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Microorganisms can produce bio-hydrogen (H2) during their metabolism, and especially dark fermentative production from carbohydrates is a promising option. However, the technology still suffers from low H2 production rates due to product inhibition of the H2-producing enzymes. In this study we investigate if subsurface (reservoir) microbes can be relevant bio-H2 producers and a source of high-pressure adapted enzymes. In addition, we also investigate the potential H2 productivity using an indigenous reservoir community, thereby proposing a preliminary screening workflow for mature oil fields to be used as potential in-situ H2 bioreactors. We cultured two H2-producers Marinitoga hydrogenitolerans and Thermotoga napthophila, which were isolated from high-pressure environments, and a freshly obtained oil reservoir brine containing a fermentative community under atmospheric and pressurized conditions (15–16 bar). All cultures produced H2 under atmospheric conditions with different productivities. Marinitoga shows the same cell number increase and glucose consumption under high pressure compared to atmospheric but with a significantly decreased H2 productivity of −86%. The productivity decline of the reservoir community was less pronounced with −35%. The mole fraction of H2 in the produced gas was in both cases high (0.64 and 0.82 respectively). Our results indicate strong differences in the pressure-related enzyme adaptations and may be used for screening of pressure-tolerant microbes and enzymes for enhanced bio-H2 production.

H2 is a CO2 free energy carrier that can be produced biologically through dark fermentation using specific bacteria. In general, biological production of H2 needs a carbon source and is more efficient at higher temperatures. Mature petroleum reservoirs have the required high temperatures for H2 production, and they contain a significant amount of organic matter in form of residual hydrocarbons. In this work, we evaluated whether indigenous microorganisms isolated from hydrocarbon reservoirs are able to biorefine hydrocarbons to H2. We observed that two Thermotoga strains, Pseudothermotoga hypogea DSM-11164 and Pseudothermotoga elfii DSM-9442, are able to convert hydrocarbons to H2. DSM 9442 produced 0.47 and 1.02 mmol H2 per liter of growth medium from 20 ml/L of n-hexadecane or a crude oil, respectively. DSM 11164 only produced H2 from n-hexadecane (0.94 mmol/L). Addition of 25 mg/L Tween 80, to reduce phase separation, together with 1 g/L glucose increased H2 production from hydrocarbons up to 12-fold. Via an energy analysis we show that bioconversion of crude oil into H2 can be more efficient than conversion of crude oil to gasoline. Therefore, we suggest dark fermentation as a promising alternative to biorefine crude oil and unlock the energy trapped in hydrocarbon reservoirs after abandonment.

As the use of hydrogen gas (H2) as a renewable energy carrier experiences a major boost, one of the key challenges for a constant supply is safe and cost-efficient storage of surplus H2 to bridge periods with low energy demand. For this purpose, underground gas storage (UGS) in salt caverns, deep aquifers and oil-/gas reservoirs has been proposed, which are environments with potentially high microbial abundance and activity. Subsurface microorganisms can use H2 in their metabolism and thus may lead to a variety of undesired side effects such as H2 loss, H2S formation, methane formation, acid formation, clogging and corrosion. In this review, we summarize the current knowledge and methodologies on microbial related risks related to H2 UGS, including basic research on subsurface microbiology, results from field trials, and experience from other industries including oil & gas. For safe underground storage of H2 gas we recommend to analyze the relevant microbiological and geochemical characteristics of each site to be able to predict the most suitable storage strategy and establish a good monitoring and mitigation approach to follow and counter potential microbial side effects.

Fe(II) oxidation coupled to nitrate reduction is a widely observed metabolism. However, to what extent the observed Fe(II) oxidation is driven enzymatically or abiotically by metabolically produced nitrite remains puzzling. To distinguish between biotic and abiotic reactions, we cultivated the mixotrophic nitrate-reducing Fe(II)-oxidizing Acidovorax strain BoFeN1 over a wide range of temperatures and compared it to abiotic Fe(II) oxidation by nitrite at temperatures up to 60°C. The collected experimental data were subsequently analyzed through biogeochemical modeling. At 5°C, BoFeN1 cultures consumed acetate and reduced nitrate but did not significantly oxidize Fe(II). Abiotic Fe(II) oxidation by nitrite at different temperatures showed an Arrhenius-type behavior with an activation energy of 80±7 kJ/mol. Above 40°C, the kinetics of Fe(II) oxidation were abiotically driven, whereas at 30°C, where BoFeN1 can actively metabolize, the model-based interpretation strongly suggested that an enzymatic pathway was responsible for a large fraction (ca. 62%) of the oxidation. This result was reproduced even when no additional carbon source was present. Our results show that at below 30°C, i.e. at temperatures representing most natural environments, biological Fe(II) oxidation was largely responsible for overall Fe(II) oxidation, while abiotic Fe(II) oxidation by nitrite played a less important role.

Souring induced by sulfate reducing microorganisms (SRM) represents a severe problem in the petroleum industry. In addition to conventional biocides and nitrate, alternative SRM inhibitors such as molybdate have been proposed recently for controlling microbial souring. We used oilfield-derived microbial consortia, rock and fluids to test molybdate as a specific SRM inhibitor for a microbial enhanced oil recovery (MEOR) application where souring might occur as a side effect. SRM cells were quantified and dissolved molybdate, sulfate and gaseous hydrogen sulfide were measured under different dynamic conditions in sandpacks with and without residual oil. In batch experiments, 0.5 mM molybdate inhibited SRM growth whereas hydrogen sulfide and mineral precipitations were observed in bottles amended with 100 mM nitrate. However, significant molybdate adsorption onto reservoir rock occurred and maximum Langmuir saturation was estimated to be ≥ 34 μmol g−1. Residual oil allowed a further propagation of molybdate into sandpacks, but a pH < 6 and sulfide concentrations >11 μMH2S aq limited the efficiency of molybdate due to rapid adsorption. Under favorable souring conditions, we also observed the localized formation of macroscopic iron sulfide precipitations. These resulted in a four-fold permeability decrease after the injection of SRM substrates for 40 days and a calculated mean sulfate reduction rate of 52 μM h−1. However, delayed sulfate reduction in molybdate-preflushed sandpacks suggests an inhibitory effect even if molybdate is partially adsorbed. Sulfate reduction was not detected when molybdate was continuously injected with MEOR nutrients into sandpacks demonstrating its inhibitory efficiency in case it is applied in early phases of field operations with a potential risk of souring.

In the course of the submission, revision and publication of the research paper, author affiliations changed to some extent. The authors regret that these affiliation changes are not reflected in the final publication and would like to apologise for any inconvenience and confusion caused thereby. The corrected affected affiliations are as follows: Felix Kögler, RWS IP Services, Chalfont St. Peter, UK; formerly Wintershall, Ante Borovina, OMV AG, Austria; formerly Wintershall, Foppe Visser, retired; formerly Wintershall

Polymer flooding is a widely applied enhanced oil recovery (EOR) technique using soluble polymers to increase the injection water viscosity and therefore enhance the sweep efficiency in the field. Biopolymers are an environmentally friendly alternative to synthetic polymers and can have good salt- and temperature tolerance like the polymer Schizophyllan. Because biopolymers are often biodegradable, it is important to protect them against potential microbial degradation at subsurface conditions, especially in the near wellbore region consisting of the first few meters after injection. Three individual sandpack experiments were performed to assess biodegradation of Schizophyllan at original reservoir conditions. An enriched, biodegrading microbial community was injected into all sand packs and two treatment options were tested: a) adding the biocide Bacillat after a mature degrading biofilm was developed; b) adding biocide prior to mature biofilm formation. Different biocide concentrations were tested. Rest viscosity (i.e. level of biodegradation) was determined by measuring viscosity injected and produced from the sandpack columns. Various microbial (cell numbers, metabolite production and identity) and petrophysical (differential pressure, permeability) parameters were assessed during the experiments. The results show that the relative loss in the effluent viscosity was lower than 10 % when 375 ppm biocide was added to the injection fluid 24 hours after the inoculation period (prior to mature biofilm development). Microbial cell counts were low and byproducts because of degradation could not be measured even after 80 days of injection. The same concentration proved to be ineffective to improve the effluent viscosity (90 % viscosity loss) measured in the column with a mature biofilm. Successive concentration increase (375, 750 and 1900 ppm) did not have a significant effect on viscosity maintenance and were not able to inactivate biodegradation. Initial high biocide concentrations (750 ppm and 1900 ppm) could not protect the polymer in the presence of an active biofilm Furthermore, degradation of the biopolymer could not be prevented by using 200 ppm biocide at the start of injection before biofilm buildup. This shows a strong resistance and adaptability of the biofilm towards the used biocide. Permeability of the sand packs containing a growing biofilm decreased drastically, indicated by a continuous increase in differential pressure. Our study shows that Schizophyllan could be protected from bio-degradation if the right biocide concentration is used at the beginning of the injection period. The sandpack study shows the importance of a well-designed biopolymer protection strategy prior to field implementation and the need for an early mitigation of biodegradation and/or biofilm formation. Such experiments enable the possibility to test the different biocidal treatments under reservoir-like conditions and predict biopolymer stability in the field.

Microbiologically active environments, like oil reservoirs, can suffer a range of problems due to the presence of sulfate-reducing microorganisms. These include, but are not limited to: H2S formation, souring and corrosion. To prevent biogenic sulfate reduction, nitrate can be used as an environmentally friendly alternative to biocides. However, side effects of nitrate injection are sometimes observed but not well understood. In our study, we used original water from an onshore reservoir, where a microbial enhanced oil recovery (MEOR) application is planned, and investigated the effects of nitrate addition on H2S generation, mineral precipitation and microbiology. We observed that nitrate did inhibit H2S formation in most, but not all cases. During nitrate reduction, iron and calcite minerals precipitated over short- and long-term (10 or 160 days) incubations. This was caused by a nitrate-reducing group of bacteria belonging to the family Deferribacteraceae. Using dynamic sandpack setups and numerical modeling approaches with the simulator TOUGHREACT, we observed significant reduction in permeabilities (∼44×) suggesting injectivity issues over time in case nitrate is continuously added to the reservoir. Our study shows that nitrate-dependent processes, which were described separately for pure-cultures before, are also valid for natural mixed communities present in oil fields and underlines the complex interplay of microbial metabolisms associated with those communities.

Utilisation of microorganisms as an enhanced oil recovery (EOR) method has attracted much attention in recent years because it is a low-cost and environmentally friendly technology. However, the pore-scale mechanisms involved in MEOR that contribute to an additional oil recovery are not fully understood so far. This work aims to investigate the MEOR mechanisms using microfluidic technology, among others bioplugging and changes in fluid mobilities. Further, the contribution of these mechanisms to additional oil recovery was quantified. A novel experimental setup that enables investigation of MEOR in micromodels under elevated pressure, reservoir temperature and anaerobic and sterile conditions was developed. Initially, single-phase experiments were performed with fluids from a German high-salinity oil field selected for a potential MEOR application: Brine containing bacteria and nutrients was injected into the micromodel. During ten days of static incubation, bacterial cells and in-situ gas production were visualised and quantified by using an image processing algorithm. After that, injection of tracer particles and particle image velocimetry were performed to evaluate flow diversion in the micromodel due to bioplugging. Differential and absolute pressures were measured throughout the experiments. Further, two-phase flooding experiments were performed in oil wet and water wet micromodels to investigate the effect of in-situ microbial growth on oil recovery. In-situ bacteria growth was observed in the micromodel for both single and two-phase flooding experiments. During the injection, cells were partly transported through the micromodel but also remained attached to the model surface. The increase in differential pressure confirmed these microscopic observations of bioplugging. Also, the resulting permeability reduction factor correlated with calculations based on the Kozeny-Carman approach using the total number of bacteria attached. The flow diversion of the tracer particles and the differences in velocity field also confirmed that bioplugging occurred in the micromodel may lead to an improved conformance control. Oil viscosity reduction due to gas dissolution as well as changes in the wettability were also identified to contribute on the incremental oil. Two-phase flow experiments in a newly designed heterogeneous micromodel showed a significant effect of bioplugging and improved the macroscopic conformance of oil displacement process. This work gives new insights into the pore-scale mechanisms of MEOR processes in porous media. The new experimental microfluidic setup enables the investigation of these mechanisms under defined reservoir conditions, i.e., elevated pressure, reservoir temperature and anaerobic conditions.

Microbial enhanced oil recovery (MEOR) is an economically attractive tertiary recovery technique and fermentative bacteria are frequently suggested for MEOR, partly because microbially produced organic acids have the potential for rock matrix dissolution. In this study, the metabolic activity and the community shift of a diverse microbiome of a high-salinity oilfield upon supplying MEOR nutrients was investigated in dynamic sandpacks set-up with and without crude oil using pure quartz sand and two types of reservoir rock. Geochemical characterization of the porous media included specific surface area (SSA), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). During the experiments, substrate and metabolites, incremental oil and differential pressure were monitored and the microbial community shift was investigated via Illumina sequencing. Fermentative Halanaerobiales outcompeted other microbes and led to an incremental oil recovery of 24.5 ± 9.6 %OOIP in reservoir rock. Microbial-induced dissolution of surface minerals was indicated by a decrease in SSA and surface-bound ferrous iron and concluded to be an important MEOR mechanism. Fermentation of sucrose was primarily limited by an insufficient acid neutralization capacity (ANC), but a carbonate content of 12% sustainably buffered the pH in a favorable growth range. Even minor amounts of other non-inert minerals (1% pyrite and calcite) facilitated microbial growth significantly, resulting in six-fold higher acetate production rates compared to quartz sand. Besides emphasizing the relevance of accessory minerals in MEOR, our results imply that the ANC could serve as potential screening parameter for predicting the performance of fermentation - based MEOR in the field.

As a dynamic screening tool for a high-salinity oilfield (186 g/L), anaerobic sandpacks were established to simulate Microbial Enhanced Oil Recovery (MEOR) under defined laboratory conditions. Glass beads, quartz sand or crushed reservoir rock were used to produce porous media which varied in permeability, wettability, homogeneity and geochemistry. In total, 14 sandpacks were flooded with oil and inoculated with indigenous fermentative bacteria of the order Halanaerobiales. After waterflooding, these were treated with nutrients in different injection scenarios during which incremental oil recovery, permeability, microbial activity and produced metabolites were measured. Our results indicate that the efficiency of MEOR is dependent on the type of porous medium used: Both glass beads and outcrop quartz sand were found to be no suitable analogue to reservoir material because not all potential MEOR effects were accounted for. MEOR was least efficient in quartz sandpacks with a recovery factor of 7.0 ± 1.7% with respect to the original oil in place (IRFOOIP), attributed mainly to fluid-fluid interactions. In sandpacks with reservoir rock, wettability alteration, matrix dissolution and bioplugging were additional MEOR mechanisms and resulted in an incremental recovery which was almost three-fold higher compared to pure quartz sandpacks (IRFOOIP = 23.2 ± 6.4%). Bioplugging was not detected in sandpacks with a permeability of 8–10 D, although cell retention was observed. Mean pore sizes of these sandpacks were calculated to be in the range of 100 μm, thus considered to be too large to allow for significant plugging. Our findings support the use of MEOR as potential tertiary recovery method but also emphasize the importance of carefully designing laboratory experiments. We argue that porous medium properties such as permeability, pore size, wettability and mineralogy play a crucial role during dynamic MEOR feasibility studies, because they directly influence incremental recovery.

Control of microbial activity and biofilm growth is an essential factor in a variety of medical and industrial applications. As microbial biofilms can protect cells within from physical or chemical threats, biocide treatments are often limited in their efficiency due to the low permeability of the biofilm. Therefore, finding new treatment options against biofilms remains an issue of high importance. We explored the option of using osmotic pressure to physically deform the biofilm matrix and increase detachment from a glass surface. Our model system is a single-species biofilm of Thalassospira sp. growing in a microfluidic set-up, enabling the observation of changes in biofilm volume and detachment rate. We observed that injection of a high salinity brine (NaCl) led to a reduction of biofilm volume (water efflux), and that subsequent injection of low salinity brine caused biofilm expansion (water influx). The differential osmotic pressure imposed on the biofilm was sufficient to weaken its adhesive strength to the glass surface, causing major detachment. The current approach has value as a method for cleaning glass microfluidic channels and other glass tubing or equipment clogged by biofilms, thus avoiding the use of harsh chemicals. As the basic physical principle of osmotic pressure initiating biofilm expansion and detachment is universal, our findings could be applied to other forms of biofilm growth.

The increasing demand for oil and also increasing numbers of mature oil reservoirs have attracted attention to enhanced oil recovery (EOR) in which microbial enhanced oil recovery (MEOR) is one of the commercial options. In order to develop a MEOR strategy for a given field it is extremely important to evaluate the microbial community present, as this will be the basis for a customized nutrient mixture. The objective of this study was to assess how sampling strategies (surface vs. subsurface) can accurately reflect the microbial community and microbial activity in a reservoir. In addition to fluid samples the analysis of sand deposits, also collected from the well bottom, was included to get insight into presence and identity of attached microbes. Fluid samples were collected at the wellhead (surface) and in-situ at a depth of approximately 780 m tvdss (true vertical depth subsea) under a reservoir pressure of 31 bars (subsurface). The pressure was released either quickly (within minutes) or slowly (stepwise, within 11 days) to assess the potential activity loss of microorganisms sensitive towards rapid pressure changes. Quantitative PCR (polymerase chain reaction) for determining cell numbers showed higher amounts of microbial cells in the subsurface sample compared to the surface water. The composition of the microbial community was only slightly more diverse in the subsurface sample than in the surface sample. Sand material, however, showed the most diverse community. Biogenic CO 2 formation rates were determined under atmospheric and high pressure conditions with highest rates, i.e. highest microbial activity, in the wellhead fluid and the solid sand sample. In conclusion no substantial differences were observed between fast and slowly depressurized subsurface fluids and also between surface and subsurface fluid samples in terms of microbiological activity and diversity. Therefore, using wellhead fluids for microbiological analysis and MEOR development seems to be a valid option although it has always to be carefully considered not to miss out on information of solid samples regarding various aspects such as community and biofilm formation.

Sulfate-reducing bacteria (SRB) can be commonly found in oil reservoirs causing H2S formation and subsequently corrosion and souring issues in associated production/injection water systems. To control and mitigate the microbial H2S formation, the production-injection loop should be monitored and the results should be implemented for effective control of H2S formation. One mitigation measure can be the injection of a biocide into produced and/or injected water killing or inactivating SRBs and other microorganisms responsible for the souring and fouling. However, the challenge often occurs to define the suitable injection spot and the required biocide concentration; not well defined biocide concentration and/or injection location can even favor SRB proliferation causing for example fouling of tanks or filters, which themselves can continously "inoculate" the system. At the water treatment plant of of one of Wintershall's onshore oil field in Northern Germany, formaldehyde injection was started in the end of 2015. The scope of the project was to investigate the effectivness of the formaldehyde regarding the bacterial- and SRB count and to analyse the MIC (Microbially influenced corrosion) and souring potential in and around the treatment plant including the waterflood injection line. Water samples were taken under sterile and anoxic conditions at various points including production lines from the three fields, selected spots of the water treatment system and injection lines to the fields. The samples were analysed for key chemical- (pH, salt content, sulfate, formaldehyde, organic acids, H2S) and microbial parameters (bacterial, archaeal via qPCR (quantitative polymerase chain reaction) and SRB cell numbers via MPN (Most Probable Number). The obtained data can not only help the operator to optimize the biocide injection to reduce souring and fouling, but also to better understand the complex microbial growth occuring in the production facilities.

Wintershall is conducting a technology project for development and field application of MEOR (Microbiologically Enhanced Oil Recovery) in collaboration with BASF. The successful results of the laboratory phase led to a first small confined pilot Huff'n’ Puff (HnP) in a Wintershall mature oil field to prove that the laboratory-developed concept works in the field under reservoir conditions. A suitable well for the MEOR operation was selected in the studied field based on selection criteria. The selected well is a former producer approximately 900 m deep. After a USIT run it was decided to recomplete it. Prior to MEOR HnP pilot, an injectivity test was performed to allow for re-assessment of the current petrophysical and geological properties around the well. In order to establish the baseline for the pilot evaluation, a comprehensive monitoring program consisting of microbiological, chemical and petrophysical surveys commenced just after the well recompletion. The surface set-up designed for follow-up MEOR field operations was installed in the field. The mixing of the MEOR solution with the injection water was regulated automatically by measuring the injection rate. The injection took four days, followed by an incubation period of five weeks. During the nutrient injection, the injectivity was significantly lower than the one obtained from a previous injectivity test. As a result, the total volume of injected nutrient was lower than initially planned. Nevertheless, the volume was sufficient to achieve the pilot objectives. The injection was carried out under matrix conditions by keeping the pressure below the fracture pressure. The injected fluid temperature was somewhat lower than planned, but according to downhole measurements, still high enough for microbial growth. It was observed that there was an oxygen ingress into the system through the injection pump, however no detrimental effect was seen on microbial activity. After the shut in period, a comparable volume of the injection fluid was produced back. The tracer concentration in the back produced fluid was used to calibrate the chemical and microbiological effects of MEOR.

Under certain conditions, injection of water or other solutions may result in H2S production, which creates additional challenges for EOR applications. Besides serious health and safety concerns, this can cause significant production problems such as reduced quality of hydrocarbons, reduced productivity of wells and increased corrosion. The numerical assessment of the souring and its successful inhibition for an MEOR (Microbial Enhanced Oil Recovery) application in a Wintershall mature oil field is the subject of this paper. The numerical assessment of the biogenic souring for the field studied was performed in three successive steps. In the first step the potential H2S generation as well as its partitioning between various phases was calculated by analytical methods. The spatial distribution of the potential H2S generation was predicted by using the CMG Stars reservoir simulator on a sector model representing the reservoir portion relevant for the application. In a third step a reactive transport model was used to evaluate the implications of an H2S inhibitor injection due to reactions with formation water and rock. A significant H2S formation potential was determined considering the formation water sulfate content and the MEOR metabolism. Its portitioning in various phases was calculated for various thermodynamic conditions of the production sequences. The spatial modeling of the H2S generation was done in a previously performed CMG Stars implementation of the MEOR simulation for the case studied. The stoichiometry of the reactions applied was calibrated by using laboratory growth and metabolism data. Various strategies were studied to inhibit the souring at laboratory scale with nitrate being one of the inhibitors tested. Potential microbial induced calcite precipitation (MICP) when using nitrate and its effect on the storage and conductivity properties of a reservoir was modelled using the reactive transport computer code TOUGHREACT. The results obtained led to the key decision for testing other souring inhibitors for the field application. The results of the numerical assessment are discussed in terms of its contribution to the decision quality of the risk management concept developed and applied in the field pilot.

Microbial Enhanced Oil Recovery (MEOR) is a cost-effective and environmentally friendly method for mature reservoirs, exploiting indigenous microorganisms that can be stimulated in the reservoir. As MEOR relies of the combination of various mechanisms a very well designed screening procedure is necessary for a successful field application. In a MEOR project started 2011 by Wintershall and BASF, we established dynamic sandpacks to investigate microorganisms sampled from Wintershall fields. Requirement for the setups are strictly anaerobic and sterile conditions. Original fluids including oil, injection water and reservoir microbes are used together with different materials to create the porous media consisting of either glass beads, quartz sand or crushed reservoir rock in order to produce sandpacks with permeabilities ranging from 1 to 13 D. Analytics included petrophysical aspects(permeability, pososity, fluid saturations) as well as microbial methods (e.g. 16S sequencing). In more than 20 dynamic MEOR experiments we observed that the choice of the porous medium is crucial for dynamic screenings and affects both microbial growth as well as oil recovery. Our study contributes to the improvement of MEOR screening methods by conducting reliable dynamic experiments, which will help having more accurate predictions for MEOR field applications in the future.

This paper explores the potential of spontaneous imbibition (SI) induced by wettability alteration to improve incremental oil recovery during microbial enhanced oil recovery (MEOR). A laboratory work is presented that consists of SI and contact angle (CA) tests on reservoir and outcrop sandstone cores. The SI and CA measurements were conducted with aqueous phases with/without MEOR to assess the incremental recovery mechanism triggered by the wettability alteration during MEOR. It was observed that the oil recovery in the SI experiments with the MEOR nutrient solution reached a plateau at 0.62 ± 0.05% of original oil in place (OOIP) while it was 0.32 ± 0.02% of OOIP with the non-MEOR reference SI experiment conducted in outcrop sandstone cores. The CA of the oil droplet measured on polished reservoir rock sample decreased from 120° to 60° when exposed to the MEOR nutrient solutions. The observed changes occurred in parallel to the growth process of the bacteria in both SI and CA tests. The experimental data shows that the wettability alteration needs cell growth because SI experiments performed only with metabolites and without microbial cells exhibited negligible incremental recovery. Numerical models calibrated with SI experiments demonstrated the wettability alteration as one of the potential MEOR mechanisms. The bacterial network that forms a biofilm at the oil-water interface, and hence, creates a viscoelastic layer is thought to be the main cause of wettability alteration, which is supported by the oscillating pendant drop measurements.