Biostimulation of biogas producing microcosm for enhancing oil recovery in low-permeability oil reservoir

Abstract

Indigenous microbial enhanced oil recovery (IMEOR) has been successfully applied in conventional oil reservoirs, however the mechanism in low-permeability oil reservoirs is still misunderstood. In order to profile the role of indigenous microcosms in oil recovery, the phylogenetic diversity of the microbial community inhibited in the reservoir by stimulation with optimized nutrients in vitro were investigated by MiSeq platforms sequencing 16S rRNA gene amplicons. Results showed that the microbial community after stimulation was dramatically changed and an increasing abundance of functional microorganisms with the ability to producing biogas, biosolvent and biosurfactant was clearly detected under anaerobic conditions: such as the genus of Clostridium, Bacillaceae, Enterobacteriaceae, Oleomonas, Marinobacter, Pseudomonas, Marinobacterium and Dietzia. Core flooding tests within sandstone were implemented and indicate that these enriched microorganisms were closely related to incremental oil recovery. In particular, biogas-producing bacteria made the most significant contribution with obvious evidence of a pressure increase during the core flooding test with no observation of decreasing surface tension and emulsification. These results suggest that the stimulation of indigenous biogas producers is a promising strategy for improving oil recovery in low-permeability oil reservoirs.

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1 Introduction

The number of low-permeability reservoirs have become a technical hurdle to enhanced oil recovery (EOR) in China. The characteristics of low-permeability reservoirs are low stratum pressure, low permeability, good water absorbing capacity and difficulty in recovering oil.¹ Water flooding is by far the most commonly used and lowest-cost approach in petroleum industry.² Problems like injected water channeling along the high permeability zone, fast rising water-cut and low yield often occur during water flooding in low-permeability reservoirs.^3,4 In order to solve them and enhance oil recovery, various approaches have been tried to improve the oil recovery on these reservoirs; such as chemical flooding, CO₂ foaming, flooding, and fracturing acidification. However, these technologies require more effort and have higher energetic, economic, and environmental costs. In addition, the feasibility of injecting chemical agents into low-permeability formations sometimes remains challenging. Therefore, indigenous microbial enhanced oil recovery (IMEOR) technology is an economically-efficient and environmental-friendly candidate through the stimulation of indigenous microorganisms by introducing nutrition to improve oil recovery has gained increasing attentions in the academic and industrial field.^5,6

MEOR technology is an environmentally friendly tertiary recovery method which involves the application of the microbial community and their metabolic products including biogas, biosurfactants, biomass and acids to extend the production life of oil wells. These metabolic products play indispensable roles with multiple mechanisms for improving oil recovery, especially biosurfactants and biogas.^7–9 Indigenous microbes, having better adaptability to the oil reservoir environment, were widely used in MEOR process.¹⁰ Numerous indigenous species, such as Pseudomonas sp., Acinetobacter sp., Bacillus sp., Rhodococcus sp., Clostridium sp. and Arthrobacter sp., have the ability to degrade crude oil and produce biosurfactants and/or biogas, and play a dominant role in enhancing oil recovery.^6,11 Thus, the diversity of the microbial community was always investigated to evaluate the feasibility or potential of IMEOR, particularly with less energy consumption and cost than exogenous technology.^12–14

It is well known that oil degraders and biosurfactant-producing bacteria play important roles in the MEOR processes;^15,16 however, the low oxidation reduction potential of petroleum reservoirs generally limited this property. Although oxygen was strategically introduced with injection water, it was rapidly consumed by the aerobic microorganisms near the wellbore area prior to entering the deep subterranean areas where the anaerobic biogas-producing bacteria, which could be good alternates for the IMEOR process, were prevalent. Nevertheless, it is possible that anaerobic microorganisms could produce an amount of biosurfactant.

Lack of nutrients is the main factor that impedes the massive propagation of the microorganisms in the reservoir, such as carbon, nitrogen, and phosphorus sources, although crude oil in reservoir could be used as carbon source.¹⁷ With the injection of nutrients, microbes in the reservoir could be stimulated and produce useful metabolites to improve oil recovery. Extensive researches have been conducted to investigate the microbial diversity of water sample from reservoir to target the beneficial microorganisms which then are stimulated by the well-designed nutrients. However, most of the research generally focuses on the bio-stimulation of biosurfactant-producing microorganism not only in the laboratory studies but also in numerous field tests with negligible interest in biogas in medium or high permeability reservoirs.^18–20 It is worthy to highlight that few detailed reports have demonstrated how the nutrients influence the microbial community and how the functional microbial groups, particularly biosurfactant-producing bacteria and biogas-producing bacteria, could be directly activated in low-permeability oil reservoirs. Therefore, it is important to figure out the possibility of IMEOR in low-permeability oil reservoirs by stimulating biosurfactant and/or biogas producing anaerobic microorganisms.

The objective of the present study is to profile the phylogenetic diversity of indigenous microorganisms in water samples from low-permeability after bio-stimulations, and find out the possible mechanism and potentials of EOR by these stimulated microorganisms in low permeability oil reservoir.

2 Experimental

2.1 Samples

Three samples of oil and formation water were collected at the heads of injection well (IW) and production wells (P1 and P2) from a low-permeability sandstone reservoir in Jing’an oilfield located in the Erdos Basin of Shanxi Province, Northwest China. 10 L of each sample was stored in hermetically sterilized plastic bottles at 4 °C, and immediately transferred to the laboratory for further analysis.

The number of microorganisms within various physiological groups such as hydrocarbon oxidation bacteria (HOB), fermentation bacteria (FMB), nitrate reducing bacteria (NRB), sulfate reducing bacteria (SRB) and methane producing bacteria (MPB) in the three samples was determined by the most-probable-number method (MPN).^21,22 The medium and culture method used for each physiological bacteria group were the same as that used by Nazina et al. and Acosta-González et al.^6,24

The physical and chemical parameters of the obtained water samples and the MPN analysis of indigenous microorganisms were showed in Table 1.

Table 1 Characteristics of water samples and SARA content of crude oil in the Jing’an oilfield

Parameter	P1	P2	IW
Category	Production well	Production well	Injection well
T (°C)	40	40	—
Water content (%)	88.1	78.4
Characteristics of the formation water
Salinity (mg L^−1)	14 457	24 341	9720
C (%)	1.42	1.23	2.41
N (%)	—	0.618	—
O (%)	4.97	5.27	3.910
Na (%)	17.4	15.8	7.850
Mg (%)	0.869	0.960	0.260
P (%)	0.0421	0.0361	0.0143
S (%)	0.0327	0.0161	0.108
Cl (%)	55.8	55.9	56.10
K (%)	0.789	0.722	0.153
Ca (%)	13.8	13.4	25.70
HOB (cell per mL)	5 × 10^2		2.5 × 10^1
FMB (cell per mL)	5 × 10^3		2 × 10^1
NRB (cell per mL)	2 × 10^2		5 × 10^4
TGB (cell per mL)	7 × 10^3		1.1 × 10^3
SRB (cell per mL)	7 × 10^2		1.1 × 10^3

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SARA content of the oil
Saturated hydrocarbon (%)	70.11
Aromatic hydrocarbon (%)	17.39
Resins (%)	6.25
Asphaltene (%)	6.25

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2.2 Nutrient optimization and culturing techniques

Based on the MPN analysis of indigenous microorganisms showed in Table 1, the biogas-producing microorganism was prevalent in the collected samples and thus was targeted for stimulation for EOR.^7,9,24 The effects of carbon, nitrogen, and yeast extract on the stimulation of the biogas producing microorganism were investigated via single-factor experiments. 100 mL of brine supplemented with selected nutrients and 2% crude oil was sealed in a 250 mL anaerobic bottle, and the anaerobic cultivation was conducted following the previous method at 40 °C for 7 days.⁷ All materials were sterilized at 121 °C for 30 min (except for production brine). The culturing with sterilized production brine was a control. All experiments were conducted in triplicate, and the data were presented as means. The population of microorganisms was determined via plate-counting method.²¹ The biogas was qualified by gas chromatography and quantified by using the drainage gas-collecting method.^7,26

2.3 Phylogenetic analysis

The above enriched culture of microorganisms stimulated with optimized nutrients were transferred into medium bottles of HOB, FMB, NRB, SRB and MPB as previous, respectively.^6,23 After 14-day incubation, cells in these medium bottles were obtained by centrifuging for DNA extraction and high-throughput microbial community analysis.

DNA was extracted using the FastDNA Spin Kit for Soil (MP Biomedicals, Cleveland, USA). The 16S rRNA gene was amplified with the universal primer set 104F (5′-GGCGVACGGGTGAGTAA-3′), and 530R (5′-CCGCNGCNGCTGGCAC-3′) in a 50 μL PCR mixture containing 25 μL of Taq PCR Mastermix (TIANGEN, Beijing, China), 6 μL of DNA template, 1 μL of primer 104F, 1 μL of primer 530R, and 17 μL of ddH₂O. The PCR program was conducted as following: initial denaturation at 95 °C for 2 min, 18 cycles beginning with denaturation at 95 °C for 30 s, annealing for 30 s at a temperature gradient ranging from 61 °C to 53 °C (1 °C touchdown every cycle) and extended at 72 °C for 30 s; with a final extension period at 72 °C for 5 min. The PCR product was purified using the E.Z.N.A Cycle-Pure Kit (Omega Bio-Tek, Inc., Norcross, USA) and then sequenced on the Illumina Miseq platform.²⁷

2.4 Bioinformatics and statistical analyses

For all data sets, reads containing one or more uncalled bases and bases with low-quality scores were removed. The FLASH method, described by Magoč and Salzberg,²⁸ was used to merge the forward and reverse reads when a correct overlap was found. The operational taxonomic unit (OTU) analysis, taxonomic richness, and diversity analysis were conducted according to Caporaso.²⁹ All sequences were assigned taxonomic affiliations with an assignment cutoff of 0.03. The Ribosomal Database Project (RDP) classifier was used to assign taxonomic data to each representative sequence. The phylogenetic analysis was performed using PyNAST.^30,31

2.5 Core flooding test

The enhancement of the recovery associated with the optimized nutrients was tested using a core flooding approach which stimulated the oil reservoir environment of Jing’an oilfield.^32,33 A schematic of the dynamic experimental setup for the physical simulation experiment is shown in Fig. 1. The cylindrical cores were 20 cm in length, 2.5 cm in diameter and packed with 100 mesh sized acid-washed silica sand with a permeability in the range of 128 to 164 mD, as show in Table 2. The core models were saturated with injection water from Jing’an. Each core model was then flooded with crude oil from Jing’an until a residual brine saturation was achieved. After aging at 40 °C for 24 h, the core models were flooded again with injection water until the water cut in the effluent of core models was higher than 98% which means that the core reached its residual oil saturation. The residual oil was then calculated by measuring the amount of oil produced during the water flooding process.

Fig. 1 Schematic of the dynamic experimental setup for physical simulation experiment.

Table 2 The parameters of sand pack columns used in the oil displacement study

Test project	Diameter (D, cm)	Length (L, cm)	Porous volume (PV, mL)	Porosity (Φ, %)	Permeability to water (Kw, mD)
Control	2.5	20	42.6	43.39	128
Nutrients	2.5	20	43.3	44.12	164

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The next steps in the experimental work depend on the type of the experiment. One set of experiments were designed as control groups, the core were shut in for 10 days at 40 °C after the first water flooding without injecting nutrients. The other set of experiments were designed to access the potential of nutrients system as an in situ MEOR. The experiments were performed with 0.4 PV prepared formation brine containing optimized nutrients, the cores were sealed for 10 days at 40 °C after nutrient injection. The amount of oil recovered in this stage was measured.

3 Results and discussion

3.1 Screening and evaluation of nutritional system for direct stimulation

Based on the MPN results of three water samples, the biogas-producing microorganism was strategically selected to be stimulated. Sample P2 was chosen to screen the nutritional system for direct stimulation. Considering the cost and H₂S hazards,^6,34 molasses, nitrate and yeast were chose as nutrients and the effects of their concentrations on biostimulation are shown in Fig. 2. The quantity of cells and the biogas production remained unchanged without the addition of molasses (in Fig. 2(a)). But enhancement was observed in samples with molasses during 7 days incubation, especially when 1.2% molasses were added. A continuous increase in the amount of molasses led to a decrease in both gas production and microorganism population. HOB was generally activated using petroleum or carbohydrate, but the growth of the microorganisms was slow when petroleum was the sole carbon source, especially for subterranean microorganisms. Compared with petroleum, molasses can be easily utilized by indigenous microorganisms and the nutritional systems containing molasses results in the rapid growth of microorganisms, especially FMB. Therefore, molasses are beneficial to microorganism growth and biogas production.

Fig. 2 The effects of the concentrations of carbon, nitrogen and yeast on biostimulation. (a) The concentration of molasses was variable, the basic medium contained (g L⁻¹): NaNO₃ 2.0, yeast 0.1; (b) the concentration of NaNO₃ was variable, the basic medium contained (g L⁻¹): molasses 12, yeast 0.1; (c) the concentration of yeast was variable, the basic medium contained (g L⁻¹): molasses 12, NaNO₃ 2.5; ▲ the number of bacteria in the culture; ▼ gas production of each 100 mL culture; ■ the number of SRB in the culture.

Up to 80% of all corrosion damage in oilfield-operating machinery is attributed to the metabolic activity of sulfate-reducing bacteria (SRB), which results in severe economic losses.³⁵ Nitrate was used to inhibit SRB growth by stimulating NRB in the petroleum reservoirs. A fine balance between carbon and nitrogen is also required for cell growth and biogas production. The effect of nitrate addition on the microorganism growth and gas production was shown in Fig. 2(b). The optimal SRB inhibition was observed when the NaNO₃ concentration was in the range of 0.2 to 0.3% and the maximum biogas and biomass production were also obtained. Nazina reported field trials in which the injection of water with 100 to 150 mg L⁻¹ of nitrate caused SRB inhibition in a reservoir containing low levels of sulfate and sulfide.²⁵ However, a higher nitrate concentration is needed in the Jing’an oil reservoir.

The effect of yeast concentration on biostimulation is shown in Fig. 2(c). Although the microorganism growth became relatively stable when the yeast concentration is above 0.06%, the maximum biogas production appeared at yeast concentration 0.08%.

Therefore, the optimized nutrition system included 1.2% molasses, 0.25% NaNO₃, and 0.08% yeast. The population of microorganisms in various physiological groups was determined after stimulation via the MPN method. The results showed that after stimulation, the population of microorganisms increased rapidly, and the population of SRB maintained a low level (Table S1†). After biostimulation, the content of volatile fatty acids increased rapidly (Table S2†), which is similar to previous studies.²² Fatty acids with small molecules can stimulate the growth of biogas-producing microorganisms, whereas one of the MEOR mechanisms, which has an important function in the improvement of oil recovery especially in carbonate reservoirs, is acid production.

3.2 Diversity analysis of enriched functional microbes

A total of 403 602 high quality sequences were obtained from 16 libraries. The sequence data quality was analyzed using FastQC, as shown in Fig. S1.† The rarefaction analysis based on OTUs at a 0.03 cut-off level shows that the curves become flat at high values of sequence numbers, which indicates a good coverage of the species in the samples.

The classification analysis of bacterial sequences was presented in Fig. 3. The population of the HOB group in P1, P2 and IW were 1.3 × 10³, 5 × 10² and 1.1 × 10² after biostimulation, respectively. After biostimulation, HOB refers to a bacteria that can use oil as a substrate at aerobic conditions.^36,37 The HOB group detected in three enriched samples were mainly categorized into two phyla, Proteobacteria and Actinobacteria. For Proteobacteria, it mainly included Phaeospirillum, Oleomonas, Pseudomonas, Marinobacter, Thalassospira, Dietzia and Parvibaculum. Compared with other genus, Marinobacter, which has been reported as a halophilic oil degrader, has a relatively high abundance in the production-water samples with an abundance of 48.3% in P2 sample and 5.2% in P1 sample, while only a negligible amount is present in the IW sample. Dietzia has been previously reported as an excellent oil-degrader and biosurfactant-producer,³⁸ was only detected in the production-water sample. Phaeospirillum with abundance of 33.9% in IW sample, has been reported as neutrophilic facultative-anaerobic, Fe(II)-oxidizing bacteria and denitrificans, but the ability for hydrocarbon degradation by this genus is still unknown.³⁹

Fig. 3 Taxonomic classification of bacterial reads retrieved from different samples at genus level from 16S rRNA gene pyrosequencing. IW, P1 and P2 refer to injection water, production water 1, and production water 2, respectively.

FMB, an important functional microbial group in the reservoir ecology, can produce a short-chain fatty acid and biogas (H₂ and CO₂). The population of the FMB group in P1, P2 and IW were 2 × 10⁹, 7 × 10⁸ and 2 × 10⁷ after biostimulation, respectively. In three brine samples after stimulation, FMB is the most abundant with Oleomonas (65.1% in IW sample), Desulfovibrionaceae (38.0%, in P1 sample), and Bacillaceae (55.7%, in P2 sample), respectively. Enterobacteriacea, Pseudomonas, and Marinobacter were also detected in three stimulated samples when analyzed with MPN. Oleomonas can degrade crude oil and has been recently described as an aerobic biosurfactant-producing bacteria.⁴⁰ The genera of Desulfovibrionaceae have been reported to have an extremely high hydrogenase activity and can produce hydrogen in natural habitats with limited sulfate.⁴¹ Enterobacteriaceae is the most prevalent in the PW sample, with an abundance of 22.1%. It can produce 1.6 moles of gas by per mole of utilized sucrose, which has great potential in oilfield applications.⁴² Bacillaceae is one of the most widely distributed bacteria in reservoirs and can produce a great amount of gas at actual oil reservoir stimulation conditions.⁴³

The population of the NRB group in P1, P2 and IW were 7 × 10⁷, 1.1 × 10⁸ and 1.1 × 10⁷ after biostimulation, respectively. For the NRB group, the dominant sequence-types in the three stimulated cultures were Hyphomicrobiaceae (61.3% in IW sample), Soehngenia (33.1% in P1 sample), Vibrionales (37.7% in P2 sample). Fusibacter, Marinobacterium, Paenibacillus, Pseudomonas, and Marinobacter were detected in relatively low amounts. Hyphomicrobiaceae dominated in the IW sample but were not detected in the PW samples. In fact, Vibrio sp. were found to be the most proficient gas-producing strains under conditions that simulated actual oil reservoir conditions. In situ growth of Vibrio in sand-packed columns produced gas (CO₂, H₂) and large recoveries of residual oil occurred.^44,45 Many species of Hyphomicrobiaceae were reported to be denitrification bacteria.⁴⁶Marinobacterium and Marinobacter had a high abundance in the PW samples. Marinobacterium and Marinobacter are nitrate-reducing, sulfide-oxidizing bacteria (NR-SOB), which contribute to the increase in redox potential through the biological oxidation of sulfide,^34,47,48Pseudomonas is one of the most common microorganisms in reservoirs and a kind of NRB, such as Pseudomonas denitrificans, Pseudomonas stutzeri, and Pseudomonas fluorescens, which were isolated from many soil and marine samples.

SRB is generally restricted in MEOR as these bacteria lead to corrosion, reservoir souring, as well as the deterioration of oil and gas. SRB had a relatively high abundance in production water (PW) samples and was undetected in the cultures of IW samples. The population of the NRB group in P1, P2 and IW were 0.5 × 10², 1.3 × 10¹ and 0.9 × 10¹ after biostimulation, respectively. Members of SRB in the PW samples were mainly Desulfovibrionaceae (84.4% in P2 and 53.1% in P1) and Fusibacter (0.15% in P2 and 24.5% in P1) followed by Sphaerochaeta. Desulfovibrionaceae was reported to be a major SRB frequently recovered from oilfields.¹⁴Fusibacter, which was first isolated from an African saline oil-producing well and has been detected in many oil reservoirs, and can reduce thiosulfate to sulfide.⁴⁹

3.3 Functional analysis of enriched microorganisms for EOR

In many studies, incremental oil production was associated with oil degraders and biosurfactant producing bacteria.^50,51 These microorganisms and their metabolites were always found in oilfield environments and play important roles in the MEOR process. The oil-degrading and biosurfactant-producing bacteria in this study are mainly related to the genus of Oleomonas, Marinobacter, Marinobacterium and Dietzia which were detected in enriched cultures, as shown above. HOB includes a small amount of the whole bacteria community and has a weak ability to produce biosurfactants under anoxic/anaerobic condition with evidence that no obvious oil emulsification was observed. We observed a high abundance of Clostridium, Bacillaceae, Enterobacteriaceae, Pseudomonas and Vibrionales in the samples, which were often reported as biogas producing bacteria.

Bacillaceae appeared frequently in the FMB, MPB culture of production water. Bacillaceae accounted for 55.7% and 8.4% in the P2.FM and P2.MP, respectively. Bacillus sp. were the most common microorganisms used for gas production for MEOR processes. Spore production by these species is also beneficial because spores survive harsh conditions and penetrate deep into the petroleum reservoir. Bacillus sp. also produce oil displacement agents such as acids, gases and alcohols.⁵²

Clostridium sp. is one of the most common and effective hydrogen producers. Clostridium sp. appeared in many samples cultured under anoxic conditions. Accounting for 29.5% in the P2.MB. It is also the dominant species existing in the microflora of the anaerobic fermentation processes. Many species of Clostridium are strong and efficient producers of hydrogen, including Clostridium butyricum, Clostridium beijerinckii and so on.⁵³

Pseudomonas had a relatively high abundance in the culture of NRB. Pseudomonas is one of the most common microorganisms in reservoirs and a kind of NRB, some species of Pseudomonas such as Pseudomonas aeruginosa, Pseudomonas stutzeri and Pseudomonas fluorescens possess the ability to denitrify nitrate compounds and produce nitrogen in anaerobic condition by the following reaction:⁵⁴

NO₃⁻ → NO₂⁻ → NO → N₂O → N₂.

Desulfovibrionaceae and Fusibacter had a relatively high abundance in the culture which limited S. Desulfovibrionaceae and Fusibacter which were reported as sulfate and thiosulfate reducing bacteria. They were a major group of environmental anaerobic bacteria that play a key role in the global cycle of carbon and sulfur. They also have the ability to use simple organic compound such as lactate, ethanol, formate and butyrate to produce H₂ through fermentation in limiting sulfur conditions.⁵⁵

Biogas producers were closely linked to incremental oil production during the MEOR process. Metabolites of biogas producers include gases (CO₂ and H₂), acids and solvents that were used to improve oil production from individual wells or to mobilize entrapped oil during water floods. If sufficient CO₂ and CH₄ are created, these gases will result in the swelling of crude oil and a reduction in its viscosity. In situ gas production may also lead to repressurization of oil reservoirs and hence improve oil recovery, especially in mature reservoirs. Organic acid production can lead to the dissolution of carbonates in source rocks, increasing porosity and permeability and enhancing oil migration. In this study, the biogas producing bacteria appeared frequently in different samples. They were easy to active in the limited oxygen environment when supplied with carbohydrates and low molecular weight organic matter. Biogas producing bacteria would be a potential microorganism in the MEOR.

3.4 Core flooding test

The core flooding test was designed to simulate the IMEOR process. Two groups of tests were designed to evaluate the influence of the optimized nutrients on oil recovery. The results of the core flooding tests are shown in Fig. 4.

Fig. 4 Change in oil recovery versus injected PV in core model.

During the shut-in period after sample injection, the inner pressure of the microbial core holder increased and reached maximum value with 0.65 MPa, while there was no significant increase of pressure in the control core holder, indicating that biogas was clearly produced under the anaerobic conditions.

As mentioned earlier, the first experiment was designated as control groups. It is shown that water flooding resulted in the recovery of 37.62% of original oil in place (OOIP) due to its volumetric sweep efficiency and the results from the second water flooding revealed that very little oil recovery (0.39% of residual oil) occurred. The other experiment was designed to evaluate the effectiveness of nutrition injection in IMEOR. It is shown that water flooding resulted in the recovery of 37.75% of OOIP due to its volumetric sweep efficiency and also the results from the second water flooding revealed that 3.7% of residual oil was obtained.

Biogas-producing bacteria were used in many MEOR field trials which have resulted in large increases in pressure and decreases in oil viscosity,⁵⁷ meanwhile, fermented CO₂, acid and solvent production at the sand surface may have led to oil release.

Arief Nuryadi et al. reported oil recovery enhanced in situ by anaerobic denitrifying medium injection.⁵⁸ Additional oil recovery in the core flooding experiments was predicted to be the result of re-pressurization by nitrogen biogas production. Macroscopic observation revealed that the injection of Bacillus subtilis resulted in more residual oil released than the injection of only the nutrient solution.^56,58 Previous research has provided evidence that stimulation or injection of bio-gas producing bacteria in the field or core flooding experiments could increase oil production with varied dynamics. The shut-in test experiment with the injection of Clostridium botulinum (CO₂ producing bacteria) resulted in 43% oil recovery from OOIP with around 0.35 MPa pressure increment.⁵⁷ Compared to this experiment, the resulting oil recovery involving nutrition injection was low but reasonable.

In general, the results indicate that biogas-producing bacteria stimulated by optimized nutrition are the reason for additional oil recovery during stimulation. Therefore, nutrient injection can provide a potential stimulation-based MEOR application in the reservoir.

4 Conclusion

The results show that the potential microbes effective for IMEOR in the investigated oilfield belong to biogas-producing bacteria. The main functional microbes include Clostridium, Bacillaceae, Enterobacteriaceae, Pseudomonas and Vibrionales. The optimized nutrition system can efficiently stimulate the growth of gas-producing bacteria as proven via the core flooding experiment. Although substantial fundamental studies of oil displacing mechanism need further testing, low permeability reservoir could be clearly developed with indigenous MEOR technology to improve oil recovery by stimulating bio-gas producing microorganism.

Acknowledgements

The authors are grateful to the State Key Laboratory of Heavy Oil of China University of Petroleum. This paper is supported by National Science and Technology Major Project (No. 2011ZX05009-004), by the National Natural Science Foundation of China (No. 41403068), the Major State Basic Research Development Program of China (2011CB200906).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra18089a

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