An Introduction to EOR

F E AT U R E

JURUTERA, February 20068

INTRODUCTIONAfter discovery, most of oil reservoirstypically undergo primary recoveryperiod in which natural energyassociated with a reservoir is used torecover a portion of the oil. Mechanismsat the early production stage includefluid and rock expansion. In its original,undisturbed condition, the reservoirrock and fluid are under high pressure.When the initial reservoir pressurebegins to drop due to oil production,both rock and fluid expand. Fluid willexpand much more as compared to rock.This concept is known as compressibility.Energy from the rock and fluid (oil andgas) expansion will push the oil towardsthe producers.

After pressure falls below the bubblepoint pressure (pressure below which thefirst gas bubble appears from the oilsolution) due to fluid withdrawal fromthe reservoir, additional recoveryresulted from gas liberation andexpansion (secondary gas cap) can beachieved. For reservoirs connected toaquifers, water encroachment from anaquifer can both displace oil fromreservoir pore space and help moderatethe pressure decline caused by fluidwithdrawal. A typical range for primaryrecovery efficiency is 12 to 15%4 oforiginal oil in-place (OOIP).

In order to decrease the rate ofpressure decline, gas is injected into thegas cap and/or water is injected into theaquifer so that oil production could be continued. This prolonged period ofprimary recovery is called secondaryrecovery. Typical recovery factor forsecondary recovery is additional

15 20%4 over primary recovery. A schematic diagram of this nature isshown in Figure 1.

The fluid injection after thissecondary recovery is referred toEnhanced Oil Recovery (EOR), ortraditionally beingreferred to tertiaryrecovery as illustratedin Figure 2. Obviously,economics always playthe major role in go -no go decisions forEOR projects as theyinvolve substantialamount of investment.A cursory examinationwith the technicalcriteria is helpful torule out the less-likelycandidates before anyexpensive reservoirdescription and econo-mic evaluations aredone.

CONCEPTEOR refer to the processes ofproducing liquid hydrocarbon(oil) by methods other thanusing natural reservoir energyand reservoir repressurisingscheme with gas or water1. Itinvolves injection of fluidsinto a reservoir to recovermore oil. The injected fluidsinteract with the reservoirrock/oil system, creatingmore favourable conditionsto displace or push the oiltowards producer wells. Theresults of the interaction

might be in terms of oil swelling,viscosity reduction, lowering ofinterfacial tension, wettability modifi-cation, or favourable mobility ratiobetween the displacing and thedisplaced fluid.

An Introduction to Enhanced Oil Recovery .....................................................................................................................................................................................................................

By: Sdr. Arif Azhan bin Abdul Manap

ABSTRACT

The objective of this paper is to introduce the concept of Enhanced Oil Recovery or EOR to the readers.It will focus its discussion on EOR concept and various processes currently applied in the oil and gasindustry.

Figure 1: Secondary recovery by water injection

Figure 2: Oil production

Oil, Gas and Mining Technical Division

008010012-013 enhanced oil 1/24/06 5:14 PM Page 8

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To understand EOR better, it isessential to understand the concept of fluid displacement. The overalldisplacement or recovery efficiency is the product of microscopic andmacroscopic displacement efficienciesas illustrated in Figure 3.

The microscopic (pore scale) displace-ment efficiency is a measure of howwell the displacing fluid mobilisedresidual oil in pore space once the fluidcontacted the oil droplets within thereservoir rock. Pore space is the voidarea in between the reservoir rockgrains. Normally, oil and waterdroplets are found within the porespace. In parts of the reservoir, whichhave been swept, some oil remainstrapped. The trapped oil is known asresidual oil, which can range between10% to 40% in saturation, depending onpore space sizes, rock wettability andcapillary pressure.

On the contrary, the macroscopic(volumetric) displacement efficiency is ameasure of how well the displacingfluid sweep the oil-bearing parts of thereservoir, areal and vertical. The bulkvolume is huge, thus the namemacroscopic or volumetric.

The recovery efficiency can berepresented in equation form:

RE = De * As * VsWhere RE = overall recovery efficiency De = microscopic displacement

efficiencyAs = areal sweep efficiencyVs = vertical sweep efficiency

All the efficiencies are stated infractions with values between 0 to 1.Thus, the closer De, As and Vs values to1.0, the more efficient the displacementprocess. For instance:

If De = 0.9, As = 0.7 and Vs = 0.8, then RE = 0.9 * 0.7 * 0.8 = 0.5

PROCESSESEOR processes can be classified into fourmain categories:

1. Chemical EORChemical EOR or chemical flooding pro-cesses are injection of liquid chemicalsinto the reservoir to reduce interfacialtension (IFT) between oil/rock andstabilise flood front. Chemicals such assurfactants (surface active agents),alkaline and polymers are normally usedfor this purpose. Soaps and detergents

are examples of surfactants/alkaline.The chemical flooding processes, asillustrated in Figure 4, can be furthersubdivided into four systems:a) Polymer assisted/augmented water

flooding b) Alkaline-polymer water floodingc) Surfactant-polymer or micellar polymer

water floodingd) Alkaline-surfactant-polymer (ASP) water

flooding

Basically, in each system, eachchemical has its own function. Alkalinepre-conditioned the flooding bycontrolling pH of the injected water,surfactant reduces oil-rock IFT, whilepolymer improves reservoir contact andflood efficiency.

2. Miscible gas EORIn miscible gas EOR or flooding, displacing

fluids in the formof gas (hydro-carbon gases, car-bon dioxide, nitro-gen or flue gas)are injected to mixwith oil to form asingle phase. Byminimising theIFT (IFT = 0 insingle-phase fluid),residual oil satu-ration will belowered whilem i c r o s c o p i cdisplacement effi-ciency (De) ismaximised.

Figure 3: Displacement efficiencyLeft pore displacement (microscopic)Centre & right aeal and vertical displacement (macroscopic)

Figure 4: Chemical flooding using alkaline/surfactant/polymer(Source: NIPER EOR Information, 1986)

Figure 5: Miscible gas flooding (water-alternating-gas or WAG mode) (Source: NIPER EOR Information, 1986)



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The carbon dioxide gas (CO2) mis-cible process is an example. Throughmultiple contact between CO2 and oil,CO2 rich phase will reach a compositionthat is miscible with oil. However, inmost cases, CO2 is very mobile (lowviscosity) at injection condition. Oil andwater are displaced by CO2, which leadsto fingering through oil phase and poorsweep efficiency. An approach toovercome this difficulty is to inject slugsof CO2 and water alternately. Thismethod is called water-alternating-gas orWAG process (refer Figure 5). Water helpsto stabilise the flood front, reduce CO2

fingering and improve overall sweepefficiency. Other methods of mobilitycontrol are being tested, such as usingfoam and polymer with CO2.

3. Thermal EORThermal EOR processes are those inwhich heat is added into the reservoir.Additional heat will result in thermalexpansion and viscosity reduction of oil,using these techniques:a) In-situ combustion ignition of a

mixture of oxygen-rich hydrocarbongas in the reservoir.

b) Hot water / steam flooding - injection ofhot water or steam intoreservoir (Figure 6)

4. Microbial EORMicrobial EOR processes involveinjection of microbes into areservoir or injection ofnutrients to stimulate growthof indigenous microbes in thereservoir. The microbes colo-nise the reservoir rock porethroat and produce various by-products such as gas, acids,biomass and biochemical(surfactants, polymer, andsolvents) to further improvereservoir conditions for oilrecovery, as illustrated inFigure 7. This technique is stillhighly experimental comparedto the previously mentionedtechniques.

Gases such as carbondioxide, hydrogen and methane,subsequently increase thepressure and expels oil from

micro traps in oil reservoir. Biomassworks by selective plugging, which isplugging wider pore throat and helps todivert displacement fluids into oilmicrotraps, resulting in less bypass oiland lower residual oil saturation. Bio-polymer improves sweep efficiencywhile biosurfactant reduces theinterfacial tensions between oil andwater. Biosolvent thins the oil bylowering its viscosity.

CLOSUREIs EOR necessary?There are four main reasons why EOR isnecessary for mature oil fields. Unsustainable production more often

production rate is difficult to maintain as field matures.

Low reserve replenishment ratio cumulative production is more thanreserve replenishment.

Low recovery from existing fields. Smaller discoveries new

discoveries tend to be smallerthan existing fields.

EOR Application around the WorldEOR have been applied globally asshown in Table 1 and Figure 8. NorthAmerica (United States of America andCanada) have the most numbers of EORapplication and highest EORproduction. The reason is because theirhave faced declining reserve since1970s. EOR was applied as analternative method to sustain their oilproduction. Gas and thermal floodingare the most applied process there.Other countries have followed in EORapplication or R&D ever since.

Figure 7: Microbial EOR processes

Figure 8: 2002 Worldwide EOR Production(Source: OGJ APR 2002)

Figure 6: Hot water/steam flooding (Source: NIPER EOR Information, 1986)

Region/Country

U.S.A. Canada Rest ofWorld

Thermal 65 16 77 158Chemical 4 22 26

Gas (all types) 78 31 17 126Microbial 3 3

Total Projects 147 47 119 313

EOR ProjectsSurvey by

Process

WorldwideTotal

Table 1 : 2002 Worldwide EOR Survey (Source: OGJ Apr 2002) )



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JURUTERA, January 2006 13

All EOR processes have been appliedsuccessfully in other parts of the world,meaning that it is a proven technology.However, almost all of EOR projects arefor onshore fields application. InMalaysia, all our fields are locatedoffshore. There are challenges to EORapplication such as large well spacing(distance in between wells), limited spaceat surface (offshore platforms),complicated reservoir geology, andhigher EOR cost compared to landoperations. A comprehensive anddetailed feasibility study is requiredbefore embarking on EOR technology forany particular field or reservoir.

ACKNOWLEDGEMENTThe author wishes to thank Dr Nasir binHj. Darman, Principal EOR Engineer(Petroleum Management Unit,

PETRONAS), En. Jamil Jelani, SeniorManager, Petroleum Engineering Group(PRSS), Hj. Mohd. Ismail bin Omar,Principal Reservoir Engineer, (PRSS),peer members and colleagues fromPetroleum Engineering Group, andManagement of PETRONAS Research &Scientific Services and for their assistanceand support in preparing this article.

REFERENCES

[1] D.W. Green and G.P. Willhite(1998). Enhanced Oil RecoverySociety of Petroleum Engineers,Richardson, Texas.

[2] B.C. Craft and M.F. Hawkins,revised by R.E. Terry (1991)."Applied Petroleum ReservoirEngineering", 2nd edition,Prentice-Hall Inc., New Jersey.

[3] "Introduction to Oil & GasProduction", AmericanPetroleum Institute (1983),Production Department, Dallas,Texas.

[4] "Enhanced Oil Recovery",National Institute for Petroleumand Energy Research (1986),Bartlesville, Oklahoma.

[5] "2002 Worldwide EOR Survey",Oil & Gas Journal (Apr 2002),71-83.

[6] D.M. Updegraff (1990). "EarlyResearch on Microbial EnhancedOil Recovery", Development inIndustrial Microbiology, 31: 135 142.



An Introduction to EOR

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