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SPE 105785-PP

Utilizing the Effect of Nitrogen to Implement Light Oil Air Injection in

Malaysian Oil FieldsZeeshan Mohiuddin, SPE, D.M Anwar Raja, SPE, Ismail Mohd Saaid, SPE, Universiti Teknologi

PETRONAS

Copyright 2007, Society of Petroleum Engineers

This paper was prepared for presentation at the 15th SPE Middle East Oil & Gas Show and Conference held in Bahrain International Exhibition Centre, Kingdom of Bahrain, 1114March 2007.

This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of thepaper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the au thor(s). The material, as presented, does notnecessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by EditorialCommittees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent ofthe Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O.Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract

Air injection has been used as one of the enhanced oil recovery (EOR) techniques especially to extract the heavy oil.

Nevertheless, a number of applications of in-situ combustion (ISC) for light oil reservoirs have been also reported

successful. In light oil reservoirs, the thermal method of EOR is known as light oil air injection (LOAI). This paper

discusses the research work carried out to find the potentials of LOAI in Malaysian light oil reservoirs. Screening

criteria has been developed from which Dulang E12-14 reservoirs were short listed for further study. In research

study, combustion was assumed to occur in low temperature oxidation (LTO) to eliminate the complexities of

combustion process. In simulation studies, effect of nitrogen was utilized in place of air, as nitrogen has various

common properties with air due to the higher percentage in it. By this assumption, the early potentials of LOAI can

be identified without performing experimental studies. This method could provide an easy alternative to assess the

potential application of the LOAI technique for depleting Malaysian light oil reservoirs. Experimental studies may

need to be carried out if simulation results show the significant amount of incremental oil.

Results of the simulation work seemed to suggest that the LOAI could significantly increase the oil recovery factor

from the depleting Malaysian light-oil reservoirs. The EOR screening software PRIze also reflected this trend.

Besides this incremental recovery, the volume of produced gas also increased.

Introduction

After natural depletion and water flooding, Enhanced Oil Recovery (EOR) techniques are

implemented to recover the remaining oil from a reservoir. These EOR techniques include thermal

methods, miscible gas injection, immiscible gas injection, chemical flooding, polymer flooding and microbial

flooding. The contribution from thermal methods is nearly 41% of all EOR techniques [1]. Unlike other EOR

methods that require special and possibly non-available injecting fluids, the thermal method (e.g. air injection and

insitu combustion) utilizes the readily available air. In the past, EOR using thermal methods had been commerciallyapplied to non carbonate heavy-oil reservoirs [2]. Main oil recovery mechanism of thermal EOR methods is reduction

of oil viscosity through in-situ generation of heat and steam. Presently, the scope of thermal methods has extended

from heavy-oil to light-oil reservoirs. This thermal method in light-oil reservoir is known as Light Oil Air Injection

(LOAI). The LOAI contributes not only to the viscosity reduction as in ISC, it also provides additional driving

mechanisms [2]. In the LOAI, air is injected into the deep, hot and high pressure reservoir where combustion occurs

which consumes about 5-10% of the residual oil. The resulting flue gas, which is primarily nitrogen and carbon

oxides provides the mobilizing force to the downstream reaction region, sweeping it to production wells. Crude oil

combustion is made-up of two types of reactions: Low Temperature Oxidation (LTO) and High Temperature

Oxidation (HTO) [2]. In light oils, LTO extends from the ignition temperature to approximately 150C and then

followed by HTO until the temperature reaches 300C [3]. Several studies reported that LTO in light-oil reservoir at

temperature of 80 - 130 oC [2, 4] can be sufficient to consume all oxygen in the injecting air. This temperature range

in LTO is considered safe whereby thermal effects are no longer significant [2, 5].


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The purpose of the present study is to determine technically, the success level of LOAI in Malaysia by utilizing

optimal resources. The research work involved screening of potential reservoirs for the LOAI project and black oil

simulation of identified reservoirs. Two assumptions were made which were LTO-combustion mode and nitrogen

injection in place of air. These assumptions were used to avoid complex thermal studies and to identify the early

potential of the LOAI method.

LTO Air Injection Vs Nitrogen Injection

Air is a mixture of gases, in which nitrogen varies from 78% to 80 % of its composition. Physical properties of

nitrogen and air are almost similar. Table 1 compares physical properties of air and nitrogen at atmospheric

conditions.

|Table 1: Comparison of physical properties of |

|air and N2. (Source : Fluidprops1.1 software of|

|Bhavya-Tech) |

|Property |Air |Nitrogen |

|Molecular Weight |28.9625 |28.0134 |

|Boiling Point (oF) |-317.8 |-320 |

|Freezing Point oF |-353.1 |-346 |

|Critical Pressure |534 |478 ||psi | | |

|Critical Temperature|-221.3 |-232.5 |

|oF | | |

|Critical Volume |0.0517 |0.051 |

|ft3/lbm | | |

|Acentric Factor |-0.00187 |0.0372 |

|Specific Gravity |1 |0.9672 |

Table 2 shows the effect of varying pressure on the compressibility factor of air and nitrogen at a fixed temperature

of 302 oF (approximate temperature after combustion in LTO range). It also shows that the difference of

compressibility factors between air and nitrogen is almost negligible.

|Table 2: Comparison of compressibility factor of |

|air andN2 at 302 oF (Source : Fluidprops1.1 |

|software of Bhavya-Tech) |

|Pressure (psi) |Compressibility |Compressibility |

| |Factor (Air) |Factor |

| | |(Nitrogen) |

|0 |1.000054 |1.000194 |

|200 |1.001016 |1.003058 |

|400 |1.002406 |1.006315 |

|600 |1.004203 |1.009945 |

|800 |1.006391 |1.013926 |

|1000 |1.008949 |1.018237 |

|1200 |1.011862 |1.022858 |

|1400 |1.01511 |1.027772 |

|1600 |1.018677 |1.03296 |

|1800 |1.022546 |1.038405 ||2000 |1.026701 |1.044091 |

Sakthikumar performed a series of tests in his studies for air injection [5]. One of the tests consisted of 2 nitrogen

injection cases and one air injection case using sandstone cores saturated with stock tank oil. Results from the

experiment showed that difference of approximately 3% to 4% oil recovery was obtained when nitrogen is injected in

place of air in consolidated porous medium with light oil under reservoir conditions [5]. Since nitrogen is a majorcomponent in air and physical properties of air are close to that of nitrogen, the detailed simulation study in the

present research work was based on nitrogen as an injectant.

Reservoir Screening

In the present study, identification of suitable Malaysian reservoirs for the LOAI project was done with the help of

screening criteria, which was developed by evaluating successful LOAI projects around the world and consulting

industry experts [6]. Table 3 summarizes the basic requirement of the candidate reservoir for the process of LOAI.


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Based on the availability of the data of 22 Malaysian light oil reservoirs, E12-14 reservoirs of Dulang field were

selected for further simulation study.

|Table 3: Screening Criteria of LOAI to Select |

|Candidate Reservoir |

|Required Parameter |Criteria developed |

|Current Reservoir Pressure |Moderate (1200-2500 ||(P) |psi) |

|Current reservoir |Higher than 100 oC |

|Temperature (T) | |

|Current Oil Saturation (So)|Minimum 30% |

|Pay thickness (t) |Not less than 8m |

|Formation Depth (d) |Minimum 200 m |

|Porosity (?) |Minimum 20% |

|Vertical Permeability |Maximum 0.4 |

|/Horizontal Permeability | |

|(kv/kh) | |

|Oil Gravity |Minimum 30o API |

|Current Water Saturation |Maximum 60% |

|(So) | |

|Homogeneity |Preferred |

Production History of Dulang E12-14 ReservoirsDulang E12-14 reservoirs lie in S3 fault block. This block was developed with a total of 6 wells which includes two

down-dip water injectors. The first production from the block started in year 1991. The initial depletion strategy for

Dulang S3 block was by natural depletion. Subsequently, falling reservoir pressures coupled with decreasing

production rates had led to the implementation of a peripheral water injection scheme through down-dip wells in year

1996. Feasibility studies in year 2002 identified re-injection of the produced gas as an EOR option [7]. Pilot project of

Water Alternating Gas (WAG) was initiated in year 2002 with an attempt to improve recovery from the E12/13 and

E14 reservoirs in S3 fault block and also, to evaluate its suitability as future EOR process for the rest of Dulang field.

Miscibility Studies

In Dulang E12-14 reservoirs, miscibility is difficult to achieve for carbon dioxide gas injection [8]. Nitrogen

miscibility pressure in oil is higher than carbon dioxide [9, 10]. In addition, for a mixture of nitrogen and carbon

dioxide, miscibility is difficult to achieve if the percentage of carbon dioxide is less than 30% [9].Therefore, the

present study considered an immiscible effect. This is in agreement with a reported study which suggests that

immiscible gas displacement needs to be evaluated when miscibility with nitrogen could not be achieved [9]. It

represents the application of air injection as an immiscible gas displacement EOR method.

Verification of Screening Criteria

The selection of Dulang E12-14 as candidate reservoirs for the LOAI was further verified using PRIze, an EOR

screening software by Alberta Research Council. The single cell model in PRIze was developed by assigning detailed

information of required parameters of selected E12/13 and E14 reservoirs. Two different modes were tested in

prediction studies. These two modes were immiscible nitrogen gas injection with and without water effect of WAG.


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Figure 1: Results obtained using PRIze for Immiscible N2 gas injection with and without WAG

Figure 1 shows that nitrogen injection with WAG could produce oil recovery of almost 18% OOIP, in contrast to the

oil recovery of almost 8% OOIP without WAG. Figure 1 also shows that immiscible nitrogen injection without gas

injection cannot be continued after 3 years of production. This is because of early breakthrough of nitrogen due to its

high mobility ratio. However, nitrogen injection with WAG produces good sweep, where it could last for almost 17

years. Results from this screening test showed that the water for pressure maintenance achieved a better oil recovery.

Sensitivity Analysis

WAG is an ongoing EOR method in Dulang E12-14 reservoirs in which produced hydrocarbon gas is reinjected into

the reservoir with alternate water injection. Air is lighter than hydrocarbon gas due to the high percentage of N 2 in it.

Sensitivity study was carried out to find out the effect of density variation of the injecting gas in the reservoirs. In this

study the history matched model of 2003 was used. This model was built on a regular 25m x 25m grid oriented in aNorth South direction. A total of 24 layers used to model the E12/13 and E14 reservoirs. However in the sensitivity

analysis, the number of layers in z direction was increased from 24 to 72. Figure 2 represents the comparison of

model with 72 and 24 layers.

Figure 2: Side view of the model - with and without refinement.

These increased number of layers helped in finding the effect of different production parameters (e.g. oil production,

GOR etc), due to density variation of the injected fluid at fine scale. Extrapolation of history matched model with

WAG was carried out from year 2003 to 2006. Two different cases were then simulated, restarting the extrapolated

history matched model in year 2006. Both cases were configured to inject 4000 Mscf/day of gas. In the first case,injection of gas with hydrocarbon gas density (0.0815 lb/ ft3) was started in 2006 which was continued to 2020. In the

second case, injection of gas with nitrogen gas density (0.07907 lb/ft3) was started in 2006 which was continued to


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2020. Figures 3, 4 and 5 shows the simulation results. Results comparison of these two cases clearly indicates that

injection of gas with reduced density produced very little effect on oil, water and gas production.

Figure 3: Comparison of Oil production using hydrocarbon and N2 gas density.

Figure 4: Comparison of Water production using hydrocarbon and N2 gas density.


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Figure 5: Comparison of Gas Oil Ratio using hydrocarbon and N2 gas density.

Simulation Studies

Figure 6 shows the top view of the simulation model of Dulang E12-14 reservoirs. The Dulang S3 fault block was

developed with a total of 7 wells. It includes two down-dip water injectors i.e. A29L and A31L, two gas injectors 14L

and 10L and three up dip producer wells 5S, 16S and 2L. In the detailed simulation studies, black oil simulation was

for the following reasons:

1. Nitrogen effect is utilized in place of air.

2. Immiscible effect of combustion mixture (CO2, N2) is considered.

3. Non sensitive behavior.

4. Thermal effects are not considered due to LTO.

To compare the results in the simulation studies, a base case is required so that performance of the process can be

estimated. Therefore actual production scenario (i.e. application of WAG in 2002 after secondary recovery) was

chosen as the base case. In the base case, restarting of history matched model was carried out from year 2003. Effect

of WAG on different production parameters (e.g. oil production, GOR etc) was predicted till year 2020. Upper target

for the gas injection rate at surface was set 4000Mscf/day with voidage replacement fraction of 0.7. The target for the

water injection at surface was set 10000 stb/day, with no immediate control. WAG time period was set 90 days to

allow alternate cycling of water and gas.


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Figure 6: Top view of the simulation model

Optimization Studies

In optimization studies, different configurations were simulated as illustrated in Table 4. These configurations were

based on changing injection and production status. In all of these configurations, restart of WAG base case was made

from April 2006.

|Table 4: Description of Simulation Cases |

|Case |Gas Injection|Water |Production |

|No |Well(s) |Injection |well(s) |

| | |Well(s) | |

|1 |14L, 10G |31L, 29L |2L, 5S ,16S |

| |(Group inj |(Group inj |(Group prod. |

| |target = |target = |target = |

| |4000Mscf/day)|10,000stb/day|3000stb/day) |

| | |) | |

|2 |14L, 10G |31L, 29L |2L, 5S ,16S |




| | |) | |

|3 |2L, 14L, 10G |31L, 29L | 5S ,16S || | |(Group inj |(Group prod. |

| |(Group inj |target = |target = |

| |target = |10,000stb/day|3500stb/day) |

| |4000Mscf/day)|) | |

| | | | |

|4 |14L, 10G |31L, 29L | 5S ,16S |




| | |) | |

Figure 7 shows the comparison of total oil production among the simulated case 1, case 2 and case 3 against the base

case. It shows that the ultimate recovery obtained in case 1, case 2 and case 3 are very close to each other, producing

about 10.4MMstb. This cumulative amount indicates that the increment of gas injection rate or conversion of well 2L

into injection well does not produce significant effect on the oil production. Figure 8 also shows the same trend in

recovery factor (RF) of case 1, 2 and 3 having ultimate RF of 35.6%, 35.8% and 35.2% respectively. Besides this

increment in oil recovery, gas oil ratio is also increased as shown in Figure 9, which is considered to be unfavorable.

This might be due to the reservoir heterogeneities and unfavorable mobility ratio between the oil and the injected gas.

High amount of gas production is considered to be very critical in LOAI. If the produced gas contains large amount of

unburned oxygen, it would create safety problems at the surface during breakthrough [3]. Furthermore, the unburned

oxygen would create some corrosion related problems.


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Figure 7: Comparison of total oil production, obtained in simulated cases

Expected recovery obtained from case 4 is 9.6 MMstb with a RF of 33.4% as shown in Figure 7 and Figure 8

respectively. The decrement in the oil production could be due to shutting the production well 2L. Figure 9 shows that

shutting of well 2L would decrease the amount of produced gas. This might also suggest that well 2L contributed to

the significant gas production in case 1, case 2 and case 3. However, the peak GOR of 4.3 MSCF/stb is still

considerably high.

In contrast to all simulated cases, the base case has significantly low and uniform GOR of 0.5 MSCF/stb as shown inFigure 9. It could be due to the better sweep in the WAG process. The projected cumulative oil production for the

base case is about 9.4 MMstb with RF of 32.7% as shown in Figure 7 and 8 respectively. This value, however, is

smaller than that of simulated cases.

Figure 8: Comparison of recovery factor in simulated cases


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Figure 9: Comparison of GOR, obtained in simulated cases

Conclusions

1. Preliminary investigation on the potential application of LOAI for Malaysian reservoirs can be determined by

using nitrogen modeling in place of air.

2. A potential field for LOAI could be identified using a set of screening criteria.

3. Black oil simulation studies on Dulang E12-14 reservoirs show that incremental recovery could be obtained at the

cost of high GOR.

4. Presence of significant amount of unburned oxygen could create safety related problems especially during

breakthrough.

Acknowledgments

The authors acknowledge Dr. Nasir Haji Darman (Petroleum Management Unit, PETRONAS), Mr. Noorisman

Maroop (PETRONAS Carigali, Malaysia), Nor Aidil Anua (Group Research, PETRONAS), Dr. Myron I

Kuhlman(M.K.Tech. Solutions, USA),Mr. Darrell Davis (Petroleum Development Oman) and Mr. Bharath Rao(Bhavya Tech., USA) for their consultation.

Reference1. Schulte, W.M.: Challenges and Strategy for Increased Oil Recovery., paper IPTC 10146 presented at the 2005 International

Petroleum Engineering Conference, Doha, Qatar, 21-23 November 2005.

2. Ren, S.R. et al.: Air Injection LTO Process: An IOR Technique for Light-Oil Reservoirs. SPE Journal (March 2002).

3. Moore, R.G. et al.: Air InjectionBased IOR for Light Oil Reservoirs. Platform, Journal of Universiti Teknologi PETRONAS

(January 2004).

4. Giliham, T.H. et al.: Keys to Increasing Production Via Air Injection in Gulf Coast Light Oil Reservoirs. paper SPE 38848

presented at the 1997 SPE Annual Technical Conference and Exhibition, San Antonio, 5-8 October 1997.

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SPE 29806 presented at the 1995 SPE Middle East Oil Show, Behrain, 11-14 March 1995.

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Paper presented at International Conference on Environment (ICENV), Penang, Malaysia, 13-15 November 2006.

7. Neve, P : Fault block S3 WAG scheme Reservoir Simulation Modeling Report. Study was conducted for PETRONAS

Research and Scientific Services in March 2004.8. Zain, Zahidah Md. et al.: Evaluation of CO2 gas injection for major oil production fields in Malaysia experimental approach

case study: Dulang Field. paper SPE 72106 presented at the SPE Asia Pascific Improved Oil Recovery Conference, Kuala


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Lumpur, Malaysia, 8-9 October 2001.

9. Turta, A.T. et al.: Reservoir engineering aspects of light oil recovery by air injection. SPE Reservoir Evaluation &

Engineering (August 2001)

10.Yellig. W.F. et al.: Determination and prediction of CO2 minimum miscibility pressures (MMP). Journal of Petroleum

Technology (January 1980)

SI Metric Conversion Factors

ft x 3.048 E-01 = m

inches x 2.54 E-02 = m

bbl x 1.588 E-01 = m3

lbs x 4.535 E-01 = Kg

psi x 6.894 E+03 = Pa

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