactor; 3, oil sands; 4, temperature sensor (precision of 0.1 °C); 5,valve; 6, four-way valve; 7, pressure sensor (precision of 0.01 MPa); 8,acquisition system; 9, personal computer (PC); 10, high-pressure airtank; and 11, gas sample connection.

Figure 5. Temperature and pressure graph of LTO experiments at different temperatures.



whether there was a temperature rise or not is dependent uponnot only heat generation during the oxidation but also the heatloss of the reactor system, which is analyzed in the followingsections.4.3. Exothermic and Thermal Effect Analysis. For the

reactions described in above, which were all conducted at a nearisothermal condition, the heat generated in the LTO reactioncan be lost through the oil sands system to the metal wall of thereactor as well as the oil bath medium. For the experimentsdescribed in Figure 5, at the low temperature range (up to 160°C), the reaction rate and heat-generating rate were low;therefore, the resulting temperature rise inside the reactor canbe very small (e.g., smaller than 0.1 °C, the precision limit ofthe temperature sensor used) or too small to be detected by thesensor. Only at a relatively higher temperature (over 170 °C),although the heat loss still existed, the reaction rate and heat-generating rate can be large enough to cause a detectabletemperature rise (e.g., 3−4 °C) inside the reactor.In previous experiments, a near ARC has been used.13,18 The

ARC device used a small sphere-shaped reactor (with an insidediameter of 2.54 cm). During the experiments, the oil and airmixture was loaded into the reactor at a required pressure andset to a starting temperature. To maintain an adiabaticcondition, the ARC has a better temperature controlmechanism that can track the temperature rising (because ofthe oxidation heat) inside the reactor. When a temperaturerising rate (at a certain temperature) greater than 0.025 °C/minis detected, the system will start to heat up to keep thetemperature outside of reactor equal to that of the reactorinside. Please note that it is the rate of 0.025 °C/min thatmakes the ARC a near ARC; therefore, it cannot achieve acomplete adiabatic reaction. The experimental results showedthat, for the typical light oil used, only at temperatures higherthan 168 °C, an accelerating effect was detected. In otherwords, the exothermal phenomenon or heating effect was onlydetected at higher temperatures, which is similar to theexperimental results obtained using the isothermally controlledreactor, as described above.A detailed analysis of heat generation and loss of the reactor

systems can be conducted to understand the thermal effect ofthe LTO reaction.LTO reaction rates can be expressed using an average O2

consumption rate and an average CO2 production rate.

ν = Δn t/O O2 2 (4)

ν = Δn t/CO CO2 2 (5)

Reaction enthalpies of the reactions can be calculated from thebond energy analysis based on the reaction schemes outlined ineqs 1 and 2.21

ΔH1 = 658.1 kJ/mol (exothermal reaction), and ΔH2 = 11.9kJ/mol (endothermic reaction).The calculated results of the reaction rate and heat-

generating rate in the oxidation experiments at different

temperatures are illustrated in Table 2. Both oxidation anddecarboxylation have been considered in calculating the heatgeneration rates.

On the other hand, if all of the heat generated during thereaction process is absorbed by the oil sands and surroundingmedia without heat loss, the temperature increase in the reactor(ΔT inside the oil sands) can be calculated as

Δ = ′+ +

TQ t

C m C m C mo o s s w w (6)

where Q′ is the heat-generating rate (J/min) and t is thereaction time (h). Co, Cs, and Cw are the heat capacities of crudeoil, quartz sand, and water, respectively (J g−1 °C−1), and mo,ms, and mw are the masses of crude oil, quartz sand, and water(g). ΔT is the temperature rise (°C).The calculation results show that there could be a

temperature rise of 70−100 °C, clearly indicating that heatloss is very important in analyzing the thermal effect of LTOreactions.In fact, in either experimental reactors or real reservoirs, part

of the heat generated during oxidation will be inevitably lost tothe surrounding environment, including rocks, oil, and watersystems. To illustrate the influence of heat loss on the detectionof the thermal effect on the isothermal experiments conductedin this study, it can be assumed that the heat-transfer rate equalsthe heat-generating rate when the system achieves equilibriumon heat transmission; that is, the heat flux is equal to the heat-generating rate. It is also assumed that the temperature isuniform in all parts of the oil sands, and there is heat loss toboth radial and vertical directions through the reactant mediainside the reactor, the wall of the reactor, and surroundingsoutside the reactor. The heat loss model is showed in Figure 6.The dashed area stands for the wall of the reactor.The heat-transfer rate model is shown in Figure 7. In the

model, Φ is the heat-transfer rate or heat-generating rate (W),T is the temperature (°C), R is the thermal resistance, λ is thethermal conductivity of the wall of the reactor (W m−1 K−1), his the convective heat-transfer coefficient between one kind offluid (oil bath or air) and another solid material (W m−2 K−1).h1 refers to h between air and oil sands; h2 refers to h betweenair and the wall of the reactor; and h3 refers to h between oilbath and the wall of the reactor. r1 and r2 are the inside and

Table 1. LTO Experimental Results under 23.5 MPa and 140−170 °C

T(°C)

initial pressure, Po(MPa)

terminal pressure, Pt(MPa)

reaction time(h)

average reaction rate(MPa/h)

O2 produced gas(%)

CO2 produced gas(%)

CO2 conversionratio

140 23.74 19.82 108.00 0.0363 1.0 6.8 0.287150 23.25 19.75 26.00 0.1346 0.9 7.5 0.317160 23.43 20.52 11.60 0.2509 0.7 8.2 0.355170 23.36 20.72 5.60 0.4714 0.2 11.3 0.481

Table 2. Calculated Results of the Reaction Rate andThermal Effect for the Experiments in Figure 5

T(°C)

O2 consumption rate(×10−3, mol/h)

CO2 production rate(×10−3, mol/h)

heat-generating rateof LTO (J/h)

140 0.462 0.13 201150 1.854 0.580 806160 4.101 1.442 1782170 8.500 4.056 3681



outside radii of the reactor, respectively, and l1 and l2 are theheights of the oil sands and air columns, respectively.Taking into consideration the geometric parameters of the

reactor and the heat transmission properties of the materialsinvolved, the temperature difference between oil sands and theouter boundary of the reactor can be obtained and is listed inTable 3.

As described above, the precision of the temperature sensorused is 0.1 °C, while the heat generated in the LTO reactioncould only achieve a temperature rise of 0.007−0.07 °C at140−160 °C, which was not detectable. However, at 170 °C,the generated temperature rise (around 3 °C) was high enoughto be detected. It is understood that the heat loss during all ofthe LTO experiments (isothermal controlled or at nearadiabatic conditions) plays a very important role in theobservation of the thermal effect, in which reservoir simulationcan be a useful method to study the thermal effect of the airinjection process.

5. REACTION KINETICS MODELThe experimental results indicated that the LTO reaction ratewas very low; therefore, the reactants (oil and oxygen) can beconsidered to be in excess at certain conditions of the reaction(the reaction order is zero). On the basis of the Arrheniusequation of reaction kinetics (eq 7), the LTO reaction rate canbe described as a function of the temperature only and can becalculated on the basis of the data of pressure reduction and gascomposition.5,6,21 The kinetic parameters can then becalculated on the basis of the results at different temperatures.For the oxidation reaction, the calculated activation energy E is146 kJ/mol and the pre-exponential factor k is 1.35 × 1015 mol/h. For the decarboxylation reaction, the activation energy is 171kJ/mol and the pre-exponential factor k is 6.25 × 1017 mol/h.Therefore, for the oxidation reaction, the O2 consumption rateis as follow:

= ×n

t

d

d1.35 10 e TO 15 17560.7/2

(7)

The diagram of the reaction rate is plotted in Figure 8a basedon eq 7 for the oil sample tested. Obviously, when thetemperature is under 160 °C, the reaction rate is very low;therefore, a significant thermal effect could not be expected.However, when the temperature is over 160 °C, the reactionrate increased rapidly, which is the so-called “acceleratingstage”. The temperature for the start of the “accelerating stage”is mainly dependent upon the values of the activation energy,which varies with different oils and oil components. Figure 8bshows the reaction rates plotted with three different values ofthe activation energy (E = 130, 146, and 160 kJ/mol). It clearlyindicates the effect of the values of the activation energy on thereaction rate and the start temperature for the “acceleratingrange”.

6. RESERVOIR SIMULATION OF THE LTO PROCESSA three-dimensional oil reservoir model was built to study theair injection LTO process, which is 150 m long, 150 m wide,and 5 m thick, with a porosity of 0.3, oil saturation of 0.7, andwater saturation of 0.3. The injection and production wellswere set at the diagonal position, and an air injection rate of6000 m3/day was used. The two-step reaction model and itskinetic parameters calculated in section 5 were used in thesimulation. The heat loss to the upper and under layer of thereservoir was considered accordingly in the simulator. Thereservoir simulation results showed that, after air injection at16.2 MPa and reservoir temperature at 70 °C, there occurred atemperature rise around 40−50 °C at the reaction zone andoxygen can be effectively consumed. In comparison to N2injection (no thermal effect), 2−3% OOIP incremental oil canbe produced in the air injection process, which can beattributed to the thermal effect of the LTO reaction. Thethermal effect of the air injection process can vaporize somelight oil compounds into the gas phase to generate a flue gasflooding (consisting of N2, CO2, and light hydrocarbons). Ahigh-temperature zone and a thermal flooding front can beformed behind the gas flooding front. A higher temperature canfurther reduce oil viscosity and make the oil swell, which canoffer an additional effect on IOR of the air injection LTOprocess.Therefore, in comparison to simple N2 or flue gas flooding,

the thermal effect on the air injection process can have thefollowing benefits and advantages: (1) It can make oil swell and

Figure 6. Simplified heat loss model in the LTO experiment.

Figure 7. Heat-transfer rate model.

Table 3. Calculated Results for the Potential TemperatureRise Inside the Reactor at the Assumed Heat TransmissionConditions

experimentaltemperature (°C)

heat-generatingrate (W)

potential temperature rise insidethe reactor (°C)

140 0.0558 0.007150 0.224 0.03160 0.495 0.07170 1.0225 2.9



become less viscous and cause light oil components toevaporate, leading to the increase of movable oil and thedecrease of residual oil. (2) A higher temperature can promotea further decarboxylation reaction, which generates more CO2,enhancing the oil displacement effect of flue gas.

7. CONCLUSION

A simplified two-step reaction model is proposed, including anoxidation reaction (exothermic), consuming oxygen, and adecarboxylation reaction (endothermic), producing carbondioxide. The kinetic parameters (activation energy E and pre-exponential factor k) have been obtained on the basis of theexperimental results.The LTO experimental results and reaction model indicated

that the whole LTO reaction scheme is an exothermal process,while under the laboratory conditions (using isothermal andARC facilities), no temperature rise could be observed becauseof the low reaction rate and heat loss during the reactions atlow temperatures (<160 °C). The experimental results haveconfirmed that a significant temperature rise occurred at hightemperatures (170 °C) because of the thermal effect of theLTO reaction. Model calculation results showed that theremight be a temperature rise of over 100 °C in the reactor ifthere was no heat loss.An air injection reservoir simulation model was established

on the basis of the kinetics model of the LTO reactions tostudy the gas flooding process and its thermal effect at realreservoir conditions. The simulation results indicate thatoxygen can be consumed in the LTO reaction at the originalreservoir temperature after air is injected and that there wouldbe a temperature rise up to 40−50 °C in the reaction zone,which is beneficial to the oil production.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors thank the China Petroleum and ChemicalCorporation (SINOPEC) and China National PetroleumCorporation (CNPC) for the financial support of this researchand allowing this paper to be published. The assistance of Yang

Zhang, Bailian Chen, and Jianhua Ren in experimental work isalso greatly appreciated.

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Figure 8. Diagrams of the oxidation reaction rate with different values of the activation energy.



mailto:[email protected]

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High-Pressure Air Injection for Improved Oil Recovery: Low-Temperature Oxidation Models and Thermal Effect