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International Biodeterioration & Biodegradation 97 (2015) 188e194

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International Biodeterioration & Biodegradation

journal homepage: www.elsevier .com/locate/ ibiod

Review

Progress in pilot testing of microbial-enhanced oil recovery in theDaqing oilfield of north China

Jian-Jun Le*, Xiao-Lin Wu, Rui Wang, Ji-Yuan Zhang, Lu-Lu Bai, Zhao-Wei HouExploration and Development Research Institute of Daqing Oilfield Company Ltd., Daqing 163712, China

a r t i c l e i n f o

Article history:Received 31 December 2013Received in revised form24 October 2014Accepted 24 October 2014Available online

Keywords:Daqing oilfieldMicrobial huff-and-puffMicrobial floodingMicrobial profile modificationPilot test

* Corresponding author. Tel.: þ86 0459 5508342.E-mail address: lejj@petrochina.com.cn (J.-J. Le).

http://dx.doi.org/10.1016/j.ibiod.2014.10.0140964-8305/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

Microbial-enhanced oil recovery (MEOR) is a biotechnology that utilizes biogas, biosurfactants, andbiopolymers produced by, and degradation by subsurface fermentation process of microorganisms forenhancing oil production from substratum reservoir. In this report, we describe the progress made inpilot testing of MEOR in the Daqing oilfield of north China in recent decades and analyze suitablereservoir conditions and application requirements. By the end of 2012, cumulative oil incrementsreached 1.2 � 105 t using MEOR in the Daqing oilfield; of the oil produced, 518 wells of single-wellmicrobial huff-and-puff yielded a cumulative incremental oil production of 6.3 � 104 t, and 10 pro-jects with 45 well patterns adopted microbial flooding and profile modification to achieve a cumulativeincremental oil production of 5.7 � 104 t. Thus, MEOR has proven to be an effective technique for routineoperations in enhanced oil production in oil fields.

© 2014 Elsevier Ltd. All rights reserved.

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Microbial huff-and-puff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Basic information about field tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Application characteristics of microbial huff-and-puff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Microbial flooding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Basic information about field tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Application characteristics of microbial flooding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Existing problems and research directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Existing problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Research directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Introduction

Microbial-enhanced oil recovery (MEOR) is a new promisingtertiary recovery process after water flooding that can be used toenhance oil extraction from subsurface strata. Research on MEORhas been carried out in some major Chinese oil fields previously(Guo, 2011;Wang, 2012a) and promising results had been observed

and reported in recent years (Huang, 2011; Wang, 2012b). Pre-liminary studies on MEOR were initially performed in the Daqingoilfield in the mid-1960s for assessment of flooded layers toimprove yield (Chen et al., 1980). Subsequently, the first fieldtesting of single-well microbial huff-and-puff technology was car-ried out in the late 1980s; these data confirmed that increased oilproduction could be achieved by injection of microorganisms in anattempt to promote microbial subsurface fermentation and alter-ation of physiochemical condition in favor of oil mobility (Zhanget al., 1995).

J.-J. Le et al. / International Biodeterioration & Biodegradation 97 (2015) 188e194 189

Additionally, later studies have shown that microbes cantransform hydrocarbons from the indigenous chemical forms tofatty acids and carbohydrates, enhancing oil production andimproving the test results (Sui et al., 2001). Microbial processes areresponsible for biochemical transformation of oil hydrocarbons areknown and have other potential in conversion of immobilized oilsto smaller molecular weight molecular and methane (Mbadingaet al., 2012). Further investigations on production water from oilfields also support the view that microbial community in the pro-duction system are rich and diverse with capabilities of methano-genesis (Mbadinga et al., 2012; Wang et al., 2012a,b, 2013; Zhouet al., 2012, 2013). Within the last decade, field trials of microbialhuff-and-puff and microbial flooding technologies have been per-formed, and the characteristics and laws of MEOR have beenanalyzed, improving our understanding of some key techniquesand providing an experimental and theoretical basis for futurelarge-scale field applications.

Microbial huff-and-puff

Microbial huff-and-puff technology involves injecting microbesand nutrient solutions after culturing in reactor into productionwells, closing off the wells for a period of time, and then continuingto produce oil. The aim of the microbial huff-and-puff method is tofacilitate the growth and reproduction of microbes in the reservoirenvironment, leading to degradation of paraffin and recalcitrantmolecular components via microbial metabolism, fermentation,sulfate-reducing and methanogenesis. Microbial metabolic pro-cesses can produce organic acids, organic solvent molecules, andbiosurfactants, which can reduce the viscosity of reservoir oil andoilewater interfacial tension, thereby increasing the solubility andalso mobility of a fraction of the oil. As a result, oil production nearwellbore zones can be increased and further extension of suchbiologically influenced area/zone can be visualized with increasingtime of incubation and proper management of the productionprocess. This technology has been widely used in China in partic-ular and abroad, and it was recently applied to field tests in theDaqing oilfield.

Basic information about field tests

According to statistical analysis (Table 1), field tests of microbialhuff-and-puff for 518 wells allowed for a cumulative incremental

Table 1Statistics of single-well microbial huff-and-puff test wells in Daqing oilfield.

Applied blocks Well number(well)

Placanticline Lamadian transition zone 4

Sabei transition zone 12Sanan transition zone 69

Sazhong transition zone 30

Xingnan ultra-low permeability reservoir 25

Pubei fault block 67

Peripheral Songfangtun low permeability reservoir 16

Low permeability þ heavy oil reservoir 84Chaoyanggou ultra-low permeability reservoir 123

Toutai ultra-low permeability reservoir 88

Total 518

oil production of 6.3 � 104 t in the Daqing oilfield from 1998 to2012, including 2.0 � 104 t of incremental oil from 140 wells in theplacanticline transition zone, with an average of single-wellincrement of 141.2 t (Zhang et al., 1999; Hou et al., 2002; Le et al.,2005; Zhong and Ma (2007); Liu, 2008; Bai, 2012), and3.6 � 104 t of incremental oil production from 378 wells in pe-ripheral low-permeability oil fields, with 115.4 t of average single-well increments (Xing et al., 2000; Han et al., 2001; Li et al., 2003;Le et al., 2004; Zhao, 2005; Xiao, 2006; An, 2010). Testing wells formicrobial huff-and-puff were successful in more than 70% of casesand were effective for a period of 3e20 months, with aninputeoutput ratio higher than 1:5.

Application characteristics of microbial huff-and-puff

In comparison with bacterial population before microbial huff-and-puff, the injected bacteria were able to adapt to the subsur-face environment. Assessed by enumeration, average bacteriapopulation decreased from 3.0 � 106 cells/mL to 1.3 � 105 cells/mLafter 7 days of closing off the wells. The quantities furtherdecreased to 6.2�103 cells/mL after 15e30 days and 2.0�102 cells/mL after 60 days.

Oil properties were significantly altered before and after mi-crobial huff-and-puff treatment. Specifically, the average viscosityof crude oil decreased by 30.6%, the setting point decreased by3e8 �C, and the wax content decreased by 7.1%. Changes in terms ofoilewater interfacial tension, organic acid content, and pH reflectedthe effects of subsurface microbial activity and fermentation takingplace and their contribution to the changes in physiochemicalproperties of the oil.

Additionally, tubing pressure, pump efficiency, and submer-gence depth were shown to be significantly improved by microbialhuff-and-puff. In the Pubei oilfield, microbial huff-and-puff wastested in 10 wells; the average tubing pressure increased from 0.35to 0.39 MPa, the average pump efficiency increased from 33.89% to46.96%, and the average submerge depth increased from 413.12 to551.62 m. Hot wash cycles of test wells were extended, therebyhaving an energy-saving effect positively on the production system.

The successful effects of microbial huff-and-puff were alsorelated to geological conditions and production conditions of thetesting areas. The average success rate was above 70%, and theperiod of effectiveness in operation ranged from 2 to 20 monthsamong 518 wells, as shown in Fig. 1. The differences in oil

Incremental oilproduction (t)

Effectiverate (%)

Validity period(mon)

Reference

463 4e8 Hou et al. (2002)Zhang et al. (1999)

1642 3e7 Lin and Chen (2003)10,124 73.5 3e18 Liu (2008)

Le et al. (2005)4500 3e10 Zhong and Ma (2007)

Bai (2012)3032 72.0 4e12 Gai et al. (2011)

Wu et al. (2008)9240.8 73.3 6e16 An (2010)

Sun (2008)

1479 75.0 6e20 Ding (2004)Zhao (2005)

15,441.4 12 Xing et al. (2000)13,437.4 71.7 8e12 Li et al. (2002)

Ma et al. (2004)4026.3 61.5 3e8 Han et al. (2001)

63,385.9 70.2

Fig. 1. a. The production performance curve of well 7P-1. b. The production performance curve of well 7P-2. c. The production performance curve of well 7P-3. d. The productionperformance curve of well 7P-4.

J.-J. Le et al. / International Biodeterioration & Biodegradation 97 (2015) 188e194190

increments were obvious among different blocks, indicating somespecificity to the geological formation condition for the MEOR. Oilincrements in the ultra-low-permeability oilfield, low-permeabilityoilfield, and placanticline transition zone were approximately40e50, 80e100, and 120e200 t, respectively. The test results werefurther improved in wells with a water content of 50e80%compared with wells with a water content of less than 10% or morethan 90%. The test results were also better in reservoirs with largeeffective thicknesses of the oil bearing layer and varying directionsof connectivity than in reservoirs with poor effective thicknessesand one-way connections.

Themajor advantages of application of enhanced oil recovery canbe complementary to the existing facilities and the process forimplementationof themicrobial huff-and-puffmethod is simple anddoes not require themobility of pipelines or buildingof newabove orbelow-surface facilities. Bacterial strains are easy to obtained,multiply and do not damage the reservoir severely. Introduction ofmicroorganisms may have medium introduced to subsurface envi-ronment, which may have potential in promoting sulfate-reducingbacteria to grow and as a result microbial influenced corrosion maybecome evident. Moreover, oil increment production is improvedand costs are reduced in managing production reservoirs using mi-crobial huff-and-puff technologies as compared to those using othertypes of existing technologies, such as fracturing, acidizing, and otherconventional stimulation measures.

Microbial flooding

MEOR involves the use of microbes or microbial metabolites toextract the remaining oil from reservoirs. This technique has thepotential to improve the mobility of crude oil, characteristics of theoilewater interface, swept volume of oil displacement agents in thereservoirs, microscopic oil displacement efficiency, and macro-scopic sweep efficiency. In terms of applicability, MEOR can bedivided into aboveground and underground fermentation tech-nology to increase the biomass and microbial activity. Additionally,

MEOR can be further divided into endogenous and exogenous mi-crobial technology based on the source and type of microbial straintargeted and used to enhance oil recovery from subsurface reser-voirs. The MEOR system for the Daqing oilfield involves under-ground fermentation technology inwhichmicrobial strains isolatedfrom the productionwater of reservoirs throughwater and polymerflooding (Yang et al., 2006; Huang et al., 2009). These microbialstrains can be screened for oil displacement strains that producegas, degrade crude oil, and produce biosurfactants through largescale fermentation and cultivation in reactors (Hou et al., 2006a,b;Guo et al., 2008; Shi et al., 2010, 2012). Afterward, the culturemedium can be pumped into the reservoirs with existing produc-tion wells without requirements of new infrastructure.

Basic information about field tests

According to incomplete statistics from 1998 to 2012, field testsof microbial flooding on 10 projects (45 well groups) have obtaineda cumulative incremental oil production of 5.7� 104 t in the Daqingoilfield. Oil increments from 6 projects and the average single-wellincrement in the placanticline transition zone were 2.0 � 104 and1083.8 t, respectively (Ge, 2004; Li and Yuan, 2005; Wang, 2006;Guo et al., 2006b; Sui et al., 2007; Le et al., 2012), as shown inFig. 2. Additionally, oil increments from 4 projects and the average-singlewell increment in peripheral low-permeability oil fields were3.7� 104 and 1382.6 t, respectively (Wang and Liu, 2005; Guo et al.,2007; Wang et al., 2008; Hou et al., 2008, 2011; Yan, 2011). Theseresults suggested that the effects of MEOR in peripheral low-permeability oil fields were greater than those in the placanticlinetransition zone, as shown inTable 2.Microbialfloodingwas effectivefor 0.5e3 years, and the inputeoutput ratio was higher than 1:3.

Application characteristics of microbial flooding

Among 10 projects (45 well groups) performing microbialflooding field tests, the injection parameters included the

Fig. 2. Curve of variation of oil production and water content.

J.-J. Le et al. / International Biodeterioration & Biodegradation 97 (2015) 188e194 191

following: (1) slug sizes of 0.006e0.06 PV; (2) concentrations ofbacterial agents ranging from 0.3% to 5%; (3) microbial culturemedium of each slug weighing from 10 to 200 t; (4) at least oneslug, at most 3e5 slugs; and (5) an injection cycle of 30e90 days(Sui et al., 2001; Ge, 2004; Wang and Liu, 2005; Guo et al., 2006a;Hou et al., 2008).

The effective features of microbial flooding were reported asfollows: (1) the testing wells took effect after 2e5 months; (2)production wells with good connectivity status had improved re-sults, and the effective time was related to well spacing andreservoir connectivity; (3) after injection of microbes in culturemedium, the water absorption profile changed, absorbent layers

Table 2Statistics of microbial flooding in Daqing oilfield.

Applied blocks Injection wells andproduction wells

Implementation d

Chaoyanggou-Block Chao 50 2 þ 10 2004.6e2005.9

Chaoyanggou-Block Chao 80 2 þ 12 1998.12

Chaoyanggou multi-pattern pilot test 9 þ 27 2009e2012

Lamadian-reservoir after polymer flooding 6 þ 18 2002.9e112003.5e6

Lamadian transition zone 1 þ 5 2002.1.27e9.15

Sabei transitional zone 3 þ 17 2002.9.25e12.5

Sabei-reservoir after polymer flooding 2 þ 6 2004.8

Sabei eastern transition zone 3 þ 7 2006.8e2007.12

Sanan eastern transition zone 3 þ 7 2002.9.25e10.29

Songfangtun-Block Fang 6 14 þ 35 2000.11e2002.9

Total 45 þ 144

and the thickness of the absorbent sandstone increased, and theinjection pressure decreased significantly; and (4) the water con-tent of the test wells increased slowly in contrast to other wells inthe same testing block (Wang et al., 2008; Hou et al., 2011; Yan,2011).

The characteristics of microbial underground fermentation,which stemmed from sample analyses of seven test wells in theSabei transition zone (Le et al., 2012) are summarized as fol-lowings: (1) microbes possessed the capacity to grow andreproduce in the reservoir, with bacterial quantities reachingbetween 103 and 105 in 1 mL of production water; (2) the pH ofthe production liquid decreased during the test period, with thepH of the central well decreasing from 8.38 to 7.71; (3) theinterfacial tension of the production liquid also decreased,reducing by 22.9% to reach a minimum of 24.37 mN m�1; (4) thehydrocarbon components of the crude oil (

PnC21�/

PnC22þ)

increased from 1.34 to 1.61, while C23eC42 decreased by 60.6%and C11eC23 increased by 48.31%; and (5) the viscosity of thecrude oil decreased by 21.12%, the wax content decreased by 7.4%,and the gel content decreased by 15.1% (Fig. 3). These changes inphysical parameters indicate the chemical changes taking placeafter injection of active microorganisms, their metabolites andalso possibly by their activity.

For comparison of oil increments by microbial flooding in oil-field, we observed several visible trends. First, the oil increments ofthe single-well groups for B4-3-B62 and L11-B103 in the placanti-cline transition zone were 663 and 4178 t, respectively. Addition-ally, the oil increments of several well groups in the Sazhongtransition zone (4 injection wells and 9 production wells) andSanan east transition zone (3 injection wells and 7 productionwells) were 645 and 5904 t, respectively. Second, the EOR of thesingle-well group for B4-3-B62 only increased by 0.13%; however,the EOR of the well groups in the Sanan east transition zone (3injection wells and 7 production wells) increased by 4.45%. Thereare apparent variabilities in performance due to geological varia-tions underground.

ate Bacterial dosage/concentration Incremental oilproduction/EOR

Bacterial liquid 250.4 m3, concentration:5%, 0.005 PV

1.45 � 104 t, 3%

Bacterial liquid 57 m3 (first round);bacterial liquid 97 m3 (second round)

1811.5 t

Bacterial liquid 2100 m3, culture solution2100 m3, concentration: 2%. 3 injectionperiods, 0.03 PV

16453 t

Bacterial liquid 116.5 m3 (first round),concentration: 1.19%; bacterial liquid 60 m3

(second round), concentration: 1.96%

5347 t

5 Plugs, cumulative injection 2271.98 m3,bacterial liquid 23.9 m3

2318 t, 1.76%

Bacterial liquid 26 m3, cumulative injection6000 m3, 0.006 PV

1895 t

Bacterial liquid 200 m3, culture solution100 t, 0.015 PV

1300 t

Exogenous bacteria liquid 100 m3, cumulativeinjection 9600 m3, 0.05 PV

2743.8 t, 2.7%

; 2003.6.24 Bacterial liquid 399.7 m3, concentration:2%, 0.06 PV, 2 slugs, cumulative injection18,344 m3

5904 t, 4.45%

3 Plugs, leading edge slug (concentration:8.0%), main slug (concentration: 6.0%), last slug(concentration: 8.0%), bacterial liquid 700 m3

5447 t

56,837.1

a

b

Fig. 3. a. Change of wax and gel content before and after degradation by microor-ganism in B4-90-B266. b. Change of wax and gel content before and after degradationby microorganism in B4-100-B265.

J.-J. Le et al. / International Biodeterioration & Biodegradation 97 (2015) 188e194192

Existing problems and research directions

Existing problems

Several challenges still limit the wide applicability of MEORtechnology worldwide. For example, in studies performed to date,the amount of microbial slugs in completed field tests ranged fromonly 0.006 to 0.06 PV. However, the amount of chemical flooding(ASP flooding) slugs was 0.6e0.75 PV, 10e100 times that of mi-crobial slugs. Because the amount of themicrobial injectionwas toolittle, it is difficult to form “a larger biological field” or influencedzone and therefore, MEOR appeared to have a limited role in theimprovement of oil production process. Thus, it may be difficult tomeasure qualitative changes associated with MEOR with highreproducibilities.

Additionally, the existing bacterial strains selected stem fromartificial selection and in favor of domesticated strains, and thereare limited selections of bacterial strains available for such use infield tests because more than 95% of the natural population ofmicrobial community in oil reservoir system cannot be cultured onartificial medium used inmicrobiology laboratories. In addition, theculture media available are mostly for fast growing microorgan-isms, not slow growing ones, which are the major communitymembers in subsurface environment. It is difficult to develop

unified technical specifications and product quality standards dueto the lack of inspection methods and varying properties of bac-terial agents. This also creates great variabilities in the market priceof bacterial agents, making it difficult to predict costs and effec-tiveness in production.

Another key reason that the MEOR field tests did not perform sowell as expected was due to the properties of microbial strains, pop-ulation composition of the microbes, and quality of bacterial cellsfromproductionprocess. In field tests ofmicrobialflooding, althoughthe evaluations of viscosity reduction rate, wax reduction rate, gelreduction rate, interfacial tension reduction, and other indicatorswere in linewith requirements, freezingpoint reduction showedpoorperformance,whichaffected theefficiencyof the liftingcapabilityandthe improvement of crude oil flow ability to a certain extent.

From the field test performed to date, injection of selectivebacteria and nutrients into the underground reservoir leading tochanges in the community structure of different bacterial groups(i.e., cultivable and uncultivable bacteria), increases in the numberand abundance of some cultivable bacteria with function of oil re-covery, and changes in underground oil, gas, and water properties.A minimum consortium composition of microorganisms for effec-tive oil recovery is not known adequately from the informationavailable. It is a much greater challenge to culture a group of mi-croorganisms in a single reactor to achieve the appropriate pro-portion that is required for effective augmentation of subsurface oilmobility and degradation. However, owing to the constraints of thebiological process and predictivity, data analysis (i.e., continuityand integrity) needs to be further validated thoroughly to bemeaningful for production use.

For the Daqing oilfield, the temperature ranged from 45 �C to89 �C, the salinity was less than 1.5 � 104 mg/L, and the perme-ability, trace elements, pH, and other parameters corresponded tothe microbial reservoir screening standards proposed by the U.S.Department of Energy (Yang and Lou, 1997). These conditionsindicated that the Daqing oilfield was suitable for developingMEORtechnology. However, the lack of long-term planning and the poorinvestment in scientific research on basic information of microbialcommunity, composition and factors affecting their changes andapplications all have restricted the development of MEOR tech-nology in the Daqing oilfield for large scale implementation.

Research directions

Currently, MEOR has been shown to increase oil recovery byapproximately 3%; however, laboratory tests predict a 10% increasein oil recovery. The differences due to heterogeneity and variationsare fully recognized. Therefore, further studies on microbial com-munity, physiology and biochemistry are needed to reach the fullpotential of this technology. Unfortunately, the complexity of theMEOR system is much greater than that of chemical flooding, whichis currently themain contributor to the lack of further technologicaldevelopment (Wang, 2012a,b). Microbial augmentation can have aplace I the oil recovery because new innovation technology alsotargets the biological methanogenesis to yield methane fromdegradation of the residual alkanes and aromatics (Mbadinga et al.,2012; Zhou et al., 2012; Wang et al., 2012a,b).

The results of field tests collectively showed that EOR couldsignificantly increase by 4.45% when the amount of bacterial agentswere greater than 0.05 PV, with a concentration of 2%. These con-ditions allowed the bacteria to have a competitive advantage andestablish a “biological field” or ‘hot zone’ in the reservoir. For futurestudies, the amount of bacterial culture and nutritional agentsshould be optimized, and injection methods should include mul-tiple cycles to improve the full potential of MEOR in productionoperation.

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Research on MEOR and other technologies should be strength-ened in more field tests with different subsurface environmentalcondition to gather basic information on the compatibility betweengeological formation and the microbial culture used. Bacterialagents can easily penetrate oil layers with high porosity andpermeability due to long-term water flood development andreservoir heterogeneity, which are both characteristics of theDaqing oilfield. Therefore, the combination of chemical profilemodification and microbial flooding should be carried out tofurther improve MEOR (Chen et al., 2005).

Over the past decade, microbial huff-and-puff, microbialflooding, and microbial profile modification have been developedin the Daqing oilfield, and cumulative oil increments have reachedmore than 1.2 � 105 t using these methods. Among these tech-nologies, microbial huff-and-puff is a mature technology that canbe widely applied. Additionally, the scale of microbial flooding hasgradually been expanded. The success of these technologies isleading to further development of EOR technology not only inDaqing of north China and also elsewhere. During the “12th five-year plan” of China, the size of microbial huff-and-puff and mi-crobial flooding experiments will further increase to meet thechallenge of recover oil from an exhausted reservoir system aftermore than 50e60 years of production. Undoubtedly, MEOR will bean effective addition of technology to the late period of oilfielddevelopment in the future.

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