SPE-174697-MS Progress of Microbial Enhanced Oil Recovery in ...

SPE-174697-MS Progress of Microbial Enhanced Oil Recovery in China Hu Guo and Yiqiang Li, China University of Petroleum (Beijing); Zhao Yiran, Xi’an Petroleum University; Fuyong Wang, Yansheng Wang, and Zhaoyan Yu, China University of Petroleum (Beijing); She Haicheng, Yulin University; Gu Yuanyuan, Jin Chuyi, and Gao Xian, China University of Petroleum (Beijing)

Copyright 2015, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Enhanced Oil Recovery Conference held in Kuala Lumpur, Malaysia, 11–13 August 2015. 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 the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract Compared with other EOR technique like gas flooding, chemical flooding, and thermal production in heavy oil, the prominent advantages MEOR has environment-friendliness and lowest cost. MEOR has various applications not only in sandstone but also carbonate reservoirs, light oil reservoirs as well as heavy oil reservoirs. This paper mainly reviewed progress in laboratory studies and MEOR field tests including six big successful field tests in China. Present focus on MEOR has been changed from qualitative analysis to quantitative characterization, and high-tech like 16S rDNA and advanced method has being tried to investigate its mechanism on molecular level. The mechanism of microbial effects on making oil emulsification and wettability alternation was the main interest of recent study. Application of high resolution mass spectrum (HRMS) on MEOR mechanism has revealed the change of polar compound structures before and after oil degradation by the microbial on molecular level. MEOR could be divided into indigenous microorganism and exogenous microorganism flooding. The key of exogenous microorganism flooding, was to develop effective production strains, and difficulty lies in the compatibility of microorganism, performance degradation and high cost. Indigenous microorganism flooding, has good adaptation but no follow up process on production strains development, thus it represents the main direction of MEOR. China has some of the most complex and diversified reservoirs and was notable for the scale of MEOR field tests since there has been six big MEOR field tests since 1998 after many precious small-cale tests. All field tests have shown positive results in incremental oil and water cut reduction. The combination of indigenous microorganism and exogenous microorganism flooding was adopted because of the cost and difficulty of exogenous microorganism flooding. MEOR screening criteria for reservoirs has been improved. The parameters include temperature, salinity, oil viscosity, permeability, porosity, wax content, water cut, and microorganism concentration in which production fluid, temperature, and salinity were the most important three parameters. MEOR was suitable in reservoirs of which temperature lower than 80°C, salinity less than 100,000 ppm, and permeability above 50 mD. MEOR experience and study in reservoirs of 120°C, salinity more than 350,000ppm and permeability of 10 mD has expanded the reservoirs range suitable to carry out MEOR.

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Introduction Microbial flooding was a general designation of a series of technologies to increase oil production by propagation and metabolites of microbes (He et al., 2013). In the current situation of low oil price, microbial flooding was a prospective technology for enhanced oil recovery, especially for the marginal reservoir and /or uneconomical reservoir, and the microbial flooding was a potential alternative to other EOR/ IOR methods, as it was being implementing with satisfactory results not only from technology point of view but also from having lesser environmental impact. Compared with thermal flooding and gas flooding, the preeminent advantages of microbial flooding were environmentally friendly characteristics and the lowest cost for increasing oil production (He et al., 2013; Salman Zahid and Khan 2007). Compared with other technologies of EOR, the distinct advantages of microbial flooding include the low energy consumed by microorganism, the combination of multiple mechanism and the reduced loss caused by degradation by some of the endogenous microorganism.(Salman Zahid and Khan 2007). The microbial flooding was used in a wide range, including high water cut, heavy oil(Qi Yibin, et al., 2012), marginal reservoir and post- polymer flooding reservoir (Yue, et al. 2014, Liu, et al. 2008; Guo, et al. 2006; Sun, 2014; Dong et al. a2012) and so on. It could be applied to sandstone (Bryant et al., 1994), carbonate (Zekriet al., 2003; Almehaideb et al., 2002), light oil, heavy oil (Feng et al., 2005; Qi et al., 2012; Li et al., 2014), mid/high permeability and low permeability reservoir (Guo et al., 2007; Long et al., 2013; Feng et al., 2000). Microbial flooding could be divided into indigenous microbial flooding and exogenous microbial flooding according to the source of the strains(Zahid et al., 2007). Exogenous microorganism indicated that the suitable microbes screened on the similar condition as but not in the reservoirs were injected into underground, and to increased oil production by using its propagation and metabolites. Indigenous microorganism means microbes were developed by remaining oil as the carbon source on the basis of the active matter existed in formations, and introducing the air and the inorganic salt with phosphorus source and nitrogen source when injecting water. (He et al., 2013) . Indigenous microorganism flooding was the development trend with the advantages of good adaptability and avoiding of microbes culture development and production process.(He et al, 2013) . . The three basic types of microbial processes for oil recovery were well simulation, well bore clean up, and enhanced water flooding(Zahid et al., 2007; Bryant et al., 1994). The oil recovery was mainly decided by the oil displacement efficiency and sweep oil efficiency. Microorganism could not only had double mechanism of improving the displacement efficiency and sweep oil efficiency (Salman Zahid and Khan 2007; He et al., 2013). While in the laboratory experiments, the oil recovery could be increased by 10% by microorganism on the basis of water flooding(Sui et al., 2001), and increased by 5% by using microbial flooding and by 16% by combined microbial-chemical flooding after polymer flooding (Liao et al., 2003). Microorganism could be prevented from being produced by combining the nutrition gel system and microbial flooding. In the model 1400mD, the oil recovery could be increased by 18.4% (Li et al., 2014). A lot of experiments about microbial flooding and field had been studied in China (Guo et al., 2006). On the basis of the 6 field experiments, three large scale leading experiments were in process, and significant results of reducing water cut and increasing oil had been achieved. The experience of field was not only beneficial to deep recognition of microbial flooding mechanism, but also would provide evidence for industrial application to microbial flooding (He et al., 2013).

Microbial colony The study on microbial colony in oil reservoir, especially the accurate analysis of the complex structure of microbial colony and changes monitoring, had significant importance for microbial flooding (Wang et al., 2013). The aerobic bacteria, facultative bacteria and anaerobic bacteria existed in formations. The

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aerobic zone near the water injection well mainly contained the aerobic bacteria; the deep part of reservoirs was anaerobic zone that mainly contains anaerobic bacteria (Xiang et al., 2014). Some of the microbes in oil reservoir were favorable for oil production, while some were unfavorable. The indigenous microorganism in oil reservoirs relevant to oil production was divided into six categories, hydrocarbon oxidizing bacteria, saprophyte, facultative zymocyte, methanogenic bacteria, sulfate reducing bacteria and nitrate reducing bacteria. Anaerobic zymocyte microbe was the main activated target bacteria (He et al., 2013). Detecting methods of microbes, such as T-RFLP(Yue et al., 2014; Qi et al., 2012; Wang et al., 2013), gene bank, MPN (Cheng et al., 2006) were beneficial to reveal the structure of microbial colony from microbial genera. A new way relating microbes that could not be cultured in extreme environments was provided by the molecular fingerprint technology with 16S rDNA as the main aspect(Dong et al., 2012; Chen et al., 2005). In the studies of oil recovery using indigenous microorganism, denaturing gradient gel electrophoresis method was valuable for analyzing microbial community structures and monitoring community dynamics at the molecular level (Chen et al., 2005). The analysis of microbial colony with T—RFLP technology in the leading biological experiments in Sheng li Oil Filed showed (Wang et al., 2013) the real situation of the changes of microorganism in reservoirs, and the changes of diversity of indigenous microbial colony promoted by the microbial flooding technology, and diversity of the microorganism in reservoir was negatively correlated with oil production in general. The core experiments (Song et al., 2010) showed that the different distribution of crude oil and other metabolites in the core was the key factor to influence the microbial diversity in reservoir. The study on Da Qing Oil Field (Yang et al., 2006) showed the number of microorganism in water after polymer flooding was 103-104/ml which was two orders of magnitude lower than that after water flooding. A stably ecological system of authigenous microorganism could be formed after water flooding and polymer flooding. The field experiments also proved that (Chen et al., 2005; Xiang et al., 2004) the substrates could be provided with the metabolites of aerobic organism in the formations after being activated by the oxygen.

Exogenous microorganism and indigenous microorganism Microbial flooding could be divided into indigenous microbial flooding and exogenous microbial flooding according to the source of the strains(Zahid et al., 2007). Exogenous microorganism indicated that the suitable microbes screened on the similar condition as but not in the reservoirs were injected into underground, and to increase oil production by using its propagation and metabolites. Indigenous microorganism meaned microbes were developed by remaining oil as the carbon source on the basis of the active matter existed in formations, and introducing the air and the inorganic salt with phosphorus source and nitrogen source when injecting water. (He et al., 2013) . Indigenous microorganism flooding was the development trend with the advantages of good adaptability and avoiding of microbes culture development and production process.(He et al.,2013). In recent years, the studies in China were all about indigenous microorganism (Xiang et al., 2004; Xiu et al., 2010; Yang et al., 2006; Zhu et al., 2014; Song et al., 2010; Yue et al., 2014; Liu et al., 2008) . For the duality of indigenous microorganism, many scholars worked on the microorganism which took the hydrocarbon in crude oils as the only carbon source (Chen et al., 2001; Sui et al., 2001; Jing et al., 2006), while some focused on the reservoirs affected by polymer flooding, and studied the microorganism which took polyacrylamide and hydrocarbon as the carbon source(Yue et al., 2014; Liu et al., 2008; Wu et al.,2006; Liao et al., 2003). Some microbes (Clostridium. Spp) had significant influence on polymer (Liu et al., 2008) which reduced the viscosity and molecular weight of polymer(Liu et al., 2008; Liao et al., 2008). The viscosity of polymer decreased by 52.1% in 7 days when the 7d microbes cultured without additional nutrition. After sucrose being added, the viscosity of polymer reached 92%(Liu et al., 2008; Liao et al., 2003). Microbes also had influence on the molecular of polymer, which hydrolyze amide water into carboxylic

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acid(Liu et al., 2008; Liao et al., 2003). NMR showed that the polymer amide group decreased from 74.6% to 60.8%, while carboxylic group had an evident increase.

Oil degradation by microorganism Microorganism could be used for oil degradation. In macroscopic view, the concept of oil degradation represented the decrease of viscosity. In microscopic view, the change of oil composition and the change of crude oil composition often determined by gas chromatography (He et al., 2013). The laboratory experiments in Da Qing Oil Field showed (Sui et al., 2001), the content of long chain hydrocarbon in crude oil relatively decreased after microbial effect, and short and medium chain hydrocarbon relatively increased, which leaded to the light component of crude oil increased more than 30%. Organic acid was produced to reduce the PH Value from7 to 6-5.5, and active material was produced to decrease the viscosity (by more than 36%)and interfacial tension(from 35.67mN/m to 35.67mN/m). The further study showed (Liao et al., 2003) after the microbial effect on Da Qing Oil Field, the content of saturated hydrocarbon and acid value relatively increased, while aromatic, nonhydrocarbon and asphalt relatively decreased. The composition of crude oil changed obviously and the wax and gum chicle content respectively decreased by 48% and 9.68%. The average acid value of Da Qing Oil Field was 10 times higher after the effect of suitable microbes.(Wu et al., 2006). Application of high resolution mass spectrum(HRMS) on MEOR mechanism has revealed the change of polar compound structures before and after oil degradation by the microbial on molecular level(She,2010). Study shows that this degradation mainly involves polar heteroatomic compounds changing from high molecular weight compound into small compound on general, and the alkyl chains of nitrogen compounds is easy to be degraded. This degradation produces small molecular weight organic acid dissolved in water, and some of this organic acid contains nitrogen, sulfur impurity atoms and many aromatic rings. The number of organic acid even increases one or two orders of magnitude. Composition and structure of organic acid change leads to wettability alternation, and it also makes water-in-oil emulsion into oil-in-water, thus reduces the viscosity of oil, which in turn improves the flow rheological property. It accounts for the common phenomenon of emulsification in MEOR on molecular level and is considered notable progress on MEOR mechanism. Almost all the microbial laboratory and filed experiments had proved that microorganism could be used to decrease the viscosity of crude oil, but the viscosity could not be largely decreased (Chen et al., 2001). The literature showed that the viscosity and interfacial tension respectively decreased by 33.6% and 31.2% by the effect of thermophile bacteria. Two kinds of microbes were sifted and separated from Da Qing Oil Field. Only heavy hydrocarbon was degraded with oxygen by Brevibacillus brevius and Bacillus cereus, and metabolites only included low molecular weight organic acid, carbon free or low carbon saturated hydrocarbon. The viscosity and interfacial tension respectively decreased by 40% and 50% when Bacillus sp. was adopted. The viscosity could be reduced by 66% when offshore heavy oil was degraded. The degradation glial and biological emulsion could be combined to play a role after complex formulation of Pseudomonas sp, and the viscosity was decreased from 1146 to 5.11, the decreasing rate reached 99% (Qi et al., 2012). The literature also showed that after 14-hour-reaction between streptococcus (Wu et al., 2009) and crude oil, the viscosity decreased from 4000mPa.s to 500mPa.s, and the viscosity of microbes and metabolites could be decreased, but the metabolites effect played a leading role. High temperature field laboratory and field experiments (Liang et al., 2003) showed that the freezing point, viscosity of crude oil and surface tension of culture medium and PH Value decrease in different degrees after the microbial effect. The viscosity decreases by 33.6%, heavy component decreased while light component increased. The content of saturated hydrocarbon, aromatic and nonhydrocarbon decreased in different degrees after microbial effect. Different microbes brined different effects on the crude oil composition, but the saturated hydrocarbon relatively decreased, and nonhydrocarbon relatively

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increased, while the change of aromatic was not regular. As for reasons of decreased viscosity, besides the degradation glial and biological emulsion, the literature believed that with the high pressure and temperature of formations, the co2 produced by microbes was in a supercritical state, and the oil ratio was higher because of the CO2 resolved in crude oil. Dagang field experiments (Chen et al., 2001) found that wax and gel content respectively decrease 2.2% and 3.7%. The endogenous microorganism field experiments of five well groups in Da Qing Oil Field also proved that the content of saturated hydrocarbon increased from 53.53%-58.88% to 54.2961.79%, the nonhydrocarbon decreased from 5.58%-19.49% to 11.79-16.67%, which was highly consistent with the results of laboratory experiments. The degree of degradation could be measured by Pristine/C17 and Phytane/C18 which were measured parameters for microbial degradation (Liao et al., 2003). Table X showed the crude oil composition change of 12 block crude oils in North China oil field before and after MEOR (He et al., 2013). Table 1—data of crude oil composition change before and after MEOR (He et al., 2013) Oil composition Microorganism m KSH-1-1 KSH-1-2 KSH-1-3

Pristine/C17

Phytane/C18

(C21ⴙC22)/(C28ⴙC29)

before

after

before

after

before

after

0.42 0.42 0.42

0.50 0.49 0.48

0.56 0.56 0.56

0.60 0.65 0.61

1.65 1.65 1.65

1.85 1.68 1.82

Table 2—The group composition analysis of crude oil treated and untreated by microorganisms (Liao et al.,2003) Microbes name

saturated hydrocarbon,%

aromatic hydrocarbon,%

Pectin,%

Asphaltene,%

Blank oil (no microbes) gwn SHB P178 PHL MRT

61.57 67.26 63.76 78.06 72.64 69.22

17.82 16.42 16.32 13.32 15.79 17.09

17.00 12.42 11.92 8.07 10.78 10.84

3.62 0.90 3.01 0.55 0.79 0.85

Surface active substance in microbial metabolism The microbial surfactant could be metabolized. Surfactants included biosurfactants (Cheng et al., 2010; Chen et al., 2001), organic solvent (Wu et al., 2006), acids (Zhi et al., 2001)and gas (Zhi et al., 2001). Gas were mainly CO2, CH4 and a small quantity of ethane (Cheng et al., 2010). The composition of biosurfactant mainly consisted of rhamnolipid (Wang et al., 2013; Cheng et al., 2010), also the mixtured of paraffin ester and glyceride (Cheng et al., 2010) which not belonged to sugar ester but phospholipid and polyketones. The main composition of the Acid was fatty acid (Chen et al., 2001; Wu et al., 2006), acetic acid, propionic (Chen et al., 2001), butyrate (Wu et al., 2006). The mass fraction of fatty acid increased from 1% to more than 60% after the effect. The field experiments made in Da Qing Oil Field (Cheng et al., 2010) showed that the content of low fatty acid largely increased with formic acid and acetic acid increased 10 times, isobutyric acid increased 7 times. Organic solvent included alcohol, such as ethane (Wu et al., 2006). The literature(Qi et al., 2012) showed that aeruginosa could produced 6 types of rhamnolipid which could reduce oil-water interfacial tension and lead to crude oil emulsion.

Crude oil emulsion Crude oil emulsion was one of the mechanism of microbial flooding. Yet there was no satisfactory standard of capacity of microbial crude oil emulsion. At present, the simplest way to study and apply to

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microbial flooding was five levels of classification involving the direct observation of oil-water emulsion. This way was simple and practical, but the disadvantage was the lack of quantitative characterization which was on the basis of emulsion coefficient EI24 and the Tubiscan emulsion stability parameter measurement (He et al, 2013). The microscopic model flooding experiments used by the oil-water emulsion visual observation method found that crude oil was emulsified by microorganism(Sui et al., 2001). The literature (Wang et al., 2005) showed that through the micro transparent simulation models the degraded crude oil was emulsified in different degree which was in the form of oil droplets of various sizes, and the oil droplets were tensile, deformed and had seepage flows. Some experiments (Cheng et al., 2010) showed that a large number of CO2 and CH4 and other biogas without oxygen. The chromatographic gas showed that co2 accounted for 56% and ch4 accounted for 42% of biogas, the quantity of total gas was even twice to eight times higher than that of culture medium. The literature writes that the biogas produced was benefit to emulsify crude oil. Hydrocarbon bacteria (Cheng et al., 2010) was activated because of good emulsified effect of microorganism. The degree of crude oil emulsion was high correspondence to the growth rate of microorganism (Chen et al., 2001). The literature (Qi et al., 2012) showed that the emulsifying effectiveness of heavy oil could be obviously improved by the effect of complex formulation of two types of microorganism. The emulsion stability increased, and the average particle size decreased 67.3% which could evidently reduce the viscosity of heavy oil.

Change reservoir physical properties Physical properties of the reservoir could be changed by product of microbes (Salman Zahid and Khan 2007), and the porosity was likely to be increased. According to an experiment, the permeability was reduced from 284 mD to 24 mD, and the viscosity might be reduced by as large as 10 times due to the effects of acid (Salman Zahid and Khan 2007). It was found by simulation that the capillary pressure was reduced by 9%-24%, and the wettability became more water-wet(Salman Zahid and Khan 2007). The core experiment withsand filling tube model indicated that, the core permeability could be significantly reduced by 20% - 58% due to the in-situ activated endogenous microbial cells and its flowing with water phase. Two microbes were selected for cultivation from hundreds of bacteria in post polymer flooding reservoirs in Daqing oil field, and laboratory experiment indicated that the plugging rate of profile control bacteria was over 70%(Guo et al., 2006). The field test indicated that the reservoir permeability was decreased after profile control and the water injection profile was significantly improved (Guo et al., 2006). According to precious studies, the mechanism of microbial profile control could be formulated as follows: microbes produced reticular formation biofilm in porous media, microbes produced colony precipitation which was then adsorbed on other colony particles to form bridge plug, and microbial metabolism produced gas to form gas lock, while the mechanism of profile control in this research belonged to the first two. Microbes could produce surfactant through metabolism and change its wettability, and much attention had been given to its mechanism. Most researches in China focused on the surfactant produced by microbes, while quantitative characterization of wettability index after microbial mediation, contact angle and other methods was seldom used. The surfactants produced by microbes were adsorbed on the surface of porous media, the wetting state of the surface of porous media was thus changed due to the effects of amphiprotic group in surfactants (Zhu et al., 2014). Microbes could make the reservoir wettability change from oil-wet to water-wet (Salman Zahid and Khan 2007). Literature (Liu et al., 2014).

Microscopic mechanism of enhanced oil recovery Much importance was attached to the microscopic mechanism of microbes contacting with crude oil and changing the pore surface as well as crude oil properties to enhance oil recovery. Visualization experiment and microscopic simulation model (Jing et al., 2006) indicated that microorganisms consuming crude oil could migrate directionally to and contact directly with the crude oil and make the microflora consuming

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crude oil highly concentrated. The basis for microorganisms consuming crude oil to enhance oil recovery was its ability to automatically search for carbon source and directionally migrate. The bacteria concentration of crude oil (at the oil-water interface) was 5 orders of magnitude higher than that of anywhere else, and the farther from the crude oil (at the oil-water interface), the lower the concentration was. Microbes automatically migrate to crude oil, concentrate and multiply constantly. The distribution regularity of concentration of surfactant as well as the acid in its metabolites was exactly the same with that of bacteria. The automatic directional migration of microbes was attributed to its chemotaxis. The microscopic mechanism of peeling off oil film and oil droplets by microbes had also been studied (Jing et al., 2006),which could be seen in Figure 1-a,b,c. Microbes automatically searched for carbon source, migrated to crude oil, and concentrated at three-phase boundary with oil-water-pore wall. Although hydrophilic, the bacteria could be hydrophobic at one end. Due to the synergistic effect with its metabolites, the bacteria entered into the space between pore surface and oil film or oil droplets, grew and reproduced massively, entered deeply inward, and finally the oil film was peeled off. The microscopic mechanism of plugging the big channel by microbes was as follows: After injected into porous media, the bacteria liquid permeated into the big channel in priority. The bacteria automatically migrated, concentrated around crude oil, reproduced massively and produced lots of metabolites containing plenty of extracellular polysaccharide. Massive bacteria and its metabolites mixed with colloidal bitumen and other substances separated from crude oil. First the mixtures accumulated as irregular net substances and membranoid substances, and then with time passing these substances gradually formed clumps or floc, blocking or particially blocking the big channel. And finally the succeeding water entered into the small channel to drive the remaining oil. The experiment also indicated that a proper time was required for the migration and concentration of microbes and metabolites, and it would be better to adopt huff and puff or intermittent methods during field microbes injection. The experiment of two parallel connection sand-packed pipes (with respective permeability of 5.56 D and 1.84 D) (Cheng et al., 2010) indicated that the microbial system based on amylocellulose nutrient solution could effectively seal highly permeable layers. The injection pressure differential was increased by 60 times compared with that before injection.

Figure 1-a—the microbial growth and peeling-off oil film (Jing et al., 2006).

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Figure 1-b—The peeling off course of oil droplet (Jing et al., 2006).

Figure 1-c—A large amount of modular/floss stopping big orifices (Jing et al., 2006).

A microscopic photoetching simulation model (Zhu et al., 2014) was used for the study of microscopic mechanism of microbial flooding. The micro-displacement efficiency of microbes could be quantitatively studied through tests and image processing. The experiment showed that various remaining oil had been changed due to effects of microbes. These effects could be ordered ranked in a descending order as: island remaining oil, membranaceous remaining oil, columnar remaining oil, blind end remaining oil and cluster remaining oil. The experiment also found that due to the growth and metabolism of microbes attached to oil-water interface, the oil-water interfacial tension was reduced, the interface was softened, and the flowability of the remaining oil was enhanced. Due to the surfactant produced by microbial metabolism concentration of the surfactant at the interface was increased, generating surface tension gradient. Once the gradient exceeded the viscous force, spontaneous interface deformation and movement, namely Marangoni convection would appear, and together with migration behaviour of microbes and disturbance action of high pressure, positions of droplets in pores would be changed.

Microbial system optimizaiton The system of microbial flooding involved activation of material composition in the system, injection of slugs, nutrition concentration and the size of nutrition-injecting slug. Lots of researches had been conducted about the activation system. The feasibility of using corn starch as the activation system was studied in the experiment (Cheng et al., 2006). The ultimate recovery growth could be significantly influenced by the cultivation time of injecting slugs, nutrition concentration and the size of nutritioninjecting slug. It had been proved that injecting 0.4PV slugs with corn starch concentration being 10~ 20mL / L and cultivating 15 ~ 20d was the best condition. Literature (Wang et al., 2011) In view of the

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extra-low permeability reservoir in Dingbian, Changqing, the bacteria with the concentration of 10% and the slug with the size of 0.5PV were selected as the best according to the laboratory displacement experiment which could enhance the recovery by 8%. Literature (Liu et al., 2014) As to the extra-low permeability oil field in Ansai, Changqing, laboratory displacement experiment and field test had been conducted. Ansai oil field features tight sandstones of the Yanchang Formation in Triassic. The oil field was a typical extra-low permeability oil field with the averaging air permeability of 1.29 ⫻ 10⫺3 m2, effective porosity of 12.4%, average single well oil output of 2 t/d, the formation viscosity of 1. 91 mpa·S and ground viscosity of 14. 15 mPa·s.

Screening criteria Screening criteria for a reservoir varied greatly (Salman Zahid and Khan 2007). Different criteria existed due to different reservoir conditions and and research progress. When microbial flooding was conducted, the factors that must be taken into account (Salman Zahid and Khan 2007) involved remaining oil saturation, hydrocarbon compositional analysis, fluids chemistry and composition, depth of reservoir, salinity of formation water, formation water sample analysis, estimated net oil increment, economic aspects. According to laboratory research and field tests in China and with consideration of researches abroad, 8 major parameters had been selected for reservoir screening and evaluation in microbial flooding could be seen in Table 3 (He et al., 2013) Screening criteria for by SINOPEC Shengli oil field which was one of the typical and biggest oilfield in China could be seen in Table 4. Table 3—CNPC MEOR reservoir screening parameter (He et al.,2013) Parameter Formation temperature/°C Crude viscosity/ mPa.s Permeability/ D Porosity/% Brine salinity/g/L Wax conten/% Water cut/% Total bacterial concentration in produced fluid, number /ml

Value range

Optimum

20-80 10-500 ⱖ0.05 12-25 ⱖ 300 ⱖ4 40-95 ⱖ 100

30-60 30-150 ⱖ 0.15 17-25 ⱖ 100 ⱖ7 60-85 ⱖ 1000

Table 4 —MEOR reservoir screen parameter in Shengli oilfield (Sun,2014) Formation temperature/°C

Permeability/D

Formation bine/PH

Brine salinity, mg/L

Crude Viscosity on ground at 50°C

ⱖ 80

ⱖ 0.05

6~8

ⱖ150000

ⱖ3000

Temperature had direct influence on the growth of microorganism (Chen et al., 2001). There was optimum temperature for the growth of microorganism which could be largely affected when the optimum temperature was exceeded. The literature (Chen et al., 2001) also showedthat 8 facultative anaerobes could grow well under the condition of 45-60°C, while they cannot grow at temperature exceeding 75°C. The literature (Feng and Chen, 2000) showed that for the same microorganism, with the temperature increasing from 37°C to 73°C, the microorganism growth curve moved down, and the bacteria concentration reduced remarkably. The microorganism selected could be used in near well disposal at temperature lower than 101 °C and salinity lower than 300000ppm. According to the literature (Chen et al., 2001), the thermophile and optimum temperature of thermophile bacteria respectively was 60-78°C and 73°C. Microorganism migration and retention in cores at the temperature of 65°C was studied (Song et al. 2010). It was believed that reservoirs temperature below 80°C were suitable for MEOR, while if temperature exceeded 80°C, the growth rate of microorganism was slow (Sun, 2014). Pilot in Helkou-

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luo(HKL-801) whose temperature was 80°C in Shengli oilfield was was effective while the pilot tests in BIngLanLi32 (BNL-32) showed no obvious effects because the temperature was 91°C(Sun, 2014). It was indicated (Sun, 2014) that reservoirs heterogeneity affected microbial flooding significantly besides temperature. Howerver, there was no reservoirs heterogeneity parameter in current standard or codes. Jianghan oilfield (Jiang and Tan,2008; Yi et al.,2009; Fu et al., 2011) cultivated thermophilic bacteria Geobacillus kaustophilus by using indigenous microorganism. The tamed microorganism could grow in 100°C and salinity of 350000ppm and the paraffin removal pilot test in a well with 117°C temperature and 250000 ppm salinity verified satisfied paraffin and plug removal effects. Salinity was one of the important indicators affecting microbial flooding (He et al.2013; Sun,2014). High salinity and high calcium concentration formation water was not suitable to the application of the microbial flooding technology Zhang et al.,2008), and one microorganism ZLG good at degrading hydrocarbon from high saltinity (150000ppm) reservoir formation water and it could effectively emulsify the crude oil. it was reported that at salinity of 30000ppm microorganism s had good effects on crude oil (Feng and Chen, 2000) It was reported that two microorganisms separated from produced fluid grew well in salinity 100000-200000ppm, while when the salinity was higher than than 200000ppm, the growth rate of these two microorganisms got slower (Jia et al., 2014). By using 16SrDNA technology, these two microorganisms proved to be Pseudomonas aeruginosa and Bacillus subtilis (Jia et al.,2014). Pilot test (Yi and Tan,2008; Yi et al., 2009; Fu et al., 2011) indicated that microorganism cultivated in salinity of 350000ppm could remove paraffin in a well of 250000 ppm salinity with good effects.

Field test The field test was an inevitable stage for applying to microbial flooding from the laboratory to the field. The physical simulation methods was mainly learned from chemical flooding, therefore, there were some limitations, such as the impossibility of simulating the oxygen-free and endogenous environments and the length of cores, which resulted in the exclusion of microorganism without growth and propagation (Wang et al., 2004; Cheng et al., 2006). The tests in Shengli oil field showed that the results between laboratory experiments and field tests varied widely, which may be attributed to the complex reservoir conditions and physical simulation methods. Therefore, it was necessary to improve the evaluation method likechoose the low injection rate and the suitable core length to keep microorganisms staying in the core within 14 days (Cheng Haiying et al., 2006). Field tests in early times focused on microbial wax removal and plugging removal, while microbial flooding has become popular in recent years (Wang et al., 2013). Microbial profile control test after polymer flooding has been conducted in Daqing oil field (Guo et al., 2006) and obvious effects of water precipitation reduction and oil increase had been achieved. A number of field tests based on microbial flooding were carried out in Shengli oil field (Sun, 2014), involving endogenous microbes flooding and exogenous microbes flooding, and the fields for testing included only one unitization oil field and many fault block oil fields. Long duration and low cost were the most prominent effects of microbial flooding. Water cut increase rate of Luo801 oil field was controlled under 1.5% for 8 years. In the low permeability field with average gas permeability of 18mD (Cao et al., 2007), obvious effects of water precipitation reduction and oil increase were achieved after microbial flooding carried out in two well groups containing 11 wells, and 80% wells were seen effects.. Although most of the field tests based on microbial flooding was a success, some fields and wells had no obvious effect partly due to heterogeneity of the reservoir (Wang et al. 2004; Wang et al. 2005). The three-year field test based on indigenous microbes in Dagang oil field (Feng, 2005; Nazina et al., 2005) was proved a success. The field was characterized by oil-bearing area of 16 km2, formation depth of 1206.8~1412.0m, initial formation pressure of 12.37Mpa, average effective pay thickness of 23.3m, original oil in place of 6.74 million ton and recoverable geological reserve of 1.77million ton. The reservoir mainly consisted of channel sandstones and sandy conglomerate of fluvial facies with the

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porosity of 33% and the permeability of 1.878D. It was featured by viscous crude oil with low wax content, underground oil viscosity of 73mPa·s, formation water salinity of 6300mg/L, and the sewage in produced fluid was processed to serve as injection water. The formation temperature was 60°C. There were 11 water injection wells and 22 production wells with the well spacing of 300~400m. Useful indigenous microbes at deep reservoir and those near water injection wells had been activated. The number of microbes was increased by 3~7 orders of magnitude, the metabolic rate of methane by 4~78 times, and metabolites by 16 ~ 208 times after the field test. The sulfate reducing rate was raised with no hydrogen sulfide produtced. Properties of crude oil and formation water were improved, and single well production of 77% wells was increased with the input-output ratio of 1:5.2. Two microorganisms (Brevibacillus brevis and Bacillus cereus) were selected from indigenous microorganisms to conduct the field tests of single well simulation and microbial flooding in ultra-low permeability reservoirs in Daqing oil field (Guo et al., 2007; Wang et al.,2008; Gai et al.,2011). The tests gained ideal results. From 2002 to 2003,60 wells were put into stimulation tests in Daqing Chaoyanggou oilfield with an average formation permeability of 10 mD and formation temperature of 55°C. Among the 60 wells, formation permeability of 28 wells was 15-25mD, and that of 22 wells was 5-15mD, and formation permeability of 10 wells was below 5mD. The effective rate of 60 microorganism single simulation well reached 71.7%, cumulative incremental oil production was 9175.5 ton, and the inputoutput ratio was 1:8. From June 2004 to February 2005,microbial flooding tests were carried out in Chaoyanggou Chao 50 block with 2 injection wells and 10 production wells (Wang et al., 2008). The average permeability of the block was 25mD and average porosity was 17%. The formation temperature was 55°C and the fissures were developed in this area. The injection was divided into two equal slug and each slug volume was 125.2 ton, while the first and second slug bacteria concentration was 2% and 5% respectively. The pressure of injection wells decreased and injectivity increased. The liquid-producing capacity increased from 43.6t to 79.6 ton, daily oil production increased from 24.7t to 40.8t, and water cut decreased 30% and the enhanced oil recovery was 3% with a effective duration of three years. The input/output ratio was 1:6. The successful pilot tests made the reservoir permeability threshold breakthrough 50mD and also showed that microbial flooding could set effective displacement system which made the dead oil well active. On base of previous field tests, Daqing oilfield carried out expanded microbial flooding tests (Gai et al., 2011) and got obvious effects of water cut decrease and oil production increase. This field test also lead to the conclusion that injection- production relation affected microbial flooding effects significantly. Changqing oil field successfully conducted microbial flooding field tests during ultra-high water cut stage (Chen et al., 2014). The average reservoir thickness, porosity, permeability and oil saturation were 6. 8 m, 17. 5%, 75. 9 mD, 51. 1%, and the geological reserve was 1.6294 million ton. The formation salinity, PH Value, and formation temperature respectively were 28000~35000ppm, 6~8 and 55 °C. After the complex formulation, the 3 microorganisms sifted from produced liquid had great effect on viscosity reduction and emulsion, which led to the reduction of viscosity and interfacial tension by 73% and 95%. The oil displacement efficiency in laboratory experiments increased by 17.73% over water flooding. The way of “flooding after adjustment and the combination of flooding and adjustment” was applied with four-section injection technological process to 21 production wells of 4 well groups in experimental area. The slug 1 was plug slug, and the slug 2 and slug 3 respectively contained high- and low-concentration flooding agent, and the slug 4 was sealing slug of high strength. 175.8t of sustained release nutrition agent and microorganism liquid, 5 500 m3 of injection liquid with the concentration of 3% were injected, and the cumulative period was 90 days. After microorganism flooding, 100% of the producers were improved, and 75% were greatly improved. The daily production of single well was promoted from 0.33t to 0.87t at most, and the composite water cut decreased by 5.6%.MEOR in Anshai ultra-low permeability in Changqing oilfield (Liu et al., 2014) reported the water cut increasing rate of 8.1% changed to ⫺4.3% and the composite decline rate changed from13.3% to 4.4%.

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China national petroleum company(CNPC) has conducted series of field tests, and much experience has been obtained(He et al.,2013). The fundamental field tests data could be seen in Table 5. Some field tests were finished in Shengli oilfield of SINOPEC, which can be seen in Table 6. The oil sample analysis of major well in Dagang Oil Field showed that the viscosity of crude oil decreased by 7.7%, and the output of 59% of total wells increased or slowly decreased, and the original production decline law was kept unchanged in 41% wells. The output of 8 wells out of 11wells increased because of formation resistance coefficient. After implementation, crude oil of Xingjiang ordinary heavy oil reservoirs was obviously emulsified, and the stability of emulsion was better. Remarkable results of increasing oil and decreasing water were obtained, which promoted the development of water flooding in test areas. The nutritional agent could not be given full use because the water breakthrough of nutritional agent happened because of the heterogeneities of reservoirs, therefore the injection rate, and nutritional agent formulation and matching profile control were needed to be improved to increase the general results of tests. After the injection of exogenous microorganism producing polymers in Jiling oil field, the water injection profile was obviously promoted, and the liquid-produced capacity of some wells was also adjusted. But some key problems of the field application of exogenous microorganism flooding were found, such as the difficulty of controlling the mixed bacteria, the high cost of nutritional formula. The rate of kinetic viscosity reduction block oil of Baogeli oil field in the North China reached 36.1%, and reached the highest amplified reduction during 3 to 4 months. With the increase of the number of microorganism in produced liquid, the decrease amplitude of viscosity decreased. When the number of microorganism produced exceeded 105/ml, the viscosity of crude oil kept steady. The effect of viscosity reduction could maintained 3 to 4 months after the microbial flooding. The injection pressure of injection well of Y9 reservoir in Jingan Changqing oil field maintained stable with good results of plugging control, and the purposes of reaching high permeability zone plugging and starting low permeability zone were achieved. At the same time, the condition of water uptake grew better which enlarged the reservoir wave and volume. The trend of output varied from decline to increasing after microbial flooding. Ecological system has been established by microbial flooding technology in reservoirs, which could improve the oil displacement coefficient and enhance the oil recovery. In Daqing Oil Field, the polymer plugging was effectively cleaned by the microorganism in reservoirs which decomposed polymers. The purpose of enhanced oil recovery was achieved by the products of growth and metabolites of microorganism flooding and the release of residual oil.

Table 5—MEOR Pilot tests in CNPC

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Table 6 —MEOR pilot tests in Shengli Oilfield (Sun,2014)

Conclusions Compared with thermal production, gas flooding and other enhanced oil recovery methods, the prominent advantages of MEOR were much lower costs, and more environmentally friendliness. Indigenous microorganism flooding was the development trend with the advantages of good adaptability and avoiding of microbes culture development and production process compared with exogenous microbial flooding. Both laboratory and field tests had verified that the crude oil composition changed remarkably as saturated hydrocarbon proportion increased, aromatics, nonhydrocarbon and asphaltene proportion decreased, and the acid value increased while wax and pectin proportion decreased. The microbial metabolism produced surface active compounds including biosurfactants, alcohol, acid and biogases. The most common and desired biosurfactant was rhamnoilpid which could reduce interfacial tension. Bigases were mostly carbon dioxide and methane, and little ethane. The acid were mainly fatty acid like methanoic acid, acetic acid, propanoic acid. The crude became emulsified with different extent due to effects of microbes. Emulsion coefficient EI24 and the Tubiscan emulsion stability parameter measurement were used to characterize emulsion quantitatively compared with the practical but qualitative five levels classification method. Microbial products could change the wettability toward more water wet and also reduce formation permeability remarkably. The microbial profile control mechanism could be accounted into one or all the mechanisms including microbes forming reticular biofilm in porous media, precipitation of the colony and forming bridge plug due to absorbing other microbes, the biogas block effects.

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The basis for microorganisms consuming crude oil to enhance oil recovery was its ability to automatically search for carbon source and directionally migrate. Microbial effects on remaining oil could be ordered ranked in a descending order as: island remaining oil, membranaceous remaining oil, columnar remaining oil, blind end remaining oil and cluster remaining oil. Application of high resolution mass spectrum (HRMS) on MEOR mechanism has revealed the change of polar compound structures before and after oil degradation by the microbial on molecular level. The reservoir screening parameters include temperature, salinity, oil viscosity, permeability, porosity, wax content, water cut, and microorganism concentration in which production fluid, temperature, and salinity were the most important three parameters. MEOR experience and study in reservoirs of 120°C, salinity of 350,000ppm and permeability of 10 mD has expanded the reservoirs range suitable to carry out MEOR.

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